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Relays in Heterogeneous NetworksSubmitted by: Arjun BhatiaGirish AnandRajesh GuptaShivani BhatiaSunanda Mukundan1University of Maryland, ENTS, Adv. Wireless NetworksTable of ContentsIntroductionHeterogeneous NetworksRelaysImplementation of RelaysTypes of RelaysImplementation StrategiesHandoverDeployment ChallengesInterference ManagementEconomic BenefitsReferences

Fall 20112University of Maryland, ENTS, Adv. Wireless NetworksPath towards LTE Advanced The very high data rates and large coverage area envisioned for 4G wireless systems in reasonably large areas do not appear to be feasible with the conventional cellular architecture as :

They do not achieve the expected throughput to ensure seamless mobile broadband in the uplink as users move far from the base station

Increase in the inter-cell interference, as well as constraints on the transmit power of the mobile devices

Poor indoor penetration and the presence of dead-spots, which results in drastically reduced indoor coverage

Solution : To address these issues, there has been an increasing interest to deploy relays, distributed antennas and small cellular access points in residential homes, subways and offices.

PS: Deployment of more number of BTS requires cost and planning

3University of Maryland, ENTS, Adv. Wireless NetworksFall 20113Heterogeneous Networks

Network architectures with relays, picocells and femtocells overlaying the macrocell network are commonly referred as heterogeneous networks All these nodes have different characteristics of RF power and coverage area Nodes with large RF coverage area are deployed in a planned way to provide coverage in urban, suburban and rural areas For small coverage area, nodes provide coverage extension or throughput enhancement. By shrinking coverage to smaller blocks, the capacity can be shared with a fewer number of people, resulting in higher capacity and faster data speeds

4University of Maryland, ENTS, Adv. Wireless NetworksFall 20114 Relay : Background A solution for radio range extension in mobile and wireless broadband cellular networks

Time-division multiple access (TDMA)-based systems This scheme of relaying, allows for easy allocation of resources to the mobile-to-relay and relay-to-mobile links. The first system based on time-division multiplex (TDM) and relays connecting mobiles to the fixed network was proposed in 1985

Code-division multiple access (CDMA)-based systems Relaying in cellular CDMA systems has also been investigated by companies

FDD based relay systems Uplink and Downlink are separated using frequency division duplex (FDD), as is done in IS-95 and Universal Mobile Telecommunications System (UMTS) terrestrial radio access (UTRA) FDD

Fixed relays (called wireless BSs) for cordless systems The European Telecommunications Standards Institute/Digital Enhanced Cordless Telephony ( E T S I /DECT ) standard in 1998 was the first specifying fixed relays (called wireless BSs) for cordless systems using TDM channels for voice and data communications

5University of Maryland, ENTS, Adv. Wireless NetworksFall 2011RELAY NODES

Relays are expected to be a cost efficient way to fulfill requirements on high data rate coverage in next generation cellular networks, like LTE-Advanced.

Low-cost low transmit power base stations using the same spectrum as backhaul and access. And have similar power as Picos.

Enhance performance at the cell edge for a comparatively low cost.

Include peak data rates of 1 Gbps in downlink (DL) and 500 Mbps in uplink (UL), bandwidth scalability up to 100 MHz

6University of Maryland, ENTS, Adv. Wireless NetworksFall 2011Why Relays are Required Increased broadband Traffic : Increased traffic due to new applications

Increased capacity : Number of mobile users with high-end smart phones accessing social networking, video and VoIP are increasing

Provide High data rate : Path towards LTE- advanced, common convergence network

Easy Installation : Het-Nets are being deployed initially in high data traffic areas using street-level plug-and-play nodes

Low Cost : Relays can be even more cost-effective than a WLAN access point, as the former does not require a wired backhaul connection

Low Opex cost : Reducing operating expenses such as tower leasing and maintenance costs for the service provider

Work well in Non Line-of-sight operation : The spectrum that will be released for 4G systems will almost certainly be located well above the 2 GHz band. Therefore the radio propogation in these bands is significantly more vulnerable to non LOS conditions.

7University of Maryland, ENTS, Adv. Wireless NetworksFall 2011Advantages of Relays

Transmission power of user equipment and base stations can be reduced

More radio resources can be allocated to a single user Hence improved capacity

Spectrum efficiency per meter square would be improved

Higher mobility and Quality of Service to the end users

Relaying is presented as a means to reduce infrastructure deployment costs

Exploitation of spatial diversity, multihop relaying can enhance cellular capacity possible to connect BTS and relay at the same time

Networks applying relaying via fixed infrastructure do not need complicated distributed routing algorithms, while retaining the flexibility of being able to move the relays as the traffic patterns change over time

Solutions to combat shadowing at high radio frequencies

8University of Maryland, ENTS, Adv. Wireless NetworksFall 2011Relays: Network ImprovementIncrease network density : LTE relay nodes can be deployed very easily in situations where the aim is to increase network capacity by increasing the number of eNBs to ensure good signal levels are received by all users. LTE relays are easy to install as they require no separate backhaul and they are small enabling them to be installed in many convenient areas, e.g. on street lamps, on walls, etc.

9University of Maryland, ENTS, Adv. Wireless NetworksFall 2011Relays: Network ImprovementNetwork coverage extension : LTE relays can be used as a convenient method of filling small holes in coverage. With no need to install a complete base station, the relay can be quickly installed so that it fills in the coverage blackspot.

10University of Maryland, ENTS, Adv. Wireless NetworksFall 201110Relays: Network ImprovementLTE relay nodes may be used to increase the coverage outside main area. With suitable high gain antennas and also if antenna for the link to the donor eNB is placed in a suitable location it will be able to maintain good communications and provide the required coverage extension.

11University of Maryland, ENTS, Adv. Wireless NetworksFall 2011Relays: Network ImprovementRapid network roll-out: Without the need to install backhaul, or possibly install large masts, LTE relays can provide a very easy method of extending coverage during the early roll-out of a network. More traditional eNBs may be installed later as the traffic volumes increase.

12University of Maryland, ENTS, Adv. Wireless NetworksFall 2011 Relay : Improve shadowing effectUniversity of Maryland, ENTs, Adv. Wireless Networks13

Relay Types Relays are of two types :

Conventional Amplifiers Decode and Forward

Conventional Amplifiers :Used as gap fillers. Also known as repeaters

14University of Maryland, ENTS, Adv. Wireless NetworksFall 2011Benefits:

Amplify the desired signals, like repeatersPerform better than Decode and Forward relays near the receiver

Limitations:

Analog repeaters increase the noise level and suffer from the danger of instability due to their fixed gain. Also amplify both the noise and interference Suffer from Loop Interference, i.e. leakage of transmit signals to receive antenna Concurrent transmission and reception at the same frequency band requires two separated antennas in the relay

15University of Maryland, ENTS, Adv. Wireless NetworksFall 2011Decode and forward

Remove the noise by decoding the received signals and then regenerating and re-encoding the signal to be forwarded to the destination Adaptive Decode and Forward Relay: The relay forwards the signal to the destination only if it is able to decode the signal correctly More attractive in both cell middle and edge deployments Rely on full decoding at the relay, hence cannot achieve full diversity

16University of Maryland, ENTS, Adv. Wireless NetworksFall 2011Implementation Strategies

One-way relays Two-way relays Shared relays

17University of Maryland, ENTS, Adv. Wireless NetworksFall 2011

Two Way Relay18University of Maryland, ENTS, Adv. Wireless NetworksFall 2011Shared Relaying

19University of Maryland, ENTS, Adv. Wireless NetworksFall 2011

TDD and FDD RelayingTDD relaying suffers from link budget limitation because only a single node transmits at a given time.In FDD relaying, two nodes can transmit simultaneously but the scheme does not allow power and resource sharing between eNB-Relay and Relay-UE links.

20University of Maryland, ENTs, Adv. Wireless NetworksFall 2011 Fixed versus Mobile/Movable (Terminal) Relaying

Relaying through Mobile Station

In a cellular network with mobile relay station, the nonactive MS (i.e. in idle state) are potential candidate to relay the traffic of the active MS to the BS.

PICTURE

21University of Maryland, ENTS, Adv. Wireless NetworksFall 2011Benefits: Low deployment/maintenance cost, since other users terminals can potentially act as relays

The MS are able to organize themselves in order to cover some unknown dead spots, which are difficult to predict with the operators planning tool

They can also be used where it is not cost effective to install FRS such as mountainous environment or subway train platform.

They can also be used at unpredictable places like accidents or at infrequently occurring events like demonstrations or sports event

If there are a large number of idles MS, there are more choice to select a MRS that can optimize the system performance. For example, a relayed user can choose a MRS with whom it experiences a LOS link.22University of Maryland, ENTS, Adv. Wireless NetworksFall 2011Limitations Relaying opportunity depends strongly on the users density

Relaying through other users terminals can considerably decrease the battery life of the mobile relay station

The relaying system performance is highly dependent of the RS selection scheme

The signaling (new channel MS-RS, inter-relay handover, RS selection) required for the relaying system might increase considerably

The cost of the terminal to support relaying (hardware and software) will increase - the implementations at the hardware become even more difficult if a MRS needs to relay more than one MSs traffic at the same time.

Due to the mobility of the relayed MS and the MRS, frequent inter-relay handoffs might occurs, which might increase the signaling and affect the relaying system performance.

Some others issues such as fast fading, power control or security need more investigations in order to make possible the communication between MSs.23University of Maryland, ENTS, Adv. Wireless NetworksFall 2011Fixed Station Relay :

Fixed station relay (FRS) are the low cost station placed at specific location. FRS are part of the network infrastructure, therefore their deployment will be an integral part of the network planning, design and deployment process.

24University of Maryland, ENTS, Adv. Wireless NetworksFall 2011Benefits Operators will have better control of coverage and capacity expected in a specific area

The FRS can be deployed at strategic locations in order to maintain a LOS with the BS. They can also use directional antenna to improve the propagation link with the BS

The FRSs are less constrained by energy consumption. They may potentially be equipped with more advanced hardware, which enable them to operate in any frequency band as well as allow them to relay several MS at a time.

The use of FRS eases the problem of RS selection since fewer number of FRS are deployed compared to MRS. Therefore the signaling overhead required for the RS selection will not be a major issue when relaying with FRS.

The inter-relay handoffs will occur only when the relayed MS will move from one FRS to another FRS.

Due to their sophisticated hardware, it is more secure to relay through a FRS than MRS. The data are always transferred through a known (fixed) RS25University of Maryland, ENTS, Adv. Wireless NetworksFall 2011Limitations :

Infrastructures cost The dimensioning, planning, optimization and maintenance of the FRS can be expensive and cost inefficient

Furthermore it might be cost ineffective to install FRS if there are many sparse coverage holes, in which only a few mobile terminals are located

Which one is a better approach ????

26University of Maryland, ENTS, Adv. Wireless NetworksFall 2011Which one is a better approach ?

Although, it has been observed that under certain conditions the uplink capacity gain of 35% is readily achievable for both form of relay stations. But relaying with MRS is a more challenging network than FRS. Thus before a relaying system with MRS can be realized, many issues need to be investigated like :

allowing the MRSs battery to be used for someone elses call increase of the signaling overhead additional hardware and software will increase the terminals cost frequent inter-relay handoffs.

27University of Maryland, ENTS, Adv. Wireless NetworksFall 2011Pairing Scheme for Relays SelectionCentralized relay pairing In this scheme the BS will act as a control node and collects the channel and location information from all the RNs and SS and then make the pairing decision. This information must be formed as a service set and periodically updated in the local BS to capture dynamic changes of SSs This scheme requires more signaling over head, and can achieve better performance gains. Distributed relay pairingIn this scheme, RS collects the channel and location information from all the nearby SSs and then makes the pairing decision. First each RS identify its service set of neighborhood SSs and also the channel conditions between its BS as well as its SS, those RS with single service set each randomly selects a time slot from the N- slots in the pairing scheme. If multiple RS choose the same time slot then collusion occurs and those RS will be trying again in the next pairing scheme.28University of Maryland, ENTS, Adv. Wireless NetworksFall 201128Handover in Relay Enhanced LTE NetworkCentralized Relaying Handover:Source Relay Node and Target Relay Node added to the existing network elementsHandover initiated by the source eNBr. The source eNBr controls the source RN and the target eNBr controls the target RNThe UE is connected to the source eNBr via the source RNMeasurement reports are sent to the source eNBr to make the decision. Decision depends upon the UEs measurements whether a handover must be done or notHandover request is then sent over to the target eNBr, which checks whether an admission of the UE is made with the target RNAn acknowledgement is sent to the source eNBr and subsequently to the source RNAny buffered downlink packets must be allocated to the target RN via the target eNBr by the source eNBrHence, synchronization between the UE and the target RN is established and information regarding Timing Advance for the UE is sent on the uplinkThe handover is confirmed by the UE to the target RN and the target RN confirms the handover to target eNBr which in turn passes the information to the Gateway via the MMEThe centralized relaying suggests that the overall process is controlled by the eNBrs (both source and target)

29University of Maryland, ENTS, Adv. Wireless NetworksFall 201129

Centralized Handover30University of Maryland, ENTS, Adv. Wireless NetworksFall 2011Handover in Relay Enhanced LTE NetworkDistributed/Decentralized RelayingSource RN initiates the handover. Both the RN and eNBrs work in collaboration to successfully conduct the handoverTarget eNBr performs the admission control on the backhaul link. The target RN receives the handover request from the target eNBr which then performs the admission control for the relay linkHandover request is acknowledged and the handover command is sent to the UE. The downlink data is sent to the target RN through the target eNBrAll downlink packets are buffered at the target RN and synchronization is established with the UE. Timing Advance for the UE is performed at the uplink and the handover confirmation message is given to the target RN by the UEThe target RN sends the confirmation message to the target eNBr which then sends the handover complete message to the Gateway via the MMEThe target eNBr receives an acknowledgement from the Serving Gateway. The target eNBr asks the source eNBr to release resources which in turn sends a command to the source RN, which initiated the handover to release resources of the UEThe data transfer now takes place between the target RN and the UE after the handover has taken place31University of Maryland, ENTS, Adv. Wireless NetworksFall 201131Decentralized/Distributed Handover

32University of Maryland, ENTS, Adv. Wireless NetworksFall 2011University of Maryland, ENTS, Adv. Wireless Networks33Relays: Deployment ChallengesUncoordinated Deployment leads toIncrease in overall interference in the systemReduce signal to noise ratio(SINR)Lack of good models for relaying in cellular systemRouting and resource managementDelay and OverheadIncreasing number of hops in the system introduces complexity and delay

Fall 2011University of Maryland, ENTS, Adv. Wireless Networks34Advanced Interference Management Advanced Interference Management techniques such as resource coordination are needed to realize full benefits of heterogeneous deployments.

Inter-cell Interference Coordination (ICIC) Interfering base stations can coordinate on transmission powers and/or spatial beams with each other in order to enable control and data transmissions to their corresponding user terminals.

Slowly-Adaptive Interference The goal of the slowly-adaptive resource coordination algorithm is to find a combination of transmit powers for all the transmitting base stations and user terminals and over all the time and/or frequency resources that maximizes the total utility of the network. Fall 2011Economic BenefitsLower Capex and Opex costWireless backhaulLower site acquisition costLess costly antenna structureFaster deployment

University of Maryland, ENTS, Adv. Wireless Networks35

Fall 2011Improve ROIHigh grade of serviceLow incremental costCompatible with GSM, WiMAX and CDMA technologies.Support current and future services(LTE, IMS)Since consumer is always on therefore better service and lower churn rate.

University of Maryland, ENTS, Adv. Wireless Networks36Fall 2011Cost Analysis of Relay vs BSUniversity of Maryland, ENTS, Adv. Wireless Networks37

Case ScenariosHeavy Traffic, Urban DeploymentLight Traffic, Rural, Suburban DeploymentFall 2011Capex and Opex of Relay vs BS(Heavy Traffic, Urban Environment)University of Maryland, ENTS, Adv. Wireless Networks38Normalized w.r.t to 1st BarFall 2011Capex and Opex of Relay vs BS(Light Traffic, Rural, Suburban Environment)University of Maryland, ENTS, Adv. Wireless Networks39Normalized w.r.t to 1st BarNumber of RelayFall 2011Study ConclusionsConventional Base StationCapex is a significant cost relative to Opex

RelayCapex grows with decreasing BS : Relay ratioCapex only slightly larger than Opex under light loadCapex considerably less than Opex under heavy load

University of Maryland, ENTS, Adv. Wireless Networks40Fall 2011Referenceswww.qualcomm.com/.../lte-advanced-heterogeneous-networks.pdfLTE Advanced - StanfordNetworkingSeminarHeterogeneous Networks- Prof. Robert W. Heath Jr.www.radio-electronics.com Cellular telecomswww.hindawi.com/journals/wcn/2009/618787/202.194.20.8/proc/VTC09Spring/DATA/09-07-03.PDF

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