access time analysis of mcptt off-network mode over lte
TRANSCRIPT
![Page 1: Access Time Analysis of MCPTT Off-Network Mode over LTE](https://reader031.vdocuments.us/reader031/viewer/2022021713/620b940c2b21b5120d3c510e/html5/thumbnails/1.jpg)
Research ArticleAccess Time Analysis of MCPTT Off-Network Mode over LTE
Yishen Sun , Wesley Garey, Richard Rouil , and Priam Varin
Wireless Networks Division, National Institute of Standards and Technology, Gaithersburg, MD 20899, USA
Correspondence should be addressed to Yishen Sun; [email protected]
Received 30 November 2018; Accepted 27 January 2019; Published 2 April 2019
Guest Editor: Maurizio Casoni
Copyright Β© 2019 Yishen Sun et al. This is an open access article distributed under the Creative Commons Attribution License,which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
Public safety organizations around the world started migrating toward Long-Term Evolution (LTE) networks to support theincreasing needs for video and data. To address the unique voice communication requirements of first responders, the 3rdGeneration Partnership Project (3GPP) introduced new capabilities that aim at providing similar functionalities as the traditionalLand Mobile Radio (LMR) systems, namely, Direct Mode communication and mission critical push-to-talk (MCPTT). DirectMode communication, also called Proximity Services (ProSe), allows public safety users to communicate directly with each otherregardless of the network status.MCPTTwas the firstmission critical service, andfirst application, standardized by 3GPP to provideboth on- and off-network voice capability. Assessing the performance of those capabilities is critical to accelerate their deploymentand adoption by first responders. In this study, we evaluate the performance of an off-network mode MCPTT device over ProSeby focusing on the access time, a measure of the delay incurred before a user can talk. We develop analytical models for varioustypes of calls and verify the accuracy of the predicted access time using ns-3 simulations. We perform sensitivity analysis to showthe validity of the models for various scenarios. Finally, we show how the models can be used to guide parameter configuration forboth MCPTT and ProSe to optimize the performance.
1. Introduction
Public safety agencies require real-time applications, andvoice communication remains the most important means ofcommunication for first responders in emergency situations.While most existing push-to-talk (PTT) communication iscarried over Land Mobile Radios (LMRs) to achieve publicsafety grade voice quality and reliability, first responders havealready adopted broadband technology, including 3rd Gen-eration Partnership Project (3GPP) Long-Term Evolution(LTE), to access video and data. Thanks to the effort of publicsafety organizations around the world, 3GPP has extendedLTEβs capabilities to support many of the situations facedby first responders and increase the networkβs reliability. Inparticular, extensions were made to fulfill the mission criticalvoice communication requirements defined by the publicsafety community [1, 2]. Those requirements include PTT,which is the standard means of communicating amongstfirst responders (similar to walkie-talkie), and Direct Mode,which allows first responders to communicate unit-to-unitwithout using the network. Also included is the ability to talk
to multiple units at once (i.e., group call), the identificationof the person speaking, and notification of emergency situa-tions.
PTT is not new, and a lot of research has been doneon the Open Mobile Alliance (OMA) PTT over Cellular(PoC) services [3] in recent years. Early evaluations of PoCwere done over 3G technologies and the results showedsignificant setup delays [4] and talking wait time on the orderof seconds [5, 6]. Even over LTE, PoC is far from being ableto provide the performance needed by first responders. Forexample, Kuwadekar and Al-Begain evaluated PoC using areal network under different conditions [7]. The call setupdelay was measured in the testing, which is the time fromwhen the PTT button was pressed to the time when all theUEs were connected so that the user could speak. The resultsshowed that if the network was not loaded, the call setupdelay ranged from 0.7 s to 4.6 s, and that the average callsetup delay was 2.556 s, which is higher than what is seenin LMR systems. Kuwadekar and Al-Begain also showed thatPTT call setup delay increased roughly linearly with respectto the amount of emulated background calls that they injected
HindawiWireless Communications and Mobile ComputingVolume 2019, Article ID 2729370, 19 pageshttps://doi.org/10.1155/2019/2729370
![Page 2: Access Time Analysis of MCPTT Off-Network Mode over LTE](https://reader031.vdocuments.us/reader031/viewer/2022021713/620b940c2b21b5120d3c510e/html5/thumbnails/2.jpg)
2 Wireless Communications and Mobile Computing
into the network, and the call setup delay exceeded 10 s oncethe number of simultaneous background calls exceeded 700.Tests of PTT during a festival, when the local network wasbeing used by up to around 10,000 people, resulted in a100% call failure rate. Those high delays are attributed to theSession Initiation Protocol (SIP) used by PoC and providedby the Internet Protocol (IP) Multimedia Subsystem (IMS).An enhanced version of PoC for public safety, called Push-to-Communicate for Public Safety (PCPS) [8], was devel-oped and aligned with 3GPP efforts to standardize a PTTapplication capable of meeting public safety needs in termsof features and performance. This Mission Critical Push-to-Talk (MCPTT) application was introduced in LTE Release13 and further enhanced in later releases [9]. The currentefforts to evaluate MCPTT mainly focus on on-networkscenarios, where the devices are connected to the network viaa base station. In [10], Kim et al. proposed an algorithm toshorten the time of the initial uplink transmission in orderto reduce the control plane latency. In [11], Solozobal et al.looked at using mobile edge computing to provide a flexiblearchitecture in 5G networks where the user plane can bedeployed closer to the users, thus improving performance.The proposed solution would also allow the network operatorto deploy the control plane at the edge of the network toprovide a standalone service if the evolved NodeB (eNodeB)is disconnected from the core network.
To support Direct Mode capability in LTE, 3GPP intro-duced Proximity Services (ProSe) capability in Release 12 andhas continued to enhance ProSe in subsequent releases [12].ProSe allows a piece of User Equipment (UE), such as a laptopor a phone, to discover and communicate with other UEs.ProSe supports both one-to-one and one-to-many transmis-sions, thus allowing for efficient group communication. Theresearch on ProSe communication has primarily focused onthe coverage and capacity aspect of this new feature. Wangand Rouil evaluated the expected device-to-device (D2D)coverage using ProSe [13] for various channel conditions andconducted a sensitivity analysis to determine themain factorsimpacting coverage. This work considered only a single linkand did not capture the effects of multiple users accessingthe same resources. Griffith et al. developed analytical andsimulation models to evaluate the probability that a controlpacket reaches a certain number of devices assuming a par-ticular control channel resource pool configuration [14]. Inthis work, multiple devices were considered but conservativeassumptions were made regarding the channel effects. Afollow-up work by Griffith et al. was done to evaluate datatransmissions over the shared channel [15] and a similar workwas completed by Cipriano and Panaitopol [16]. The resultsof those contributions can help operators allocate resourcesbased on a desired performance target but do not considerend-to-end performance metrics such as latency.
To the best of our knowledge, this is the first workthat evaluates the performance of off-network MCPTT overProSe. Using the MCPTT access time as the key performanceindicator (KPI), we propose analytical models to estimate theaccess time under different scenarios.Themodels can be usedto guide parameter configuration, and to better understandthe interaction between MCPTT and ProSe. We have also
developed a ns-3 model for simulating MCPTT over ProSe[17], which we used to validate the predicted delays. Majorfunctions of the simulation model include floor control, callcontrol, call type control, and emergency alert. All protocoloperations are based on 3GPP MCPTT specifications [18,19]. We designed test cases and used them to check thecorrectness of the protocol models that we implemented inthe simulator [20].
The rest of the paper is organized as follows. In Section 2,we describe D2D communication over ProSe and our base-line model to estimate the transmission delay. In Section 3,we leverage the ProSe model to develop a set of theoreticalmodels to estimate the MCPTT access time for the varioustypes of calls. We included simulation results to comparewith our analytical estimates. In Section 4, we conduct asensitivity analysis of the ProSe and MCPTT parameters.Section 5 concludes the paper and outlines future work.
2. Modeling of Message TransmissionTime over ProSe
2.1. D2D Communication over ProSe Sidelink. The need forwireless communication among public safety users whileout-of-coverage can be met by utilizing Mode 2 of D2Dcommunications supported by 3GPP LTE [12]. In Mode 2,UEs operate without any network supervision, which is thecase when the UEs are out of coverage of any base stations,and voice and data communications are carried over the airinterface referred to as the ProSe sidelink, or βsidelinkβ forshort. D2D communication over sidelink is performed overperiodically repeating sets of sidelink resources in the timedomain, where a single set of sidelink resources composea sidelink period. Each sidelink period contains instancesof two channels that are separated in the time domain:the Physical Sidelink Control Channel (PSCCH) and thePhysical Sidelink Shared Channel (PSSCH), as depicted inFigure 1. Each channel is defined by a resource pool, whoseelements are defined by a combination of certain ResourceBlock (RB) indexes in the frequency domain, and certainsubframe indexes in the time domain. The transmitting UEuses the PSCCH to send a SidelinkControl Information (SCI)message to indicate to which UEs its subsequent data mes-sages are addressed, where and when the data transmissionson the PSSCH will occur (i.e., resource assignment in thetime and frequency domains), and how (i.e., the modulationand coding scheme). Upon successful reception of the SCImessage, any receiving UEs can tune to the correspondingresources in the PSSCH. Transmissions in the PSSCH use aTimeResource Pattern (TRP), which is a subframe indicationbitmap of a fixed length Nππ π (e.g., 8 subframes for LTEin Frequency Division Duplex (FDD) mode) repeated inthe time domain through the length of PSSCH, to identifywhich subframes are used by a transmitting UE. Each TRP isidentified by an index Iππ π corresponding to the predefinedsubframe indication bitmap established in [21]. Every datatransmission on the PSSCH is performed with four HARQtransmissions. In addition, if outgoing data is to be scheduledfor transmission in a given occurrence of the PSSCH, the datahas to be available at least 4ms before the beginning of the
![Page 3: Access Time Analysis of MCPTT Off-Network Mode over LTE](https://reader031.vdocuments.us/reader031/viewer/2022021713/620b940c2b21b5120d3c510e/html5/thumbnails/3.jpg)
Wireless Communications and Mobile Computing 3
Sidelink Period K+1
Scheduling cutoff time for sidelink period K
Data channel PSSCH
Control channel PSCCH Control Information for a msg tx Data tx of a msg failsData tx of a msg succeeds
time
PSSCHPSCCH PSSCHPSCCH
4 ms
3rd tx1st tx
2nd tx 4th tx
Sidelink Period K
Figure 1: D2D communication over ProSe Sidelink, with periodically repeating control and shared channels.
corresponding sidelink period. In this paper, we identify thisgap as the scheduling cutoff time. Sidelink communication isconsidered half duplex, which means that a device is not ableto receive messages if it is transmitting in the same subframe.
2.2. One Way Transmission Time. Let X be a message that atransmitting UEmust send and let the one-way transmissiontime of message X over the sidelink, denoted as ππ‘π₯(π), bethe duration from when the message π becomes available atthe transmitting UE to the time when message π is receivedsuccessfully by the receiving UE. The calculation of ππ‘π₯(π)considers the packet error rate (PER) of each transmissionattempt, the random occurrence of PTT button push timing,and ProSe configuration parameters. To be more specific,ππ‘π₯(π) can be written as a function π() that has the form:
ππ‘π₯ (π) = π (πππΏ, ππππΆπΆπ», ππππππ¦, [π1, . . . , ππ] ,ππππ. (πππ ππ’π βπππ) ,πππ π, πΌππ π, π, π) (1)
where
(i) πππΏ is the duration of a sidelink period;(ii) ππππΆπΆπ» is the duration of the control channel
(PSCCH) of a sidelink period;(iii) ππππππ¦ is the transmission plus propagation delays
from the sending UE to the targeted receiving UE;(iv) ππ, π = 1, 2, . . . , π, is the PER of the π-th transmission
attempt within a sidelink period, with π being thetotal number of transmission attempts of a packet inone sidelink period;
(v) ππππ. (πππ ππ’π βπππ) represents the distribution ofthe PTT button push timing;
(vi) πππ π and πΌππ π are ProSe time resource pattern relatedparameters;
(vii) π is an integer between 1 and πΏ, with πΏ = β(πππΏ βππππΆπΆπ»)/(π Γ πππ π)β, and the MCPTT message isincluded in the π-th Transmission Block (TB) of thesidelink period data channel (PSSCH).
Since ππ‘π₯(π) remains the same for all MCPTT messagesdiscussed in this paper and thus does not depend on π, werefer to it as ππ‘π₯ for short.
The minimum value of ππ‘π₯ that one message may experi-ence occurs when the following three conditions are met, asshown in Figure 2(a):
(1) the message becomes available for transmission rightbefore the scheduling cutoff time of the upcomingsidelink period πΎ (i.e., 4ms ahead of the start ofsidelink period πΎ);
(2) the message is scheduled to be transmitted in the firstsubframe of the data channel of the sidelink period(e.g., πΌππ π = 0); and
(3) the first transmission attempt of the message is suc-cessfully received by the targeted UE.
Thus, the minimum value for ππ‘π₯ in units of milliseconds is
min (ππ‘π₯) = 4 + ππππππ¦ + ππππΆπΆπ»; (2)
Themaximum value of ππ‘π₯ that one message may experience,assuming that only the last transmission attempt is success-fully received by the targeted UE, is shown in Figure 2(b).The message transmission time can be even larger if none ofthe transmission attempts in one sidelink period succeed, andwe will discuss this situation in Section 4. When both of thefollowing two conditions are met, the maximum value of ππ‘π₯shown in Figure 2(b) occurs:
(1) the message becomes available for transmission rightafter the scheduling cutoff time of the upcomingsidelink period πΎ, so it must wait for the full lengthof the sidelink period K and is scheduled to betransmitted in the subsequent sidelink period πΎ + 1;
(2) the last transmission attempt of the message is sched-uled to be transmitted in the last subframe of the datachannel of the sidelink (e.g., πΌππ π = 7) period.
Thus, the maximum value of ππ‘π₯ in units of milliseconds is
max (ππ‘π₯) = 3 + ππππππ¦ + 2 β πππΏ; (3)
![Page 4: Access Time Analysis of MCPTT Off-Network Mode over LTE](https://reader031.vdocuments.us/reader031/viewer/2022021713/620b940c2b21b5120d3c510e/html5/thumbnails/4.jpg)
4 Wireless Communications and Mobile Computing
Sidelink Period K+1
PTT Button Pushed(Message X available)
Sidelink Period KScheduling cutoff time for sidelink period K
Message Y sentMessage Y receivedas the response
Message Message X receivedX sent
TRTT(X,Y)
Ttx(X)
Data channel
Control channel Control Information for a msg tx Data tx of a msg failsData tx of a msg succeeds
(a) Minimum case
Sidelink Period K Sidelink Period K+1
PTT Button Pushed(Message X available)
Message Y receivedas the response
Message
Sidelink Period K+2 Sidelink Period K+3
Scheduling cutoff time for sidelink period K
Message X received
Message X Sent Y Sent
TRTT(X,Y)
Ttx(X)
Data channel
Control channel Control Information for a msg tx Data tx of a msg failsData tx of a msg succeeds
(b) Maximum case
Figure 2: Message TRANSMISSION TIME over ProSe ππ‘π₯ and Tπ ππ.
Assume the π-th TB is the TB in which message X is assignedto be transmitted. Let ππππππΏ denote the time between themoment the PTT button at the transmitting UE is pushedand the beginning of the sidelink period in which themessage is transmitted. There are four components of themessage transmission time ππ‘π₯: (1) ππππππΏ; (2) the controlchannel duration ππππΆπΆπ»; (3) the time offset of the π-thTB from the beginning of the current instance of the datachannel PSSCH which equals (π β 1) Γ πππ π Γ π; and(4) the duration from the beginning of the π-th TB to thetime that message X is sent out successfully within the π-thTB. Note that for a message to be transmitted with ProSeparameters πΌππ π = π and assuming the message is transmittedsuccessfully in the π-th attempt initially, component (4)equals (π β 1) Γ πππ π + π + ππππππ¦. This is because for each
failed transmission attempt, an additionalπππ πms delay willbe added, and within each round of transmission attempt,there is πms time offset introduced for a message transmittedwith TRP bitmap parameter πΌππ π = π. Therefore, the averageof component (4) shall be calculated by conditioning onthe possible πΌππ π values π and, given π, by additionallyconditioning on the index number of the first successfultransmission attempt π. Also, since we are looking at thedelay associated with a successful message transmission, weuse the law of total probability and condition the value ofcomponent (4) on the event that the message transmissionsucceeds in exactly π attempts for π = 1, 2, . . . , π; theprobability of this event is (1βππ)βπβ1
π=0ππ. Furthermore, theprobability of message transmission failure is the probabilityof π transmission attempt failures, which is βπ
π=1ππ. Thus,
![Page 5: Access Time Analysis of MCPTT Off-Network Mode over LTE](https://reader031.vdocuments.us/reader031/viewer/2022021713/620b940c2b21b5120d3c510e/html5/thumbnails/5.jpg)
Wireless Communications and Mobile Computing 5
the message transmission success probability is 1 β βππ=1ππ,
and the average value of ππ‘π₯ is
π΄Vπ (ππ‘π₯) = πΈ (ππππππΏ) + ππππΆπΆπ» + (π β 1) Γ πππ π Γ π + 11 β βπ
π=1ππ(πππ πβ1)β
π=0
ππππ (πΌππ π = π)
β [ πβπ=1
(((π β 1) Γ πππ π + π + ππππππ¦) (1 β ππ) πβ1βπ=0
ππ)]
= πΈ (ππππππΏ) + ππππΆπΆπ» + (π β 1) Γπππ π Γ π + (πππ πβ1)βπ=0
ππππ (πΌππ π = π)
β [(π + ππππππ¦) + πππ π1 β βππ=1ππ
πβ1βπ=1
(π (1 β ππ+1) πβπ=1
ππ)]
(4)
where π0 = 1.2.3. Roundtrip Transmission Time. The roundtrip messagetransmission time over the sidelink, denoted as ππ ππ(π, π),is defined as the duration from when message π is availableat UE A until UE A receives the response message π fromthe corresponding UE (denoted as UE B for simplicity)successfully over the sidelink channel, then
ππ ππ (π, π) = ππ‘π₯ (π) + ππ΅ + ππππ’πππππ¦+ (ππ‘π₯ (π) β ππππππΏ (π)) (5)
where ππ΅ is the time between when message π is received byUE B and the time when the response message π is availableat UE B as defined by the protocol (e.g., backoff timers);and ππππ’πππππ¦ is the time that a message has to wait beforethe beginning of the sidelink period in which the message istransmitted. This overhead time is due to the sidelink periodboundary restriction.
Note that ππ ππ(π,π) is not the simple summation ofππ‘π₯(π),ππ΅, and ππ‘π₯(π).This is because the available timing ofmessage π is dependent on the available timing of messageπ
and ππ΅, which is no longer the independent PTT push timingthat is assumed for UE B when calculating ππ‘π₯(π).
The minimum ππ ππ(π, π) that UE A may expect is
min {ππ ππ (π, π)}= 4 + βππππΆπΆπ» + ππππππ¦ +min {ππ΅} + 4
πππΏ βπππΏ+ ππππΆπΆπ»,
(6)
as shown in Figure 2(a) by assuming ππ΅ = 0.As can be seen from (6), min{ππ ππ(π, π)} and thusππ ππ(π,π) is always greater than πππΏ.The maximum ππ ππ(π, π) that UE A may experience is
max {ππ ππ (π, π)} = 3+ (1 + βππππΆπΆπ» + π Γπππ π + ππππππ¦ +max {ππ΅} + 3
πππΏ β)β πππΏ + ππππΆπΆπ» + π Γπππ π + ππππππ¦
(7)
as shown in Figure 2(b) by assuming ππ΅ = 0.The calculation of average ππ ππ(π, π) starts from rear-
ranging (5) into mutually indepenedent components:
π΄Vπ (ππ ππ (π, π)) = πΈ (ππ‘π₯ (π) + ππ΅ + ππππ’πππππ¦ + (ππ‘π₯ (π) β ππππππΏ (π)))= πΈ (ππππππΏ (π) + (ππ‘π₯ (π) βππππππΏ (π)) + ππ΅ + ππππ’πππππ¦ + (ππ‘π₯ (π) β ππππππΏ (π)))= πΈ (ππ‘π₯ (π) βππππππΏ (π) + ππ΅ + ππππ’πππππ¦) + πΈ (ππ‘π₯ (π))= πππΏ β πΈ (πΎπ ππ) + πΈ (ππ‘π₯ (π))
(8)
where πΎπ ππ is one less than the number of sidelinkperiods from the sidelink period in which message X istransmitted successfully to the sidelink period in
which message Y is transmitted successfully, soπΎπ ππ = (ππ‘π₯(π)βππππππΏ(π) + ππ΅ + ππππ’πππππ¦)/πππΏ. Similar tocomponent (4) in (4), the average of πΎπ ππ shall
![Page 6: Access Time Analysis of MCPTT Off-Network Mode over LTE](https://reader031.vdocuments.us/reader031/viewer/2022021713/620b940c2b21b5120d3c510e/html5/thumbnails/6.jpg)
6 Wireless Communications and Mobile Computing
be calculated considering both possible πΌππ π valuesπ and possible numbering of the first successful transmissionattempts π. In addition, the randomness introduced by ππ΅needs to be included as well.
πΈ (πΎπ ππ) = [ πβπ=1
(((1 β ππ) πβ1βπ=0
ππ)πΎπππ₯(π)βπ=1
(π β ππππ (πΎπ ππ = π |the first successful transmission is attempt n)))] Γ 1
1 β βππ=1ππ
= 11 β βπ
π=1ππ { πβπ=1
[((1 β ππ) πβ1βπ=0
ππ)πΎπππ₯(π)βπ=1
π(πππ πβ1)βπ=0
ππππ (πΌππ π = π)Γππππ ((π β 1) πππΏ β€ (ππππΆπΆπ» + 4 + (π β 1) Γ πππ π + i + ππππππ¦ + ππ΅ < π β πππΏ)]}
(9)
where π0 = 1; and πΎπππ₯(π) = β(ππππΆπΆπ» + π Γ πππ π β 1 +ππππππ¦ + max{ππ΅})/πππΏβ is the maximum possible πΎπ ππ fora given π. By combining (8) and (9), the average value ofππ ππ(π, π) is
π΄Vπ (ππ ππ (π,π))= [ πβ
π=1
(((1 β ππ) πβ1βπ=0
ππ)πΎπππ₯(π)βπ=1
π(πππ πβ1)βπ=0
ππππ (πΌππ π = π) Γ ππππ (ππ΅πΏ (π, π, π) β€ ππ΅ < ππ΅π (π, π, π)))]Γ πππΏ1 β βπ
π=1ππ + π΄Vπ (ππ‘π₯)(10)
where π΄Vπ(ππ‘π₯) is calculated by (4);
ππ΅πΏ (π, π, π) = (π β 1) πππΏ β ππππΆπΆπ» β 4β (π β 1)πππ π β π β ππππππ¦;
and ππ΅π (π, π, π) = πβπππΏ β ππππΆπΆπ» β 4β (π β 1)πππ π β π β ππππππ¦.
(11)
2.4. Baseline Configuration Parameters for Evaluation. Ana-lytical estimates and simulation results presented in Sections3 and 4 assume the baseline configuration shown in Table 1unless otherwise noted. Note that the PER of a single trans-mission attempt is set at 0.1 at most, since 0.1 is the commondesign assumption of LTE PER when Hybrid automaticrepeat request (HARQ) Acknowledegment (ACK)/NegativeAcknowledgement (NACK) is in use. Given that there is noHARQ ACK/NACK over the ProSe PSSCH meaning that allfour HARQ transmissions always occur, the PER should notbe set to a more aggressive value higher than 0.1.
We assume in the simulations that the timing of a PTTbutton push is uniformly distributed within a sidelink period
πππΏ, and π = 1 (i.e., the message is transmitted in the firstTB of the sidelink, which is reasonable given the importanceof public safety related communication). It is also assumedthat ProSe TRP parameters are configured as πππ π = 1 andπππ π = 8. This means that only one of the 8 subframes in aTRPwill be used to send data andππππ(πΌππ π = π) = 1/πππ π =1/8. Then (4) is simplified as
π΄Vπ (ππ‘π₯) = 7.5 + πππΏ2 + ππππΆπΆπ» + ππππππ¦+ 8 (π1 + π1π2 + π1π2π3 β 3π1π2π3π4)1 β π1π2π3π4
(12)
in units of ms.We also assume that the MCPTT user only needs to push
the PTT button to initiate the request to speak and is notrequired to follow up the request with any further actions,such as acknowledgement. In addition, since there are somevariations in MCPTT message sizes (e.g., call announcementmessage is long) and the size of resource allocated to eachUE (e.g., the resource allocated for a voice message maybe small to start with), our modeling assumes that MCPTTmessages are not piggybacked, in order to get the conservative
![Page 7: Access Time Analysis of MCPTT Off-Network Mode over LTE](https://reader031.vdocuments.us/reader031/viewer/2022021713/620b940c2b21b5120d3c510e/html5/thumbnails/7.jpg)
Wireless Communications and Mobile Computing 7
Table 1: Baseline configuration parameters used in the MCPTT access time evaluation.
Parameter Name Symbol ValueSidelink period (ms) πππΏ 40, 60, 80, 120, 160, 240, 280, 320PSCCH duration (ms) ππππΆπΆπ» 8# transmission attempts in one sidelink period π 4
Time Resource Pattern parameters πππ π 8πππ π 1PER of the 4 transmission attempts in each sidelink period ππΈπ [0.1, 0.1, 0.1, 0]Transmission + Propagation delays (ms) ππππππ¦ 0.93
Get the permission to talk
time
Press the PTT Button Call setup completed
(if not in the call already started)Call Control Access Time TCallControl Floor Control Access Time TFloorControl
MCPTT Access Time TAccessTime
Figure 3: Components of MCPTT access time.
estimates/results. When piggybacking is allowed, the accesstime may be further reduced.
3. Modeling of MCPTT Access Time
3.1. Overview. According to 3GPP LTE specifications, βTheMCPTT access time is defined as the time between when anMCPTT User requests to speak (normally by pressing theMCPTT control on the MCPTT UE) and when this user getsa signal to start speaking.β [9]. Based on protocol details, theMCPTT access time (ππ΄ππππ π ππππ) can be further divided intotwo components as illustrated in Figure 3: call control accesstime (ππΆππππΆπππ‘πππ) and floor control access time (ππΉπππππΆπππ‘πππ).Call Control Access Time (ππΆππππΆπππ‘πππ) is the time from whentheMCPTTuser presses the PTTbutton towhen theMCPTTUEestablishes a call successfully.This component occurs onlywhen the call which the MCPTT UE wants to join/start doesnot exist yet. Floor control access time (ππΉπππππΆπππ‘πππ) is thetime fromwhen theMCPTTUEestablishes a call successfully(if not already in a call) or when the MCPTT user presses thePTT button (if in a call already), whichever is later, to whenthe MCPTT UE becomes the floor arbitrator. In MCPTT off-network mode group call operation, the floor arbitrator is thecurrent speaker who is the only groupmember that is allowedto speak to the group and that also arbitrates the floor in caseothermembers of the group call (i.e., other floor participants)request that they be granted the floor so that they can talk.
MCPTT off-network mode supports the following threecategories of calls: basic group call, broadcast group call,and private call. The analytical modeling of MCPTT accesstime associated with basic group call is analyzed first inSection 3.2, because the call control procedures and floorcontrol procedures of the basic group call are very com-prehensive. Then the analytical modeling of MCPTT accesstime associated with broadcast group call and private calls is
presented in Sections 3.3 and 3.4, respectively, which reusesa subset of the modeling of basic group call procedures. Weobtain simulation results assuming two users for all three callcategories and compared these results with their analyticalestimate counterparts in the corresponding sections. Simu-lation results presented throughout this paper are generatedby running the ns-3 model for MCPTT over ProSe [17],especially the call control module and floor control module.Parameters configuration and simulation setup follow thedescription in Section 2.4, unless otherwise noted.
3.2. Basic Group Call. From the viewpoint of MCPTT accesstime performance evaluation, the various situations that anMCPTT client faces when pushing the PTT button to talk in abasic group call (with group ID ππ) can always be categorizedinto one of the five scenarios summarized in Table 2.
In Sections 3.2.1 and 3.2.2, we explain the call control andthe floor control access procedures, and access time modelsare described. In Section 3.3 we derive the total access timeππ΄ππππ π ππππ experienced by an MCPTT client under ScenariosA through E from different combinations of ππΆππππΆπππ‘πππ andππΉπππππΆπππ‘πππ. Note that the value of ππΉπππππΆπππ‘πππ may be 0 forcertain scenario(s), as will be analyzed in Section 3.2.2.
3.2.1. Call Control Access Time ππΆππππΆπππ‘πππ. When an MCPTTclient (UE A) wants to join/establish a basic group call withMCPTT group ID ππ by pushing the PTT button, it has togo through either procedure βC1: Initiate a new group callβ orprocedure βC2: Join an existing group call,β which are shownin Figure 4. In the figure, the colors of the message name andthe corresponding arrows are consistent with the color of thesending UE.
The call control access procedure we defined above hasincorporated the call probe procedure, call setup procedure,
![Page 8: Access Time Analysis of MCPTT Off-Network Mode over LTE](https://reader031.vdocuments.us/reader031/viewer/2022021713/620b940c2b21b5120d3c510e/html5/thumbnails/8.jpg)
8 Wireless Communications and Mobile Computing
Table 2: Scenarios for basic group call.
Scenario Group call ππexists?
UE A in group call ππalready?
The floor arbitrator ofgroup call ππ exists? ππ΄ππππ π ππππ
A No No No ππΆππππΆπππ‘πππ + ππΉπππππΆπππ‘πππB Yes No Yes ππΆππππΆπππ‘πππ + ππΉπππππΆπππ‘πππC Yes No No ππΆππππΆπππ‘πππ + ππΉπππππΆπππ‘πππD Yes Yes Yes ππΉπππππΆπππ‘πππE Yes Yes No ππΉπππππΆπππ‘πππ
UE A pushes PTT button:- Sends out Call Probe with group ID nn; - Starts Wait for Call Announcement Timer TFG1 and Call Probe Retransmission Timer TFG3;- Retransmits Call Probe upon expiry of TFG3, and restarts TFG3.
Call Probe(group ID nn)
Call Probe(group ID nn)
TFG1
C1: If no Call Announcement with group ID nn is received bythe expiry of timer TFG1, UE A:- Initiates the group call;- Sends the Group Call Announcement message with group ID nn;- May start talking;
Call Announcement(group ID nn)
TFG1
Call Announcement(group ID nn)
C2: If Call Announcement with group ID nn is received beforetimer TFG1 expires, UE A:- Joins the existing group call with group ID nn;- Sends Floor Request message if UE A wants to talk.
TFG1
Call Announcement(group ID nn)
Call Announcement(group ID nn )
new group call
existing group callC
A
B
C1: Initiate a
C2: Join an
A
A
B
B
C
C
Figure 4: Call control access procedures.
and periodic group call announcement procedure specified in[18].The decision to go through procedures C1 or C2 is madeby UE A (the UE with the PTT button pushed) itself basedon its own observation and may not be the correct decision.To be more specific, UE A always chooses C1 under ScenarioA, which is the right decision. However, under Scenarios B orC, UE A might make the correct decision, C2, or the wrongdecision, C1, depending on how soon UE A hears back fromother UEs with respect to the configuration of timer TFG1.
When UE A executes the C1 procedure,
ππΆππππΆπππ‘πππ (πΆ1) = ππΉπΊ1; (13)
where ππΉπΊ1 is the value of βwait for call announcementβtimer ππΉπΊ1.
When UE A executes the C2 procedure,
ππΆππππΆπππ‘πππ (πΆ2)= ππ ππ (βπΆπππ πππππβ, βπΆπππ π΄ππππ’πππππππ‘β) ; (14)
whereππ ππ is given by (5) andwheremessagesX andY are theCall Probe and Call Announcement messages, respectively.In (5), ππ΅ = ππΉπΊ2πππππ(π), where π is the number ofMCPTT UEs in the group call capable of sending out a CallAnnouncementmessage in response to receiving aCall Probe
message from another UE. Here ππΉπΊ2πππππ(π) reflects thetime that it takes for at least one UE of the group to respondto a Call Announcement message after receiving the CallProbe message from UE A. It is a random variable that is theminimum ofπ random variables, each of which is uniformlydistributed between 0 seconds and (1/12) seconds (83.3ms).Therefore,
ππΉπΊ2πππππ (π) ={{{{{{{{{{{
0, πππ100012 (π + 1) , πVπ
100012 , πππ₯(15)
Theminimum, maximum, and average of ππΆππππΆπππ‘πππ(πΆ2) canbe obtained by plugging configuration values into (6), (7), and(10) accordingly.
The simulation study of call control access time in thissection focuses on Scenarios B and C, i.e., Procedure C2.This is because there is no uncertainty in Scenario A (i.e.,Procedure C1), sinceππΆππππΆπππ‘πππ always equals ππΉπΊ1when theMCPTT UE establishes a new group call.
Figure 5 shows that, for all sidelink periods πππΏ definedby 3GPP for LTE ProSe, the analytical model provides goodestimates of the minimum, average, and maximum for the
![Page 9: Access Time Analysis of MCPTT Off-Network Mode over LTE](https://reader031.vdocuments.us/reader031/viewer/2022021713/620b940c2b21b5120d3c510e/html5/thumbnails/9.jpg)
Wireless Communications and Mobile Computing 9
est sim est sim est sim est sim est sim est sim est sim est sim
40 60 80 120 160 240 280 320
Sidelink Period (ms)
Analytical Model (est) vs Simulation Results (sim)
Min Avg Max
0
100
200
300
400
500
600
700
Acce
ss T
ime (
ms)
Figure 5: Call control access time for joining an existing group call with large value for ππΉπΊ1 with 95% Confidence Intervals for the averagevalues from the simulations.
call control access time experienced by anMCPTT client whogoes through procedure C2.The 95%Confidence Interval forthe average values obtained from simulation is also shownand is very tight. The minimum and maximum values areobserved over all simulations and therefore do not have aConfidence Interval. This is true for all the figures shown inthis document. As can be seen from the figure,ππΆππππΆπππ‘πππ (πΆ2)increases asπππΏincreases and ranges from as small as 52ms tomore than 680ms. The ππΉπΊ1 timer value in the simulationis configured to max (1 sec, 6 β πππΏ) and the ππΉπΊ3 timervalue is configured to 6 β πππΏ; thus UE A will make thecorrect decision because neither timer will expire premature-ly.
3.2.2. Floor Control Access Time ππΉπππππΆπππ‘πππ. When anMCPTT client wants to talk and thus pushes the PTT buttonon UE A, the UE will go through one of the βF0: new call,ββF1: No Response,β or βF2: Response Receivedβ proceduresdepending on the feedback that it receives from other UEs.
After the MCPTT client (UE A) establishes a group call(i.e., C1 procedure), UE A sends out a Floor Granted messageand may start talking, so the corresponding floor controlaccess procedure is as follows.
F0. Send out Floor Granted message to grant the floor tooneself.
After an MCPTT client (UE A) joins an existing groupcall (i.e., C2 procedure), or if UE A is already a part of anongoing group call, UE A sends out a Floor Request message.Depending on whether a response is received within aconfigured time window, UE A may perform the F1 or F2procedure as illustrated in Figure 6. In the figure, the colorsof the message name and the corresponding arrows areconsistent with the color of the sending UE. Depending onthe relative priority of the floor request and the queueingcapability of the current floor arbitrator, procedure F2 maybe further divided into three cases:
F2.1 (preemptive floor request). UE Aβs floor request has pre-emptive priority, so UE B must grant the floor to UE Aimmediately;
F2.2 (floor request denied). UE B is busy and does not supportqueueing requests, so UE B denies the floor to UE A;
F2.3 (floor request queued). UE B is busy but supportsqueueing of requests, so UE Aβs floor request is placed inqueue and will be handled later.
It is worth pointing out that the time it takes for UEA to receive a response is the same for F2.1, F2.2, andF2.3. However, the 3GPP definition of MCPTT access timefocuses on the case of βwhen this user gets a signal to startspeakingβ [2]. Therefore, in the following analytical modeland simulation study, among the three cases of procedure F2,we focus on procedure βF2.1: preemptive floor requestβ. It isbecause procedure F2.2 or F2.3 will not grant the floor to UEA, and thus UE Aβs access procedures are not successful. Themodel and results can be easily generalized if the MCPTTaccess time definition is rewritten as βwhen this user gets asignal whether he/she can start speakingβ.
The floor control access procedure we defined above hasincorporated several state transition procedures specified in[19]. The decision to go through procedure F0, F1, or F2is made by UE A (the UE with the pushed PTT button)based on its own observation, and this decision may or maynot be the correct one. To be more specific, UE A alwayschooses F1 under Scenario A, which is the right decision.However, under Scenarios B or D, UE A might make thecorrect decision (F2) or the wrong decision (F1), dependingthe time until UE A receives a response message from thefloor arbitrator UE B and the configuration values of theπ201timer and πΆ201 counter.The floor control access time of eachof the three procedures are as follows:
When F0 procedure is executed,
ππΉπππππΆπππ‘πππ (πΉ0) = 0; (16)
![Page 10: Access Time Analysis of MCPTT Off-Network Mode over LTE](https://reader031.vdocuments.us/reader031/viewer/2022021713/620b940c2b21b5120d3c510e/html5/thumbnails/10.jpg)
10 Wireless Communications and Mobile Computing
UE A requests the floor, with UE B being the current floor arbitrator:- Triggered by MCPTT User pushing thePTT button;- Sends out Floor Request message;- Starts Floor Request Retransmission timer T201;- Retransmits Floor Request upon T201 expiry;
Floor Request
T201 F2: Depending on whether βqueueing of floor requestβ is supported and whether the Floor Request is pre-emptive, UE A may receive from UE B one of the following floor control messages as the response:- Floor Granted (F2.1);- Floor Deny (F2.2);- Floor Queue Position Info (F2.3);
Floor control message(s)
F1: No response
F2: Response Received
F1: UE A takes the floor and declares itself as the floor arbitrator if no response from the incumbent floor arbitrator UE B is received aοΏ½er T201 expires C201 times:- Assumes the role of floor arbitrator;- Sends out Floor Taken message to all group call participants;
A requests the floor, with UE B being current floor arbitrator:iggered by MCPTT User pushing the
T button;
Floor Request
T201Floor controlmessage(s)
F1: No response
F2: Response Received
Floor Taken
Floor Taken
A
A
A
B
B
BC
C
C
Figure 6: Floor control access procedure.
est sim est sim est sim est sim est sim est sim est sim est sim
40 60 80 120 160 240 280 320Sidelink Period (ms)
Analytical Model (est) vs Simulation Results (sim)
0
100
200
300
400
500
600
700
Acce
ss T
ime (
ms)
Min Avg Max
Figure 7: Floor control access time for preemptive floor request with 95% Confidence Intervals for the average values from the simulations.
When F1 procedure is executed,
ππΉπππππΆπππ‘πππ (πΉ1) = πΆ201 β π201; (17)
where π201 is the βFloor Request Retransmissionβ timer andπΆ201 is the βFloor Request Retransmissionβ counter. The3GPP default settings of these two parameters are πππΏ and 3,respectively.
When the F2.1 procedure is executed,
ππΉπππππΆπππ‘πππ (πΉ2.1)= ππ ππ( (βπΉππππ π πππ’ππ π‘β, βπΉππππ πΊππππ‘ππβ) ; (18)
again using (5), where ππ΅ = 0. The minimum, maximum,and average of ππΉπππππΆπππ‘πππ(πΉ2.1) can be obtained by pluggingconfiguration values into (6), (7), and (10) accordingly.
The simulation study of floor control access time in thissection focuses on either Scenario B or D, i.e., procedureF2.1. This is because there is no uncertainty associated with
ππΉπππππΆπππ‘πππ for ScenariosA,C or E:ππΉπππππΆπππ‘πππ = πΆ201βπ201for Scenarios C & E (i.e., procedure F1), and ππΉπππππΆπππ‘πππ = 0for Scenario A (i.e., procedure F0). Also, we focus on thecase when UE Aβs floor request is of preemptive priority,i.e., F2.1, as explained in Section 2.4. In the simulation, itis assumed that the incumbent floor arbitrator UE B doesnot generate Real-time Transport Protocol (RTP) packetsduring UE Aβs access time. This assumption will not impactUE Aβs access time when UE A makes the right decision.We make this assumption to get a conservative estimate ofthe likelihood of UE Aβs making the wrong decision, whichleads to an undesirable situation where there are multiplefloor arbitrators. More detailed definition and analysis of themultiple floor arbitrator situation are given in Section 4.3.
Figure 7 shows that for all sidelink period lengths πππΏdefined by LTE, the analytical model provides good estimatesof the minimum, average and maximum floor control accesstimes experienced by an MCPTT client who goes throughprocedure F2.1. The π201 timer value in the simulation is
![Page 11: Access Time Analysis of MCPTT Off-Network Mode over LTE](https://reader031.vdocuments.us/reader031/viewer/2022021713/620b940c2b21b5120d3c510e/html5/thumbnails/11.jpg)
Wireless Communications and Mobile Computing 11
Table 3: MCPTT access time ππ΄ππππ π ππππ for different scenarios.Scenario Correct decision Wrong decisionA ππΆππππΆπππ‘πππ(πΆ1) + ππΉπππππΆπππ‘πππ(πΉ0) N/AB ππΆππππΆπππ‘πππ(πΆ2) + ππΉπππππΆπππ‘πππ(πΉ2) ππΆππππΆπππ‘πππ(πΆ2)+ππΉπππππΆπππ‘πππ(πΉ1) or ππΆππππΆπππ‘πππ(πΆ1)+ππΉπππππΆπππ‘πππ(πΉ0)C ππΆππππΆπππ‘πππ(πΆ2) + ππΉπππππΆπππ‘πππ(πΉ1) ππΆππππΆπππ‘πππ(πΆ1) + ππΉπππππΆπππ‘πππ(πΉ0)D ππΉπππππΆπππ‘πππ(πΉ2) ππΉπππππΆπππ‘πππ(πΉ1)E ππΉπππππΆπππ‘πππ(πΉ1) N/A
configured to 6 β πππΏ, so that UE A will make the correctdecision.
Comparing Figure 7 with Figure 5, it shows that althoughthe minimum call control access time ππΆππππΆπππ‘πππ(πΆ2) andfloor control access time ππΉπππππΆπππ‘πππ(πΉ2.1) are very similar,the average and maximum values of ππΉπππππΆπππ‘πππ(πΉ2.1) aresmaller than those of ππΆππππΆπππ‘πππ(πΆ2) for short sidelink peri-ods (small πππΏ values). This is because of the extra delayintroduced by timer ππΉπΊ2 to the round-trip time during thecall control procedure.WhenπππΏ is large, however, the impactof ππΉπΊ2 becomes negligible.
3.2.3. Group Call MCPTT Access Time ππ΄ππππ π ππππ. The ana-lytical model of the overall access times for scenarios Athrough E can be obtained by the proper combination of theanalytical models of ππΆππππΆπππ‘πππ and ππΉπππππΆπππ‘πππ derived inprevious sections, as shown in Table 3. It is worth pointingout that for Scenario A, ππΉπππππΆπππ‘πππ(πΉ0) is 0 as was explainedin Section 3.2.2.
It is worth pointing out that, for Scenarios B, C, and D,UE A may make the correct decision or the wrong decision,depending on the MCPTT parametersβ configuration andthe one-way or roundtrip message transmission time overthe sidelink. When UE A makes the correct decision, theanalytical models presented in Section 3 align quite well withthe MCPTT access time observed in our simulations. WhenUE A makes wrong decisions, the simulation results deviatefrom the analytical estimates, which we analyze in Section 4,together with the discussion of potential negative impactsfrom incorrect decisions.
3.3. Broadcast Group Call. A broadcast group call is a specialgroup call where the initiating MCPTT user expects noresponse from the other MCPTT users, so that when theinitiatorβs transmission is complete, so is the call. Therefore,the UE that initiated the broadcast group call is the onlyMCPTT UE acting as the floor arbitrator. Consequently,only one scenario needs to be evaluated to examine theaccess time required to establish a new broadcast groupcall:
Scenario F. UE A establishes a new broadcast group call bypushing the PTT button.
The value of ππΆππππΆπππ‘πππ under Scenario F is zero, becausethe UE can initiate a broadcast group call without waitingfor timer expiration or messages from other UEs. Similarly,the value of ππΉπππππΆπππ‘πππ is also zero, because the UE that
initiates a broadcast group call becomes the floor arbitratorimmediately. Consequently, the overall access time is:
ππ΄ππππ π ππππ = 0; (19)
3.4. Private Call. A private call takes place between twoMCPTT UEs. For the private call scenarios, a MCPTT UEis in one of two possible states: it is in a private call already orneeds to start a new private call; a UE cannot join an existingprivate call. Consequently, there are only two scenarios thatneed to be evaluated to obtain the access time of a private call:
Scenario G. UEA establishes a new private call by pushing thePTT button.
Scenario H. UE A is already in a private call and wants to talkby pushing the PTT button.
For Scenario G, the possible call control procedures thatUE A may experience are depicted in Figure 8. In the figure,the colors of themessage name and the corresponding arrowsare consistent with the color of the sending UE. To establisha private call successfully, procedure CP2.1 is the only path,and then UE A becomes the floor arbitrator automatically.Therefore, the overall access time for Scenario G is
ππ΄ππππ π ππππ = ππΆππππΆπππ‘πππ (πΆπ2.1)= ππ ππ (βπππVππ‘π πΆπππ πππ‘π’π π πππ’ππ π‘β,βπππVππ‘π πΆπππ π΄πππππ‘β) ;
(20)
which uses (5) to evaluate ππ ππ. The minimum, maximum,and average ofππΆππππΆπππ‘πππ(πΆπ2.1) can be obtained by pluggingconfiguration values into (6), (7), and (10), respectively, withππ΅ = 0
Under Scenario G, when the time for UE A to receive aresponse is greater than ππΉπ1 β πΆπΉπ1, UE A will make theconclusion that the other UE (UE B) cannot be reached andstop trying to establish the call, i.e., procedure CP1.
For Scenario H, the floor control procedure that UE Amay experience is very similar to the F2.1 procedure, and theoverall access time for Scenario H is from (18)
ππ΄ππππ π ππππ = ππΉπππππΆπππ‘πππ (πΉ2.1)= ππ ππ (βπΉππππ π πππ’ππ π‘β, βπΉππππ πΊππππ‘ππβ) ; (21)
The simulation study of private call access time in thissection focuses on Scenario G, i.e., procedure CP2.1. This
![Page 12: Access Time Analysis of MCPTT Off-Network Mode over LTE](https://reader031.vdocuments.us/reader031/viewer/2022021713/620b940c2b21b5120d3c510e/html5/thumbnails/12.jpg)
12 Wireless Communications and Mobile Computing
UE A requests to establish a private call with UE B:- Triggered by MCPTT User pushing the PTT button;- Sends out Private Call Setup Request message;- Starts private call request retransmission timer TFP1;- Retransmits Private Call Setup Request uponTFP1 expiry.
Private Call Setup Request
TFP1
"CP2: UE A may receive from UE B one of the following call control messages as the response:- Private Call Accept (CP2.1);- Private Call Reject (CP2.2);
Call control messages
CP1: No response
CP2: Response Received
CP1: UE A determines that the private call cannot be established if no response from UE B is received aοΏ½er TFP1 expires CFP1 times;A
B
A
B
A
B
Figure 8: Call control access procedure of private call.
0
100
200
300
400
500
600
700
est sim est sim est sim est sim est sim est sim est sim est sim
40 60 80 120 160 240 280 320
Acc
ess T
ime (
ms)
Sidelink Period (ms)
Analytical Model (est) vs Simulation Results (sim)
Min Avg Max
Figure 9: Access time of private call with 95% Confidence Intervals for the average values from the simulations.
is because the access time of Scenario H can use theresults for F2.1 presented in Section 3.2.2. Figure 9 showsthat, for all sidelink period lengths πππΏ defined by 3GPP,the analytical model provides good estimates of the min-imum, average, and maximum for access times experi-enced by an MCPTT client going through procedure CP2.1.We observe that the access time and the spread betweenthe minimum and the maximum values increases as πππΏincreases, for both analytical estimates and simulationresults.
4. Impact of MCPTT Parameters onMCPTT Access Time
As already mentioned briefly in Section 3, several importantobservations can be made based on the analytical andsimulation modeling of MCPTT access time.
Observation 1. AnMCPTTUE needs more than one sidelinkperiod to send a message and get the response message backover the sidelink.
As can be seen from (6), min{ππ ππ(π, π)} and thusππ ππ(π,π) is always longer than πππΏ. However, the default3GPP settings of several MCPTT retransmission timers, e.g.,ππΉπΊ3 and π201, is one πππΏ only. Such a small timer valuemay lead to unnecessary retransmissions of certain MCPTTmessages and may have negative impacts as illustrated inSections 4.1 and 4.3.
Observation 2. An MCPTT UE may make wrong decisionsabout the existence of call and/or floor arbitrators, undersome scenarios and with certain parameter configurations.
As summarized in Table 3, UE Aβs decision may go wrongfor Scenarios B, C, and D.These wrong decisions may lead toundesirable situations such as a multiple calls situation or a
![Page 13: Access Time Analysis of MCPTT Off-Network Mode over LTE](https://reader031.vdocuments.us/reader031/viewer/2022021713/620b940c2b21b5120d3c510e/html5/thumbnails/13.jpg)
Wireless Communications and Mobile Computing 13
6βT_
SL
T_SL
6βT_
SL
T_SL
6βT_
SL
T_SL
6βT_
SL
T_SL
6βT_
SL
T_SL
6βT_
SL
T_SL
6βT_
SL
T_SL
6βT_
SL
T_SL
40 60 80 120 160 240 280 320
Sidelink Period (ms)
Large TFG3 (6βT_SL) vs Small TFG3 (T_SL)
0100200300400500600700
Acce
ss T
ime (
ms)
Min Avg Max
Figure 10: Impact of ππΉπΊ3 Timer on call control access time with 95% Confidence Intervals for the average values from the simulations.
Table 4: Impact of ππΉπΊ3 configuration.πππΏ (ms) 40 60 80 120 160 240 280 320
Ratio of multiple occupancy tosingle occupancy
ππΉπΊ3 = πππΏ 0.510 0.667 0.670 0.576 0.573 0.524 0.530 0.534ππΉπΊ3 = 6 β πππΏ 0 0 0 0 0 0 0 0
Fraction of samples discarded ππΉπΊ3 = πππΏ 0.132 0.140 0.092 0.059 0.052 0.024 0.025 0.026ππΉπΊ3 = 6 β πππΏ 0 0 0 0 0 0 0 0
multiple floor arbitrators situation, both of which are furtheranalyzed in Sections 4.2 and 4.3, respectively.
Motivated by the above observations, in the remainderof this section, we study the impact of various MCPTTparameters to evaluate the benefits and drawbacks of differentconfigurations on MCPTT access time performance.
4.1. Impact of βCall Probe Retransmissionβ Timer ππΉπΊ3. Toevaluate the impact ofππΉπΊ3, we look at the call control accesstime for Scenarios B and C (group calls). Figure 10 shows theresults using the same assumptions as for Figure 5 but withππΉπΊ3 configured at the 3GPP default setting πππΏ instead of6 β πππΏ.
It appears that varying ππΉπΊ3 fromπππΏ to 6βπππΏ producesno significant difference in the call control access time experi-enced by an MCPTT client. To further investigate the impactof ππΉπΊ3, we define two other performance metrics calledβmultiple occupancyβ and βsingle occupancyβ to capture theimpact of the half duplex effect, which can lead to packetlosses at the receiving UE. βMultiple occupancyβ refers to asituation in whichmultiple UEs try to sendmessages over theair during the same sidelink period, while βsingle occupancyβrefers to a situation in which only one UE tries to sendmessages over the air during a sidelink period. The half-duplex effect will not occur with βsingle occupancyβ but mayoccur with βmultiple occupancyβ. As shown in Table 4, thenegative impact of ππΉπΊ3βs configuration valueβs being toosmall can be seen in the higher ratio of βmultiple occupancyβvs βsingle occupancy,β which may lead to more frequent
occurrences of the half duplex effect. This is mainly due tounnecessary retransmissions of the βCall Probeβ message.The negative impact of ππΉπΊ3βs being too small is also shownthrough the nonzero ratio of samples discarded. Note thata sample is discarded if both SCI message transmissionsin the control channel (PSCCH) fail or all four HARQtransmissions in the data channel (PSSCH) fail, due to eitherhalf-duplex effect or channel PER. Given PER = [0.1, 0.1,0.1, 0] as a simulation assumption, all samples discardedare always the consequence of the half duplex effect. Thefraction of samples discarded is dependent on the size ofthe resource pools allocated to sidelink communication. Thepositive impact of a small ππΉπΊ3 is that the βCall Probeβmessage may be retransmitted sooner if it is lost.
It is worth pointing out that the occurrence of βmultipleoccupancyβ and thus the half-duplex effect are expected tobe more frequent when the number of participating UEsin the group call increases. Therefore, although we do notsee any noticeable difference in access time when changingππΉπΊ3 value in the 2-UE system, the impact of ππΉπΊ3 willbecome more obvious for multiple-UE systems. In addition,the occurrence of the half-duplex effect can be reducedby adapting resource pool configuration to the numberof participating UEs (e.g., extending the pool in the timedomain), which is an interesting research topic by itself.
4.2. Impact of βWait for Call Announcementβ Timer ππΉπΊ1.When we run the simulation with the same assumptions asfor Figure 5 but with ππΉπΊ1 configured at the 3GPP defaultsetting of 150ms instead of max(1π , 6 β πππΏ), we can see from
![Page 14: Access Time Analysis of MCPTT Off-Network Mode over LTE](https://reader031.vdocuments.us/reader031/viewer/2022021713/620b940c2b21b5120d3c510e/html5/thumbnails/14.jpg)
14 Wireless Communications and Mobile Computing
TFG
1 la
rge
TFG
1=15
0 m
s
AοΏ½e
r mer
ge
TFG
1 la
rge
TFG
1=15
0 m
s
AοΏ½e
r mer
ge
TFG
1 la
rge
TFG
1=15
0 m
s
AοΏ½e
r mer
ge
TFG
1 la
rge
TFG
1=15
0 m
s
AοΏ½e
r mer
ge
40 60 80 120
Sidelink Period (ms)
Large TFG1 (TFG1 large, β est) vs Small TFG1 (TFG1 = 150ms)vs Small TFG1 aοΏ½er call merge (AοΏ½er merge)
0
100
200
300
400
500
Acce
ss T
ime (
ms)
Min Avg Max
Figure 11: Impact of TFG1 on call control access time with 95% Confidence Intervals for the average values from the simulations.
Figure 11 that the maximum access times experienced byan MCPTT client are reduced and capped at 150ms, andthus the average access time is reduced. As we explained inSection 3.2.1, when ππΉπΊ1 expires, UE A assumes that the callof interest does not exist and will establish a new group callby itself, which marks the end of the MCPTT access timeduration. However, given that the scenario simulated is B orC, i.e., group call ππ exists already, these ππΉπΊ1 expirationsare premature, and the capped maximum access time mightnot be an improvement. Indeed, the MCPTT client, UE A,makes the wrong decision when establishing a new groupcall instead of joining the existing call. Consequently, theMCPTT client is not aware of the existing call, and multiplecalls of the same group ID coexist. This undesirable situationof multiple calls with the same group ID will be referred to asthe βmultiple calls situationβ for the remainder of this paper.The multiple calls situation will persist until the call mergingprocedure is triggered. After taking into account the extradelay introduced by the necessary call merging procedure,the average effective call control access time is very closeto the results that we obtained when ππΉπΊ1 is large, as seenin Figure 11, so the analytical model gives a good predictionof the effective call control access time after the call mergeprocedure. Simulation results with a sidelink period longerthan 120 ms are not included in Figure 11 becauseUEAalwaysmakes the wrong decision in that case and, thus, the callmerging procedure is always triggered to fix themultiple callssituation.
The disadvantages of the multiple calls situation includenot only the extra delay due to the callmerging procedure, butalso the potential false impression/feedback to the MCPTTclient. The MCPTT user, who is very likely a first responderon an urgent task, might think that no teammate is withincommunication range, although some teammates may bedeployed nearby. Similarly, the MCPTT user may try toswitch groups if he/she is notified wrongly that the groupdoes not have an existing call. As another example, a first
responder may decide to take actions at a higher risk, insteadof waiting for support/backup from other teammates whenin a time-sensitive or life-threatening situation. Therefore,although a smaller ππΉπΊ1 configuration may reduce the waitbefore establishing a new call if the call does not exist, the3GPP default setting of ππΉπΊ1 = 150ms is worth furtherdiscussion to reduce the occurrence of the multiple callssituation, and it may be desirable to take πππΏ into accountwhen configuring ππΉπΊ1.
The negative impact of the multiple calls situationbecomes more pronounced when the channel conditionsdegrade. Figure 12 compares the impact of channel loss onthe call control access time when timer ππΉπΊ1 is configuredwith the 3GPP default setting. When the channel PER ofthe fourth transmission attempt, π4, in the Sidelink SharedChannel increases from 0 (lossless) to 0.1 (lossy), ππΆππππΆπππ‘πππremains similar. This is because the potential long delay duetomessage loss is mostly absorbed by the expiration ofππΉπΊ1.However, the value of ππΆππππΆπππ‘πππ discussed above is only thenominal access time and does not consider that the MCPTTclient might make the wrong decision to establish a newgroup call whenππΉπΊ1 expires.When comparing the effectiveaccess time experienced by the MCPTT client, i.e., consider-ing the extra delay introduced by the call merging procedurewherever applicable, the average effective ππΆππππΆπππ‘πππ valuehas increased considerably due to the channel loss. The extradelay observed, which is quite large, is mainly due to the slowperiodicity of the Call Announcement transmission cyclewhen the MCPTT UE is not responding to a Call Probe.When a Call Probe message is lost, or the responding CallAnnouncement message is lost, a UE is more likely to waitfor the periodic Call Announcement to trigger a call mergingprocedure, which is slower.
Table 5 shows the probability of the βmultiple calls situ-ationβ occurrence if ππΉπΊ1 is set to 150ms. Because of thepotential packet loss due to the unnecessary retransmissionofMCPTTmessages, an error situation ismore likely to occur
![Page 15: Access Time Analysis of MCPTT Off-Network Mode over LTE](https://reader031.vdocuments.us/reader031/viewer/2022021713/620b940c2b21b5120d3c510e/html5/thumbnails/15.jpg)
Wireless Communications and Mobile Computing 15
p4=0
p4=0
.1
AοΏ½e
r mer
ge, p
4=0
AοΏ½e
r mer
ge, p
4=0.
1
p4=0
p4=0
.1
AοΏ½e
r mer
ge, p
4=0
AοΏ½e
r mer
ge, p
4=0.
1
p4=0
p4=0
.1
AοΏ½e
r mer
ge, p
4=0
AοΏ½e
r mer
ge, p
4=0.
1
p4=0
p4=0
.1
AοΏ½e
r mer
ge, p
4=0
AοΏ½e
r mer
ge, p
4=0.
1
40 60 80 120
Sidelink Period (ms)
PER = [0.1, 0.1, 0.1, 0] (p4 = 0) vs PER = [0.1, 0.1, 0.1, 0.1] (p4 = 0)
0100200300400500600700800
Acce
ss T
ime (
ms)
Min Avg Max
Figure 12: Impact of channel loss on call control access time with TFG1 = 150ms with 95% Confidence Intervals for the average values fromthe simulations.
Table 5: Probability of βMultiple Calls Situationβ for various ππΉπΊ1 configurations and channel conditions (ππΉπΊ1 = 150 ms).
πππΏ(ms) 40 60 80 120
Probability of the occurrence ofβmultiple calls situationβ
ππΈπ = [0.1, 0.1, 0.1, 0] ππΉπΊ3 = 6 β πππΏ 0.169 0.384 0.504 0.894ππΉπΊ3 = πππΏ 0.176 0.378 0.513 0.904
ππΈπ = [0.1, 0.1, 0.1, 0.1] ππΉπΊ3 = πππΏ 0.186 0.362 0.525 0.917
when ππΉπΊ3 is set to πππΏ than when it is set to 6 β πππΏ. Whenthe channel condition is worse, the occurrence rate of theβmultiple calls situationβ also increases, as shown in the lasttwo lines of Table 5.
4.3. Impact of Floor Request Retransmission T201. Theimpacts of a smaller value for the Floor Request Retrans-mission Timer, π201, such as the 3GPP default setting πππΏ,are multifold. On the positive side, if there is no floorarbitrator present, a smaller value for π201 enables theMCPTT user to declare the floor sooner, with the accesstime equal to T201βC201, than when π201 is larger (e.g.,6 β πππΏ) as shown in Figure 13(a), or when π201βs value isbased on the provision of the near worst case as shownin Figure 13(b). In the case shown in Figure 13(b), π201 =π β πππΏ, where π is the smallest integer which is greaterthan max{ππΉπππππΆπππ‘πππ (πΉ2.1)}/πππΏ, which can be calculatedbased on (18). By comparing Figures 13(a) and 13(b), wecan see that the channel loss makes the positive impact ofa small π201 configuration more pronounced. Note that weobtained the results in Figure 13(a) by running the simulationwith the same assumption as for Figure 7, except that weconfigured π201 at the 3GPP default setting πππΏ insteadof at 6 β πππΏ. We obtained the results of Figure 13(b) byrunning the simulation with the same assumption as forFigure 7, except that we set PER to [0.1, 0.1, 0.1, 0.1] instead
of [0.1, 0.1, 0.1, 0], and we configured π201 at either πππΏ orπ β πππΏ.However, on the negative side, the decision made by
the MCPTT client (e.g., assume the floor is idle) with asmaller π201 value might be premature, and it may lead toan undesirable situation in which there are multiple floorarbitrators, which we will refer to as the βmultiple floorarbitrators situationβ. Note that this situation is neither easynor quick to resolve, given the current procedures. Table 6shows the increase in ratio of βwrong decisionβ vs βcorrectdecisionβ when comparing small π201 settings with largeπ201 settings. It is also clear from comparing Table 6(a) toTable 6(b) that the channel loss makes the negative impactof a small π201 configuration even more evident. Note thatthe results in Table 6 show the worst case outcome becausewe assumed that the incumbent floor arbitrator UE B doesnot generate RTP packets during UE Aβs access time. If thecurrent floor arbitrator UE B is actively sending RTP packets,the counter C201 at UE A is reset every time an RTP packetis received. Therefore, it will take more time and will be lesslikely for UE Aβs πΆ201 to reach the threshold and make awrong decision when UE B is talking.
We encounter the βmultiple floor arbitrators situationβwhen there is more than one floor arbitrator simultaneouslyin the same off-network group call, although the correctbehavior is that there should be at most one floor arbitrator
![Page 16: Access Time Analysis of MCPTT Off-Network Mode over LTE](https://reader031.vdocuments.us/reader031/viewer/2022021713/620b940c2b21b5120d3c510e/html5/thumbnails/16.jpg)
16 Wireless Communications and Mobile Computing
6βT_SL T_SL 6βT_SL T_SL 6βT_SL T_SL 6βT_SL T_SL
40 60 80 120
Sidelink Period (ms)
Large T201 (6βT_SL) vs Small T201 (T_SL)
0
100
200
300
400
500
600
700
Acce
ss T
ime (
ms)
Min Avg Max
(a) Access time when π201 = 6 βπππΏ, PER = [0.1, 0.1, 0.1, 0] with 95% Confidence Intervals for theaverage values from the simulations
mβT_SL T_SL mβT_SL T_SL mβT_SL T_SL mβT_SL T_SL
40 60 80 120
Sidelink Period (ms)
Large T201 (= mβT_SL) vs Small T201 (= T_SL)
0
100
200
300
400
500
600
700
Acce
ss T
ime (
ms)
Min Avg Max
(b) Access time when π201 = π β πππΏ, PER = [0.1, 0.1, 0.1, 0.1] with 95% Confidence Intervals forthe average values from the simulations
Figure 13: Impact of π201 on floor control access time.
Table 6: Impact of T201 configurationβpotential multiple floor arbitrators situation.
(a) PER = [0.1, 0.1, 0.1, 0]
πππΏ (ms) 40 60 80 120Ratio of βpremature π201 expiration caseβ (i.e., thewrong decision case) vs βthe correct decision caseβ
π201 = πππΏ 0.026 0.018 0.0007 0.0004π201 = 6 β πππΏ 0 0 0 0
(b) PER = [0.1, 0.1, 0.1, 0.1]
πππΏ (ms) 40 60 80 120Ratio of βpremature π201 expiration caseβ (i.e., thewrong decision case) vs βthe correct decision caseβ
π201 = πππΏ 0.126 0.145 0.075 0.060π201 = π β πππΏ 0 0 0 0
at any time. There are several circumstances that may leadto the βmultiple floor arbitrators situation,β such as a pre-mature π201 timer expiration or the hidden node problem.The potential disadvantages of the multiple floor arbitratorssituation are as follows:
(i) The lack of resolution procedure for βmultiple floorarbitrators situation.β This is in contrast to theβmultiple group calls situation,β for which the callmerging procedure is already defined in the callcontrol protocol as a remedy.
![Page 17: Access Time Analysis of MCPTT Off-Network Mode over LTE](https://reader031.vdocuments.us/reader031/viewer/2022021713/620b940c2b21b5120d3c510e/html5/thumbnails/17.jpg)
Wireless Communications and Mobile Computing 17
Table 7: Impact of π201 Configurationβpotential half duplex effect.
πππΏ (ms) 40 60 80 120 160 240 280 320
Ratio of double occupancyvs single occupancy
π201 = πππΏ 0.603 0.723 0.581 0.575 0.573 0.523 0.529 0.534π201 = 6 β πππΏ 0 0 0 0 0 0 0 0
Ratio of samples discardedπ201 = πππΏ 0.112 0.126 0.071 0.058 0.051 0.024 0.025 0.025
π201 = 6 β πππΏ 0 0 0 0 0 0 0 0
Table 8: Average MCPTT access time in milliseconds with 3GPP default settings.
πππΏ (ms) 40 60 80 120 160 240 280 320Scenario A 150 150 150 150 150 150 150 150Scenario B 184.46 221.95 263.45 342.81 Call Merge procedure is always triggeredScenario C 226.43 293.82 364.47 503.17 Call Merge procedure is always triggeredScenario D 78.03 108.13 138.98 199.63 260.22 379.80 440.32 500.85Scenario E 120 180 240 360 480 720 840 960
(ii) The MCPTT client who takes the floor prematurelyis not aware of the situation and might start talkingwhile not knowing that no one is listening;
(iii) The incumbent floor arbitrator is not aware of thesituation either and does not know that one or moregroup members are not listening.
As was the case in the call control study, the negative impactof the Floor Request Retransmission Timer π201 beingconfigured too small can be noticed in Table 7 and is mainlydue to the unnecessary retransmissions of the Floor Requestmessage.
When comparing the same performance metrics of theππΉπΊ1 = πππΏ case in Table 4 with the π201 = πππΏ case inTable 7, the call control procedure seems to perform slightlybetter than its floor control counterpart. This is because theππΉπΊ2πππππ timer adds a random delay to the transmissiontiming of the responding Call Announcement message.Consequently, the transmission of the Call Announcementmessage does not always compete with the retransmissionof a Call Probe message for resources in the same sidelinkperiod.
4.4. MCPTT Access Time with 3GPP Default Settings. Table 8summarizes the simulation results of the average MCPTTaccess time in milliseconds for different scenarios when theMCPTT parameters are configured according to the 3GPPdefault settings.
We obtained the values in the table assuming no hardwareor software processing time. In addition, we obtained theaccess times when UE A makes the correct decision asdescribed in Table 3. However, wrong decisions are notuncommon occurrences with the 3GPP default settings,which might lead to the βmultiple floor arbitrators situationβand/or the situation of multiple calls with the same groupID.
5. Conclusions and Future Work
We developed the analytical models and simulation toolsdescribed in this paper to estimate and evaluate the perfor-mance and user experience of MCPTT off-network modeoperations over ProSe in LTE, with a focus on access timewhen an MCPTT UE is out of coverage. We defined perfor-mance metrics and application scenarios for the access timeanalysis. Our analytical models provide good estimates ofthe minimum, average, and maximum for both call controland floor control access times experienced by an MCPTTclient under various scenarios, as shown by our simulationresults. We summarize our main observations with respect toparameter configurations in Table 9. These observations maybe used to understand the tradeoff and provide configurationguidelines for MCPTT and ProSe.
The sensitivity analysis results show that preventivesolutions, such as proper parameters configuration and/ormodified procedures, need to be investigated to reduce thepossibility of an MCPTT UEβs making wrong decisions. Inaddition, 3GPP may re-evaluate the default setting of timersTFG1, TFG3, and T201 due to the undesirable occurrenceof the βmultiple floor arbitrators situation,β the coexistence ofmultiple group callswith the same group ID, and the relativelyhigh probability of the half duplex effect.
Our future work includes enhancing the access timeanalysis to consider more aspects related to ProSe, suchas a probability model of the half duplex effect in thePSCCH. In addition, we will evaluate the MCPTT accesstime performance for the scenarios whenmultiple active UEstry to talk at nearly the same time, or there are multipleongoing group calls competing for radio resources overProSe.
Data Availability
The data used to support the findings of this study areavailable from the corresponding author upon request.
![Page 18: Access Time Analysis of MCPTT Off-Network Mode over LTE](https://reader031.vdocuments.us/reader031/viewer/2022021713/620b940c2b21b5120d3c510e/html5/thumbnails/18.jpg)
18 Wireless Communications and Mobile Computing
Table 9: Impact of parameters configuration on MCPTT access time.
Parameter Small Big
πππΏ (i) Shorter access time;(ii) More overhead;
(i) Longer access time;(ii) Less overhead;
ππΉπΊ3 (i) Faster retransmissions of Call Probe;(ii) More unnecessary retransmissions of Call Probe(e.g., with 3GPP default configuration) are likely;
(i) Reduced unnecessary retransmissions of CallProbe;
(ii) Necessary Call Probe retransmission may beslowed down;
ππΉπΊ1(i) Reduced nominal maximum and average access
time;(ii) The call merge procedure is more likely to be
triggered (e.g., with 3GPP default configuration), andresult in increased effective maximum and average
access time;
(i) Nominal maximum and average access timemay be larger;
(ii) Reduced unnecessary call merge proceduretriggering, and thus reduced effective maximum
and average access time;
π201&(πΆ201 β π201)
(i) Faster determination of idle floor;(ii) The βmultiple floor arbitrators situationβ is morelikely to occur (e.g., with 3GPP default configuration);(iii) More unnecessary retransmissions of Floor Request
(e.g., with 3GPP default configuration) are likely.
(i) Slower determination of idle floor;(ii) The βmultiple floor arbitrators situationβ can
be reduced (C201βT201);(iii) Reduced unnecessary retransmissions of
Floor Request.
Conflicts of Interest
The authors declare that they have no conflicts of interest.
References
[1] National Public Safety Telecommunications Council (NPSTC),βMission Critical Voice Communications Requirements forPublic Safety,β 2011.
[2] National Public Safety Council Telecommunications, βPublicSafety Broadband Push-to-Talk over Long Term EvolutionRequirements,β Tech. Rep., 2013.
[3] A. Rebeiro-Hargrave, OMA Push-to-Talk Architecture, Multi-media Group Communication, Chichester, UK, pp. 23β57, 2AD,2008.
[4] D. Ma, W. Huang, Q. Sun, W. Li, C. Yin, and Z. Shao, βDesignand realization of the poc system with short session-setup-delay,β in Proceedings of the 2012 International Conference onComputer Science and Service System, CSSS 2012, pp. 1075β1078,August 2012.
[5] E. Casini, N. Suri, M. Breedy, P. Budulas, J. Kovach, and R.Roy, βPeerTalk: A push-to-talk and instant messaging servicefor tactical networks,β in Proceedings of the 2013 IEEE MilitaryCommunications Conference, MILCOM 2013, pp. 1476β1481,November 2013.
[6] M.-H. Tsai and Y.-B. Lin, βTalk burst control for push-to-talkover cellular,β IEEE Transactions on Wireless Communications,vol. 7, no. 7, pp. 2612β2618, 2008.
[7] A. Kuwadekar and K. Al-Begain, βAn evaluation of push to talkservice over ims and lte for public safety systems,β in Proceedingsof the 2014 6th International Conference on ComputationalIntelligence and Communication Networks, CICN 2014, pp. 412β416, November 2014.
[8] Open Mobile Alliance (OMA), Enabler Release Definition forPush to Communicate for Public Safety (PCPS), 2017.
[9] 3GPP, βTechnical Specification Group Services and SystemAspects; Mission Critical Push To Talk (MCPTT) over LTE;Stage 1 (Release 14); TS 22.179,β Tech. Rep., 2016.
[10] J. Kim, O. Jo, and S. W. Choi, βState-based uplink-schedulingscheme for reducing control plane latency of MCPTT services,βIEEE Systems Journal, 2018.
[11] R. Solozabal, A. Sanchoyerto, E. Atxutegi, B. Blanco, J. O.Fajardo, and F. Liberal, βExploitation of mobile edge computingin 5G distributed mission-critical push-to-talk service deploy-ment,β IEEE Access, vol. 6, pp. 37665β37675, 2018.
[12] 3GPP, βTechnical Specification Group Core Network and Ter-minals; Proximity services (ProSe) User Equipment (UE) toProSe Function Protocol Aspects; Stage 3 (Release 14); TS24.334,β Tech. Rep., 2018.
[13] J. Wang and R. Rouil, βAssessing coverage and throughputfor D2D communication,β in Proceedings of the 2018 IEEEInternational Conference on Communications, ICC 2018, May2018.
[14] D. W. Griffith, F. J. Cintron, and R. A. Rouil, βPhysical sidelinkcontrol channel (PSCCH) in mode 2: performance analysis,βin Proceedings of the 2017 IEEE International Conference onCommunications, ICC 2017, May 2017.
[15] D. Griffith, F. Cintron, A. Galazka, T. Hall, and R. Rouil,βModeling and simulation analysis of the physical sidelinkshared channel (PSSCH),β in Proceedings of the 2018 IEEEInternational Conference on Communications, ICC 2018, May2018.
[16] A. M. Cipriano and D. Panaitopol, βPerformance analysisof sidelink data communications in autonomous mode,β inProceedings of the 2018 IEEE Wireless Communications andNetworking Conference, WCNC 2018, pp. 1β6, April 2018.
[17] National Institute of Standards and Technology, βPublicSafety Communication modeling tools based on ns-3,β 2018.https://github.com/usnistgov/psc-ns3.
[18] 3GPP, βTechnical Specification Group Core Networks andTerminals;MissionCritical PushToTalk (MCPTT) call control;Protocol specification (Release 14); TS 24.379,β Tech. Rep., 2018.
[19] 3GPP, βMission Critical Push To Talk (MCPTT) media planecontrol; Protocol specification (Release 14); TS 24.380,β Tech.Rep., 2018.
![Page 19: Access Time Analysis of MCPTT Off-Network Mode over LTE](https://reader031.vdocuments.us/reader031/viewer/2022021713/620b940c2b21b5120d3c510e/html5/thumbnails/19.jpg)
Wireless Communications and Mobile Computing 19
[20] P. Varin, Y. Sun, andW. Garey, βNISTIR 8236 Test Scenarios forMission Critical Push-To-Talk ( MCPTT ) Off-Network ModeProtocols Implementation,β Tech. Rep., 2018.
[21] 3GPP, βTechnical Specification Group Radio Access Network;Evolved Universal Terrestrial Radio Access (E-UTRA); Physicallayer procedures (Release 14); TS 36.213,β Tech. Rep., 2018.
![Page 20: Access Time Analysis of MCPTT Off-Network Mode over LTE](https://reader031.vdocuments.us/reader031/viewer/2022021713/620b940c2b21b5120d3c510e/html5/thumbnails/20.jpg)
International Journal of
AerospaceEngineeringHindawiwww.hindawi.com Volume 2018
RoboticsJournal of
Hindawiwww.hindawi.com Volume 2018
Hindawiwww.hindawi.com Volume 2018
Active and Passive Electronic Components
VLSI Design
Hindawiwww.hindawi.com Volume 2018
Hindawiwww.hindawi.com Volume 2018
Shock and Vibration
Hindawiwww.hindawi.com Volume 2018
Civil EngineeringAdvances in
Acoustics and VibrationAdvances in
Hindawiwww.hindawi.com Volume 2018
Hindawiwww.hindawi.com Volume 2018
Electrical and Computer Engineering
Journal of
Advances inOptoElectronics
Hindawiwww.hindawi.com
Volume 2018
Hindawi Publishing Corporation http://www.hindawi.com Volume 2013Hindawiwww.hindawi.com
The Scientific World Journal
Volume 2018
Control Scienceand Engineering
Journal of
Hindawiwww.hindawi.com Volume 2018
Hindawiwww.hindawi.com
Journal ofEngineeringVolume 2018
SensorsJournal of
Hindawiwww.hindawi.com Volume 2018
International Journal of
RotatingMachinery
Hindawiwww.hindawi.com Volume 2018
Modelling &Simulationin EngineeringHindawiwww.hindawi.com Volume 2018
Hindawiwww.hindawi.com Volume 2018
Chemical EngineeringInternational Journal of Antennas and
Propagation
International Journal of
Hindawiwww.hindawi.com Volume 2018
Hindawiwww.hindawi.com Volume 2018
Navigation and Observation
International Journal of
Hindawi
www.hindawi.com Volume 2018
Advances in
Multimedia
Submit your manuscripts atwww.hindawi.com