cooperative multicasting for wireless scalable video transmissions

3980 IEEE TRANSACTIONS ON COMMUNICATIONS, VOL. 61, NO. 9, SEPTEMBER 2013

Cooperative Multicasting forWireless Scalable Video Transmissions

Sheng-Chieh Wang, Student Member, IEEE, and Wanjiun Liao, Fellow, IEEE

Abstract—Cooperative multicasting is a promising technique tocombat severe fading and path loss for wireless video delivery.In typical wireless video multicasting, nodes under the worstchannel condition determine the video delivery quality, therebylimiting the system performance and affecting the Quality ofExperience (QoE) of the users. Existing cooperative multicastschemes may not fully exploit the characteristics of scalable videoover wireless networks. To tackle this problem, we propose twonew cooperative multicast schemes called OppCM and CodedCMfor scalable video delivery. OppCM performs opportunisticlistening and conditional demodulating to enhance the systemperformance; CodedCM delivers layer-encoded video with multi-resolution modulation through cooperative multicasting. Weanalyze the system performance in terms of average outageprobability for four cooperative multicast schemes, includingdirect multicast (for baseline comparison), pure cooperative mul-ticasting, and the two proposed schemes OppCM and CodedCM.We also evaluate their performance via simulations. The resultsshow that the proposed OppCM and CodedCM outperform theother two schemes, demonstrating that they are indeed promisingsolutions to wireless scalable video multicasting.

Index Terms—Wireless video multicast, cooperative multicast,scalable video, outage probability.

I. INTRODUCTION

IN recent years, the demand for video multicast has beenincreasing rapidly with advances in communications and

multimedia technologies. Wireless video multicast conservesbandwidth by streaming a video to multiple nodes (sub-scribers) simultaneously via wireless broadcast. However,wireless channels are error-prone and location-dependent dueto path loss and fading. Usually, the farther a node is fromthe base station, the worse the channel quality. Therefore, itis a challenge to find an efficient wireless video multicastingapproach which accommodates the requirement of all nodesin the same multicast group given that the channel conditionsamong the nodes may be different. To ensure all nodesin the same group receive video of the same quality, thesolution defined in the wireless standards (e.g., IEEE 802.16

Manuscript received April 9, 2012; revised November 4, 2012 and May 1,2013. The editor coordinating the review of this paper and approving it forpublication was M. Kieffer.

This work was supported in part by the Excellent Research Projects ofNational Taiwan University, under Grant Number AE00-00-04, and in part bythe National Science Council (NSC), Taiwan, under Grant Number NSC99-2221-E-002-030-MY3 and the Thematic Research Program, Academic Sinica,Taiwan.

S.-C. Wang is with the Graduate Institute of Communication Engineering,National Taiwan University, Taipei, Taiwan (e-mail: [email protected]).

W. Liao is with the Department of Electrical Engineering and the GraduateInstitute of Communication Engineering, National Taiwan University, Taipei,Taiwan, and with the Research Center for Information Technology Innova-tion, Academia Sinica, Taiwan (e-mail: [email protected]). W. Liao is thecorresponding author.

Digital Object Identifier 10.1109/TCOMM.2013.071013.120245

and 3GPP LTE) for multicast and broadcast service calls fortransmitting video in accordance with the nodes at the lowestrate. Accordingly, the nodes with the worst channel conditionin a group will govern the performance of wireless videomulticasting and affect the Quality of Experience (QoE) ofthe receivers.

For scalable video, each video stream is encoded at thehighest resolution and divided into layers such that eachreceiver can decode the stream at the preferred rate andresolution with a set of layers. The first layer, namely, the baselayer (BL), is the most significant layer. It contains the datawith the most important features of the video, thus ensuringthe basic quality of the video. The additional layers, namely,enhancement layers (EL), include the data that progressivelyrefine the reconstructed video program. There are many re-search efforts on scalable video multicasting over wirelessnetworks [1]–[9]. They all show that with proper designs,scalable video multicasting allows a group of multicast nodesunder different channel qualities to receive video programsof different video qualities. For example, in [9], the wirelessmulticast service for scalable video delivery is enabled viaan envelope-based adaptive resource allocation scheme overIEEE 802.16 networks.

Cooperative communications is an emerging technique toexploit spatial diversity so as to combat severe fading andpath loss in wireless environments. With cooperative commu-nications, nodes overhearing packets from other nodes mayserve as relays and help forward the overheard packets tothe target destination. Therefore, the effect of virtual an-tenna array (VAA) can be achieved [10]. As shown in [11],[12], employing cooperative communications indeed increasessystem capacity and enhances transmission reliability. Thetechnique of cooperative communications can also be usedfor video multicasting service in wireless networks. Typically,the operation in cooperative multicasting is performed in twophases. In the first phase, the base station (BS) broadcasts thevideo programs to all nodes within its transmission range. Inthe second phase, the relays (either chosen from the receivingnodes or established in advance) will help forward the video tothe nodes which fail to decode the video during the first phase.In [13], the authors analyze the performance and provideoptimal power allocation and relay location strategies forseveral wireless cooperative multicast schemes. The work in[14] is among the few on cooperative multicast for scalablevideo delivery over wireless networks. Recall that for amulticast scheme suited for scalable video delivery, nodesunder different channel qualities should receive the video withdifferent video qualities. The work in [14] achieves this goalwith a 3-phase solution for a 2-layer video delivery, i.e.,

0090-6778/13$31.00 c© 2013 IEEE

WANG and LIAO: COOPERATIVE MULTICASTING FOR WIRELESS SCALABLE VIDEO TRANSMISSIONS 3981

each video is encoded into two layers: one base layer andone enhancement layer. However, in that work, only the baselayer of the video streams is transmitted by the two-phasecooperative multicast. The enhancement layers of the videostreams are still delivered directly via traditional multicasting.As a result, it requires three phases to transmit a 2-layervideo and fails to take advantage of user cooperation for theenhancement layers of the video.

In this paper, we propose two new cooperative multicastschemes called OppCM and CodedCM for scalable videodelivery in wireless networks. We analyze the average outageprobabilities of four multicast schemes for wireless scalablevideo multicasting, namely, direct multicast (i.e., conventionalmulticast, which is defined in IEEE 802.16 and 3GPP LTE formulticast and broadcast service), pure cooperative multicastsuch as [14], and the two proposed OppCM and CodedCM.We also compare their performance via simulations. Theresults show that the proposed OppCM and CodedCM out-perform the other two schemes.

The rest of this paper is organized as follows. In Sec. II,we overview two existing cooperative multicast schemes andderive their average outage probabilities for wireless scalablevideo delivery. In Sec. III, we propose two new cooperativemulticast schemes, namely, OppCM and CodedCM, and thenderive their outage probabilities based on the same systemmodel described in Sec. II. In Sec. IV, the performance of thefour different schemes is evaluated via simulations. Finally,this paper is concluded in Sec. V.

II. EXISTING COOPERATIVE MULTICAST SCHEMES FOR

SCALABLE VIDEO DELIVERY

In this section, we analyze the performance of two existingwireless multicast schemes for scalable video delivery: 1) Di-rect Multicast1 , and 2) Pure Cooperative Multicast (PureCM)[14]. We first describe the system model and then compute theSNRs of BL and EL frames received by all nodes for the twomulticast schemes. Based on the SNRs, the average outageprobabilities for the two schemes can then be derived.

We consider an infrastructure-based wireless network witha circular cell of radius R, where the BS is located at thecenter of the cell and N nodes are uniformly independentlydistributed in the cell. In our model, all nodes are half-duplex.Node i (1 ≤ i ≤ N) is located at (ri, θi), where the radius ri(0 < ri ≤ R) is a random variable with probability densityfunction (pdf) f(ri) = 2πri/πR

2, and the polar angle θi, 0 ≤θi < 2π, is a random variable uniformly distributed within[0, 2π). Since ri and θi are independent of each other, thejoint p.d.f. of (ri, θi) is f(ri, θi) = ri/πR

2.In this system, the BS multicasts video frames to nodes

within its coverage. Each video flow is encoded by a videoencoder into two layers: BL and EL, and the bit ratio of BL/ELis 1. We assume that the channel between any two nodes hasindependent slow fading, propagation path loss, and additive

1This multicast scheme is presented solely for baseline comparison. ForDirect Multicast, there is no relay participation, i.e., no user cooperation. Thevideo programs being transmitted are not encoded into layers. This schemeincludes only one phase where BS broadcasts the video to all nodes withinits coverage.

white Gaussian noise. Slow fading here implies that the chan-nel gain over the transmission time of a single packet remainsconstant, which is reasonable for video communications [14].Nodes receiving frames with a packet error rate (PER) lessthan a sufficiently small threshold ρth are regarded to bedecodable via effective error concealment in the video encoder.The objective of a wireless scalable video multicasting schemeis to enable all nodes in the coverage range of the BS to decodethe BL frames correctly while maximizing the number ofnodes who can decode EL frames. Without loss of generality,we do not consider channel coding. Most existing codingmethods can be easily adapted to our proposed and to otherexisting multicast schemes. For fair comparison, we fix thechannel code rate for all multicast schemes we discuss inthis paper, and apply a suitable modulation such that thetransmission rates of BL frames (specifically, one BL bit persymbol per transmission is considered) for all schemes arethe same. This implies that we use the same transmission rateto multicast the most significant content of the video. Givena time period, the amount of transmitted BL frames are thesame for all multicast schemes. Note that we do not restrict thetransmission rate of EL frames. With our designs, the proposedmulticast schemes aggressively transmit both BL frames andEL frames at the same time to achieve better performance.

We assume that all channels are Gaussian. The outageprobability is defined as the probability that the set of channelrealizations cannot support reliable transmission at a rate ofRth. In other words, the outage probability is the probabilitythat the maximum mutual information I between the sourceand the destination is less than Rth, i.e., Pr(I < Rth), whereI = log2 (1 + γ) and γ is the received SNR. Given that thecoding length is sufficiently large, a lower bound of PER canbe obtained by the outage probability. Due to slow fading, theaverage outage probability can be viewed as the PER. Alsoit can be regarded as the ratio of the number of nodes thatcannot receive a frame out of all nodes in the system.

A. Direct Multicast

In this scheme, only BL frames are multicast (via one-hoptransmission) to all nodes in the group. The BS broadcasts asignal x with unit power using BPSK. From [15], the receivedsignal at node i located at (ri, θi) is then expressed by

yi =

√Pr−β

i hix+ ni, (1)

where P is the transmit power used by the BS, β is the pathloss exponent, hi is the channel gain between the BS andnode i modeled as a circularly symmetric complex Gaussianrandom variable with zero-mean and unit variance, and ni isadditive white Gaussian noise with zero-mean and varianceN0. Hence, the instantaneous received SNR γi at node i canbe expressed by

γi =Pr−β

i |hi|2N0

, (2)

which is an exponential random variable with λi =N0/(Pr−β

i ). Since only the BL frames are transmitted, theinstantaneous received SNR values of BL and EL frames areγb,i = γi, and γe,i = 0, respectively.


We next derive the outage probability for Direct Multicast.As in [10], the outage probability for node i located at ri canbe written as

Pi = Pr[Ii < Rth|ri] = 1− exp

{− (2Rth − 1)N0

Pr−βi

}, (3)

where Ii = log2(1 + γi).Therefore, the average outage probability can be written as

P =

∫ R

0

pif(ri)dri

=

∫ R

0

(1− exp

{− (2Rth − 1)N0

Pr−βi

})2riR2

dri,

(4)

and the average outage probability of BL frames as

Pb = P. (5)

B. Pure Cooperative Multicast

This scheme is a combination of cooperative communica-tions and direct multicast. The transmission includes threephases. The BL frames are transmitted in the first two phasesthrough cooperative communications, followed by the thirdphase in which the EL frames are transmitted through directmulticast.

Phase 1: The BS broadcasts BL frames with BPSK modu-lation. The instantaneous received SNR at node i in Phase 1can thus be expressed by

γp1,i =

P pBS1r

−βi |hi|2N0

, (6)

where the superscript p means PureCM, and P pBS1is the

transmit power used by the BS in Phase 1.After Phase 1, the nodes receiving the BL frames form a

set G.Phase 2: The nodes in set G are all selected to serve

as relays applying a decode-and-forward (DF) cooperativeprotocol. The received SNR at node i located at (ri, θi) inPhase 2 can be written as

γp2,i =

P pG

N0

∣∣∣∣∣∣∑j∈G

√r−βji hji

∣∣∣∣∣∣2

, i /∈ G, (7)

γp2,i = 0, i ∈ G, (8)

where P pG is the transmit power used by a single relay, rij

denotes the distance between nodes i and j obtained by r2ij =(ri cos θi − rj cos θj)

2 +(ri sin θi − rj sin θj)2, and hij is the

channel gain between nodes i and j modeled as a circularlysymmetric complex Gaussian random variable.

Phase 3: The BS broadcasts EL frames using BPSK mod-ulation. The instantaneous received SNR at node i in Phase 3is expressed by

γp3,i =

P pBS3r

−βi |hi|2N0

, (9)

where P pBS3 is the transmit power used by the BS in Phase 3.

Note that for fair comparison, the total transmit power used inthe three phases for PureCM should be less than or equal to

the power used in Direct Multicast [15], i.e., P pBS1+Np

GPpG+

P pBS3 ≤ 3P , where Np

G is the number of relays. The workin [13] shows that allocating about half of the total transmitpower to the BS in a two-phase cooperative multicast gives thebest performance. For simplicity, we only consider P p

BS1 =P , P p

G = P/E[NpG] , and P p

BS3 = P , where E[NpG] is the

expected value of the number of relays and this value can becomputed in advance.

Finally, for BL frames, the nodes not in set G combineboth signals received in Phase 1 (from the BS) and Phase 2(from the relays) by using a maximal ratio combining (MRC)technique to achieve the maximized SNR. The resulting signalat the output of MRC has a maximized SNR equal to the sumof the SNRs corresponding to each phase at node i (i /∈ G).Therefore, the SNR of the BL frames is γp

b,i = γp1,i+γp

2,i, andthe SNR of the EL frames is γp

e,i = γp3,i.

Based on the above, the outage probabilities of BL and ELframes for node i located at (ri, θi) are

P pb,i = Pr[Ipb,i < Rth|(ri, θi)], (10)

P pe,i = Pr[Ipe,i < Rth|ri], (11)

where Ipb,i =13 log2 (1+γp

b,i), and Ipe,i =13 log2(1+γp

e,i). Thenormalization factor is 1/3 due to the fact that PureCM needsthree phases to complete a transmission [13]. From (10) and(11), we can further derive the average outage probabilities ofBL and EL frames, i.e., P p

b and P pe , respectively.

III. TWO NEW COOPERATIVE MULTICAST SCHEMES FOR

SCALABLE VIDEO DELIVERY

In this section, we propose two new cooperative multicas-ting schemes for wireless scalable video delivery: 1) coop-erative multicast with opportunistic listening and conditionaldemodulating (denoted Opportunistic Cooperative Multicast,or OppCM for short), and 2) cooperative multicast with multi-resolution modulation (denoted Coded Cooperative Multicast,or CodedCM for short). The proposed schemes are describedfor 2-layered video delivery, in which each video is encodedinto two layers2. BL and EL, as in [14]. In what follows, wefirst describe the operation of the two proposed schemes. Wethen derive their SNR values based on the same system modelused in Sec. II, and calculate their average outage probabilitiesaccording to the derived SNR values.

A. Opportunistic Cooperative Multicast (OppCM)

In OppCM, the BL frames are transmitted via a 2-phasecooperative multicasting similar to PureCM. The main differ-ence between OppCM and PureCM is that OppCM takes onlytwo phases to transmit a 2-layer video. Therefore, instead oftransmitting the EL frames in an additional phase (i.e., the

2In this paper, for ease of explanation, we assume that the video is encodedinto only two layers for both OppCM and CodedCM. In practice, a videomight be encoded into more than two layers. To adapt a multi-layer videotransmission to OppCM and CodedCM illustrated in this paper, the quicksolution is categorizing all the video layers into two levels. Map the highlevel layers to the BL, and map the low level ones to the EL and then followthe procedure in the paper. However, apart from this solution, with a specialdesign we can still find a suitable modulation method which adapts to differentcircumstances while using the same concept and similar strategy as used inthis paper.


Fig. 1. The purpose-made QPSK constellation for OppCM.

third phase) as in PureCM, OppCM makes the nodes withbetter channel qualities to receive the EL frames during thesetwo phases opportunistically. The design goal of OppCM isto increase the number of nodes which receive the BL framesand the nodes which receive the EL frames. In the following,we use a QPSK-BPSK case as an example to demonstratethe procedures of OppCM. Other techniques such as 8PSK-QPSK and 16QAM-QPSK are also applicable, and may beused to transmit a video stream encoded into higher layerswith different BL/EL bit ratios.

1) Opportunistic Listening:”Opportunistic listening” is commonly regarded as overhear-ing and preserving an unintended packet (i.e., a packet notdestined to the listener) by a node for future use. However,in OppCM, opportunistic listening is performed to preserveintended but undecodable signals. In the first phase, the BSbroadcasts both BL and EL frames through a purpose-madeQPSK constellation as shown in Fig. 1. As can be seen, thispurpose-made QPSK constellation is not gray-coded. Thus, b1is for BL bits and b2 is for EL bits. After the transmission, thenodes with better channel qualities (i.e., able to demodulateQPSK symbols) might be able to decode both BL and ELbits, and thus are selected to serve as relays. Typically, thenodes who fail to demodulate the QPSK symbols will ignorethe undecodable signals. However, although these nodes can-not demodulate a QPSK symbol, some of them may havethe ability to demodulate a BPSK symbol, which impliesthat they at least have partial (i.e., half) knowledge of thereceived QPSK symbol. Hence, OppCM allows such nodes(which are called potential nodes) to perform opportunisticlistening, which preserves the undecodable signals for furtherdemodulating (called conditional demodulating).

2) Conditional Demodulating:In the second phase, the relays selected in the first phase helpforward the BL frames through BPSK modulation. For thosenodes which perform opportunistic listening (i.e., the potentialnodes or the nodes capable of demodulating a BPSK symbol)in the first phase, if they decode the BL bits successfully inthe second phase, they can utilize this information to furtherdemodulate the preserved signals from the first phase in orderto decode the EL bits.

Fig. 2. Illustration for conditional demodulation. The purpose-made QPSKconstellation can be transformed into a BPSK constellation.

Fig. 2 illustrates the procedure of conditional demodulating.Fig. 1 gives the original purpose-made QPSK constellationfor the Phase 1 transmission. In the second phase, if theBL bit successfully decoded by the potential nodes is 0,i.e., b1 = 0, the possible composition of the undecodableQPSK symbol must be either 00 or 01. By removing 10and 11 from the original QPSK constellation, the purpose-made QPSK constellation can be transformed into a BPSKconstellation including only 00 and 01. Recall that a potentialnode corresponds to the node with the ability to demodulatea BPSK symbol. It therefore can decode the EL bit (b2)according to the decision boundary depicted in Fig. 2. As forthe case that the BL bit successfully decoded by the potentialnodes is 1, the purpose-made QPSK constellation will betransformed into a BPSK constellation with 10 and 11, whichcan also be demodulated by the potential nodes.

B. Coded Cooperative Multicast (CodedCM)

Although PureCM and OppCM have different strategies fortransmitting EL frames, we can observe that both schemesallocate certain system resources to transmit the EL framesregardless of the transmission performance of the BL frames.However, for scalable video delivery, the most importantrequirement is that all subscribers in this system are en-sured to receive the basic video (e.g., BL frames). Therefore,the multicast schemes could allocate system resources fortransmitting EL frames only if all nodes can decode the BLframes successfully. Furthermore, it is proved that cooperativemulticast outperforms direct multicast. Thus, to achieve betterperformance, the BL frames and EL frames should bothbe transmitted by cooperative multicast. Based on the twoconsiderations above, we propose a new two-phase cooperativemulticast scheme called CodedCM, in which the scalablevideo is encoded by a multi-resolution modulation schemewhere the symbol is composed of both BL and EL bits.Below we briefly overview the concept of multi-resolutionmodulation, which plays an important role in CodedCM.

Multi-resolution modulation [16]–[20] is a technique thatcan be used to carry a symbol which comprises bits ofdifferent importance (level), e.g., BL bits and EL bits, forvideo programs. The basic idea of this modulation techniqueis to let the receivers under better channel conditions decodemore bits to achieve better quality while those under worsechannel conditions at least decode the most important bits to


Fig. 3. Nonuniform signal constellation with an offset angle α.

have basic quality. There are many types of multi-resolutionmodulation schemes, such as nonuniform QPSK, nonuniformQAM, and superposition modulation. Fig. 3 illustrates two ex-amples of gray-coded nonuniform PSK constellation, namely,nonuniform QPSK and nonuniform 8PSK. The offset angle αcan be used to adjust the Euclidean distance such that the bitsof different importance may have unequal error probabilities.

The multi-resolution modulation described below is basedon nonuniform QPSK modulation. Note that in this work, ourdescription is based on nonuniform PSK although other multi-resolution modulation techniques may be used. The advantageof PSK is that the receivers do not need to demodulate thereceived signal amplitude (as which might be difficult to obtainin mobile wireless networks). The design goal of CodedCM isto find a suitable nonuniform QPSK constellation that ensuresall nodes within the coverage receive the BL frames, whilemaximizing the number of nodes receiving the EL frames.We allocate system resource (i.e., the transmit power) for BLand EL frames by adjusting the offset angle. The smaller thedegree of the offset angle, the higher the probability a nodecan demodulate correctly for BL bits. Therefore, given thetotal transmit power and a packet error rate (PER) thresholdρth, we can find the optimal offset angle α that minimizesthe average outage probability of the EL frames under theconstraint that the average outage probability of the BL framesshould be less than ρth.

CodedCM comprises two phases and both BL and ELframes are transmitted. In Phase 1, the BS multicasts bothBL and EL frames to all N nodes via a nonuniform QPSKmodulation with an offset angle α(0 ≤ α ≤ π/4). After thefirst phase, all nodes in the coverage are divided into threegroups, i.e.,G1,G2,G3. The set G1 includes NG1 nodes whichreceive both the BL and EL frames. The set G2 includesNG2 nodes which only receive the BL frames, and the setG3 contains NG3 nodes which fail to receive frames of bothlayers. N = NG1+NG2+NG3 . In Phase 2, node cooperationcomes into play to improve system performance. The nodes insets G1 and G2 are all selected to serve as relays which helpforward the video frames simultaneously to nodes in set G3.The nodes in set G1 apply the DF protocol, and then multicastboth the BL and EL frames (for signal x) through nonuniformQPSK with angle α. On the other hand, the nodes in set G2

only multicast the BL frames (signal xb) using nonuniformQPSK with angle 0 (i.e., BPSK) because their received SNRsof the EL frames are too low to decode.

C. Average Outage Probabilities of OppCM and CodedCM

1) Opportunistic Cooperative Multicast : This schemecomprises two phases. The BL frames and EL frames are bothtransmitted in the first phase, while only the BL frames aretransmitted in the second phase through user cooperation.

Phase 1: The BS broadcasts both BL and EL framestogether via a purpose-made QPSK modulation as shown inFig. 1. The instantaneous received SNR at node i in Phase 1is

γo1,i =

P oBSr

−βi |hi|2N0

, (12)

where the superscript o means OppCM, and P oBS is the trans-

mit power used by the BS in Phase 1. Note that transmittingvia QPSK can be regarded as transmitting a pair of orthogonalBPSK signals simultaneously. Therefore, the total SNR can bedivided into two parts, one half for BL frames and the otherhalf for EL frames. The instantaneous received SNR valuesof BL and EL frames in Phase 1 are

γob1,i =

P oBSr

−βi |hi|22N0

, (13)

γoe1,i =

P oBSr

−βi |hi|22N0

. (14)

After Phase 1, all nodes in the coverage can be divided intothree groups, i.e., G1, G2, and G3. The set G1 includes thenodes which are able to demodulate QPSK symbols, i.e., thenodes which receive both BL frames and EL frames. The setG2 includes the potential nodes which are unable to demodu-late QPSK symbols but can decide BPSK symbols. The rest ofnodes are included in G3. The G2 nodes perform opportunisticlistening to preserve the undecodable signals received fromPhase 1. Through opportunistic listening, although the nodesin G2 and G3 all fail to receive BL and EL frames in Phase1, the G2 nodes (the potential nodes) have the chance tosuccessfully demodulate the preserved signals after Phase 2.

Phase 2: The nodes in set G1 are all selected to serveas relays applying a decode-and-forward (DF) cooperativeprotocol. The relays transmit the BL frames via BPSK. Sinceonly the BL frames are transmitted, the total received SNR isall from BL frames. The instantaneous received SNR of theBL frames at node i located at (ri, θi) in Phase 2 can thereforebe written as

γob2,i = γo

2,i =P oG1

N0

∣∣∣∣∣∣∑j∈G1

√r−βji hji

∣∣∣∣∣∣2

, i /∈ G1, (15)

γob2,i = 0, i ∈ G1, (16)

where P oG1

is the transmit power used by a single relay. Otherparameters are the same as in (7).

For fair comparison, the total transmit power used in Phase1 and Phase 2 for OppCM should be less than or equal to thepower used in Direct Multicast, i.e., P o

BS + NoG1

P oG1

≤ 2P .We set P o

BS = P , and P oG1

= P/E[NoG1

] , where E[NoG1

]is the expected values of the number of relays, which can becomputed in advance.

After the second phase, the nodes not in G1 (i.e., i /∈ G1)can be further divided into two sets, say H1 and H2. Thenodes which receive the BL frames from relays in Phase 2


form set H1, the other nodes (which fail to receive the BLframes) are included into H2. The H1 nodes then performconditional demodulating to further demodulate the preservedsignal from Phase 1 for obtaining EL frames.

Finally, it can be observed that during the entire process,a node can receive the BL frames either from the BS duringPhase 1 or from the relays during Phase 2, while a node canreceive the EL frames only if it receives the EL frames inPhase 1 or it is remarked as a potential node in Phase 1 andalso receives the BL frames in Phase 2. The receiving statusof a node is summarized as follows:

Node i receives BL frames successfully,

if{i ∈ G1} ∪ {i ∈ H1},(17)

Node i receives EL frames successfully,

if{i ∈ G1} ∪ {i ∈ G2 ∩H1}.(18)

Based on the received SNR values derived above, we cancalculate the average outage probability for OppCM. We firstderive the outage probability of BL frames. There are onlytwo ways for nodes to receive BL frames. One way is toreceive these from the BS during Phase 1, as G1 nodes do.The other way is from the relays during Phase 2 as H1 nodesdo. Therefore, the outage probability of the BL frames fornode i located at (ri, θi) can be written as

P ob,i = 1− Pr[{i ∈ G1} ∪ {i ∈ H1}|(ri, θi)]

= 1− Pr[{Iob1,i ≥ Rth} ∪ {Iob2,i ≥ Rth}|(ri, θi)],(19)

where Iob1,i =12 log2(1 + γo

b1,i), and Iob2,i =12 log2(1 + γo

b2,i).The normalization factor is 1/2 due to the fact that OppCMneeds two phases to complete a transmission.

As for the EL frames, except receiving them in Phase 1as for G1 nodes, a node can receive EL frames if 1) it isremarked as a potential node in Phase 1 (i.e., included in G2)and 2) it receives the BL frames in Phase 2 (i.e., included inH1). Therefore, the outage probability of the EL frames fornode i is expressed by

P oe,i = 1− Pr[{i ∈ G1} ∪ {{i ∈ G2} ∩ {i ∈ H1}} |ri]

= 1− Pr[{Ioe1,i ≥ Rth}∪{{Io1,i ≥ Rth} ∩ {Iob2,i ≥ Rth}}|ri],

(20)where Ioe1,i =

12 log2(1 + γo

e1,i), and Io1,i =12 log2(1 + γo

1,i).From (19) and (20), we can further derive the average outageprobabilities of BL and EL frames, i.e., P o

b and P oe , respec-

tively.2) Coded Cooperative Multicast:

This scheme comprises two phases and both BL and ELframes are transmitted. The multi-resolution modulation de-scribed below is based on nonuniform QPSK modulation.Note that in this work, we focus only on nonuniform PSKalthough other multi-resolution modulation techniques are alsoapplicable. The advantage of PSK is that the receivers neednot demodulate the received signal amplitude, which might bedifficult to obtain in mobile wireless networks.

1) Phase 1: The BS broadcasts both BL and EL framestogether through a nonuniform QPSK modulation with an

offset angle α(0 ≤ α ≤ π/4). The received signal at nodei located at (ri, θi) in Phase 1 is

yc1,i =

√PBSr

−βi hix+ ni, (21)

where c means CodedCM, PBS is the transmit power usedby the BS, and other parameters are the same as in (1). Theinstantaneous received SNR γc

1,i at node i in Phase 1 thus canbe expressed by:

γc1,i =

PBSr−βi |hi|2N0

. (22)

For multi-resolution modulation, the bits carried by thesame symbol might even experience different error probabil-ities which are highly dependent on the symbol power andthe applied modulation scheme. To better reflect this effect,we define effective power, denoted by Peff . Transmitting a bitcarried by a symbol with a symbol power via a certain modu-lation can be viewed as the bit being individually transmittedwith Peff via BPSK modulation such that the error probabilityof the bit resulted from the above two cases are the same.Therefore, through nonuniform QPSK depicted in Fig. 3, theeffective powers of BL and EL bit are Pb1,eff = PBS cos2 α,and Pe1,eff = PBS sin2 α, respectively, in Phase 1. Conse-quently, we can further obtain the instantaneous received SNRvalues of the BL and EL frames in Phase 1, respectively, as

γcb1,i = Pb1,eff r−β

i |hi|2/N0 = γc1,i cos

2 α, (23)

γce1,i = Pe1,eff r−β

i |hi|2/N0 = γc1,i sin

2 α. (24)

After the first phase, all nodes in the coverage are dividedinto three groups, i.e., G1,G2,G3. The set G1 includes NG1

nodes which receive both the BL and EL frames. The set G2

includes NG2 nodes which only receive the BL frames, andthe set G3 contains NG3 nodes which fail to receive framesof both layers. N = NG1 +NG2 +NG3 .

3) Phase 2: In the second phase, node cooperation comesinto play to improve system performance. The nodes in sets G1

and G2 are all selected to serve as relays which help forwardthe video frames simultaneously to nodes in set G3. The nodesin set G1 apply the DF protocol, and then multicast both theBL and EL frames (for signal x) through nonuniform QPSKwith angle α. On the other hand, the nodes in set G2 onlymulticast the BL frames (signal xb) using nonuniform QPSKwith angle 0 (i.e., BPSK) because their received SNRs of theEL frames are too low to decode. The received signal at nodei (i ∈ G3) located at (ri, θi) in Phase 2 can be written as

yc2,i =∑j∈G1

√PG1r

−βji hjix+

∑k∈G2

√PG2r

−βki hkixb + ni,

(25)where PG1 and PG2 denote the transmit power used by nodesin G1 and G2, respectively. All the channel gains {hji, hki}between nodes j, k and i are assumed to be mutually inde-pendent, and other parameters are the same as in (7).

For fair comparison, the transmit power used by the BSin Phase 1 plus the total power consumed by all relaysin sets G1 and G2 in Phase 2 should be less than orequal to the power used in conventional direct multicast,i.e., PBS + NG1PG1 + NG2PG2 ≤ 2P . The work in [13]


shows that allocating about half of the total transmit power tothe BS in a two-phase cooperative multicast gives the bestperformance. For simplicity, we consider PBS = P , andPG1 = PG2 = P/(E[NG1 ] + E[NG2 ]), where E[NG1 ] andE[NG2 ] are the expected values of NG1 and NG2 , respectively,which can be computed in advance.

The instantaneous received SNRs of the BL and EL framesat node i in Phase 2 can be expressed as

γcb2,i = γc

e2,i = 0, i ∈ {G1, G2}, (26)

γcb2,i =

1

N0

∣∣∣∣∣∣∑j∈G1

√Pb2,eff r−β

ji hji +∑k∈G2

√PG2r

−βki hki

∣∣∣∣∣∣2

, i ∈ G3,(27)

γce2,i =

1

N0

∣∣∣∣∣∣∑j∈G1

√Pe2,eff r−β

ji hji

∣∣∣∣∣∣2

, i ∈ G3, (28)

where Pb2,eff = PG1 cos2 α and Pe2,eff = PG1 sin

2 α denotethe effective powers of BL and EL bit, respectively, throughnonuniform QPSK in Phase 2.

Finally, the nodes in set G3 combine both signals receivedin the first phase and the second phase by an MRC technique.Thus, we have,

γcb,i = γc

b1,i + γcb2,i, (29)

γce,i = γc

e1,i + γce2,i. (30)

The outage probabilities of the BL and EL frames for nodei located at (ri, θi) are expressed by

P cb,i = Pr[Icb,i < Rth|{(ri, θi), α}], (31)

P ce,i = Pr[Ice,i < Rth|{(ri, θi), α}], (32)

where Icb,i = 12 log2(1 + γc

b,i), and Ice,i = 12 log2(1 + γc

e,i).The normalization factor 1/2 is due to the fact that the codedcooperative multicast needs a two-hop transmission.

Again, based on (31) and (32) and following the similar stepas in Direct Multicast, we can derive the average outage prob-abilities of BL and EL frames, i.e., P c

b and P ce , respectively,

for CodedCM.

IV. PERFORMANCE EVALUATION

In this section, we conduct simulations to compare theperformance of different multicast schemes for scalable videoover wireless networks. In the simulations, the BS is locatedat the center of a cell with radius R = 100m. There areN = 100 nodes with coordinates (ri, θi) generated inde-pendently according to the joint probability density functionf(ri, θi) = ri/πR

2. The channel gains are generated indepen-dently following the complex Gaussian distribution CN (0, 1),and the propagation path loss parameter is set to β = 2.6,which is suitable for an urban area. To compute the outageprobability, we set Rth = 1bps/Hz. To find the optimal offsetangle α for CodedCM so as to ensure all nodes within thecoverage receive the BL frames, we set ρth = 0.5% [11],which means compressed data with a PER less than ρth isdecodable. Each simulation data value under a specific settingis the mean value based on over 106 simulation runs.

Fig. 4. The receiving status of nodes after Phase 1 in OppCM.

A. Opportunistic Cooperative Multicast

Fig. 4 shows the receiving status after Phase 1 in OppCM.As can be seen, the ratio of G1 nodes (which are able todemodulate QPSK symbols and thus decode both the BL andEL frames correctly) increases with the increase in P/N0 .The G2 nodes (which are unable to demodulate QPSK butcan decide BPSK symbols) are the potential nodes regardedas having half of the information of the undecodable signalsreceived from Phase 1. The G2 nodes perform opportunisticlistening. Fig. 5 shows the receiving status after Phase 2 inOppCM so as to demonstrate how opportunistic listening andconditional demodulating work jointly to improve the systemperformance. In Phase 2, G1 nodes serve as relays to helpforward the BL frames cooperatively. We observe that viaconditional demodulating, there is a high percentage of nodesdecoding the preserved signals for EL frames successfully(i.e., G2∩H1 nodes) among those potential nodes (G2 nodes)after Phase 2. As for G3 nodes, due to user cooperation,there is also a high percentage of nodes receiving the BLframes (i.e., G3 ∩ H1 nodes) in Phase 2. Fig. 6 shows thefinal receiving status in OppCM. Almost all nodes within thecoverage range receive the BL frames when the value of P/N0

is sufficiently high. Besides, the ratio of nodes receiving bothBL and EL frames increases with the increase in P/N0.

B. Coded Cooperative Multicast

CodedCM can aggressively allocate the resource to transmitBL and EL frames of the video to meet the requirement thatusers in the system should all receive the BL frames. Thisimplies all resources should be used to transmit the BL framesfirst, and the EL frames are transmitted only if all nodes cansurely receive the BL frames. We first examine the result ofapplying nonuniform QPSK modulation in CodedCM. Fig. 7shows the average outage probability for different degrees ofoffset angle α when P/N0 = 62.5dB. As can be seen, theaverage outage probability of the BL frames increases with theincrease of α because larger α results in lower effective powerof the BL frames. Moreover, given a value of P/N0 , theoptimal α can be obtained when the average outage probability


Fig. 5. The receiving status of nodes after Phase 2 in OppCM.

Fig. 6. The final receiving status after two phases in OppCM.

of the BL frames reaches the PER threshold (= 0.5%). In Fig.8, the optimal offset α is plotted for different power-to-noiseratios in CodedCM. When P/N0 < 60dB, the optimal α is0◦, which means at this time the average outage probabilityof the BL frames is still higher than the PER threshold. Underthis circumstance, all resource, i.e., transmit power, should beallocated to transmit BL frames. With an increase in P/N0

from about 60dB to 63dB, the optimal angle increases from0◦ to 45◦. When PN0 > 63dB, the optimal α is 45◦ becauseat this time the average outage probabilities of the BL and ELframes are both lower than the PER threshold even thoughα has reached the maximum value of 45◦. Note that in thiscase, the system can afford at least a double rate transmissionfor BL frames under the given PER threshold. Therefore,there must be other multi-resolution modulation schemes (e.g.,nonuniform 8PSK) which can be used to improve the bit rateas compared to the nonuniform QPSK.

C. Performance Comparison

The performance of different multicast schemes is comparedvia simulations. Fig. 9 plots the average outage probability

Fig. 7. The average outage probability with different offset angle α inCodedCM via nonuniform QPSK with P/N0 = 62.5dB.

of each scheme (i.e., Direct Multicast, PureCM, OppCM,and CodedCM) with different SNRs. We first observe theadvantage of cooperative communications. In Fig. 9, allcooperative multicast schemes (including PureCM, OppCM,and CodedCM) outperform Direct Multicast even though theformer three carry both BL and EL frames while the latteronly carries BL frames. It can also be observed that OppCMoutperforms PureCM in terms of both BL frames and ELframes. The average outage probabilities of BL and EL framesfor OppCM are both lower than those for PureCM in all cases.

For CodedCM, the average outage probability of BL framesis significantly reduced to the PER threshold (which meansat this time all nodes are ensured to receive the BL frames)at lower P/N0 compared with other cooperative multicastschemes. It is thanks to the proper resource allocation throughmulti-resolution modulation with the optimal offset angle.Moreover, CodedCM outperforms PureCM in the region withsufficiently high SNR values. For example, with P/N0 =62.5dB, CodedCM ensures the average outage probability ofthe BL frames is equal to the PER threshold, while reducingthe average outage probability of the EL frames to about0.046, which are both lower than the average outage probabil-ities of the BL and EL frames in PureCM. Furthermore, sinceCodedCM transmits the EL frames via cooperative multicast,the average outage probability of EL frames in CodedCM isthe lowest among all multicast schemes at sufficiently highP/N0 values.

V. CONCLUSIONS

In this paper, we explore cooperative multicasting forscalable video in wireless networks. Specifically, we proposetwo new cooperative multicast schemes called OppCM andCodedCM for scalable video delivery. We analyze the averageoutage probabilities of these two and the other multicastschemes. We then evaluate and compare their performancewith direct multicast and PureCM via simulations. The resultsshow that OppCM outperforms PureCM in the transmissionsof both BL and EL frames. Moreover, through nonuniformQPSK with the optimal offset angle α, CodedCM ensures


Fig. 8. The optimal offset angle for CodedCM via nonuniform QPSK.

Fig. 9. Performance comparison of different multicast schemes.

that the PER of BL frames is equal to or less than a giventhreshold while reducing the average outage probability of ELframes. In summary, the proposed OppCM and CodedCM bothoutperform the other two schemes. Therefore, they are indeedpromising solutions to scalable video multicasting in wirelessnetworks.

In this work, in order to convey our idea more easily, weassume that the transmitted video is encoded into only one BLand one EL. For a higher-layer video stream, we should followthe main idea but redesign the operations when applyingOppCM. As for CodedCM, a higher-resolution modulationscheme might be applied (In this paper, we use a nonuni-form QPSK which is a two-resolution modulation scheme).Furthermore, in this paper we also assume the number ofBL bits equals the number of EL bits for both OppCM andCodedCM. In practice, the BL/EL bit ratio might not be 1and it might vary with time. However, with a special design,we can still find a suitable modulation method to adapt todifferent circumstances while using the same concept and

similar strategy. For example, if the ratio of BL and EL bitnumber becomes 2, for OppCM, BS can use a purpose-made8PSK modulation scheme (i.e., 2 bits for BL, 1 bit for EL)to send video bit streams in the first phase. In the secondphase, relay nodes might use QPSK (i.e., 2 bits for eachBL) to transmit BL frames. In this strategy, opportunisticlistening and conditional demodulating are also performed andthe operation procedures are similar to the one described inthe paper. As for CodedCM, if the layer ratio is also assumedto be 2, BS can multicast the video streams based on a non-uniform 8PSK in which a BL bit and an EL bit might havedifferent effective powers. Following a similar procedure asthe example demonstrated for CodedCM in the paper, we canthen find the best way to allocate the effective power to BLbits and EL bits, and that optimal constellation will be theoptimal solution when applying CodedCM.

In the future, we will further study the QoE metrics otherthan outage probability for wireless cooperative multicastingof scalable video streams. We will also take into accountthe characteristics of higher-layer video streams and variousBL/EL bit ratios in our design.

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Sheng-Chieh Wang Sheng-Chieh Wang was bornin Taipei, Taiwan. He received his BS degree inElectrical Engineering and MS degree in Commu-nication Engineering both from National TaiwanUniversity (NTU), Taipei, Taiwan in 2009 and 2011,respectively. His research interests include wirelessnetworks and 3GPP Long Term Evolution (LTE)protocols. He has been working in MediaTek Inc.,Taiwan since 2011.

Wanjiun Liao Wanjiun Liao received her Ph.D.degree in Electrical Engineering from the Universityof Southern California, USA, in 1997. She is aDistinguished Professor of Electrical Engineering,National Taiwan University (NTU), Taipei, Taiwan,and an Adjunct Research Fellow of Research Centerfor Information Technology Innovation, AcademiaSinica, Taiwan. Her research focuses on the designand analysis of wireless multimedia networking,cloud datacenter networking and green communica-tions.

Prof. Liao was on the editorial boards of IEEE TRANSACTIONS ON

WIRELESS COMMUNICATIONS and IEEE TRANSACTIONS ON MULTIME-DIA. She has also served on the organizing committees of many internationalconferences, including serving as Tutorial Co-Chair of IEEE INFOCOM 2004,TPC Area Chair of IEEE International Conferences on Multimedia and Expo(ICME) 2004, TPC Vice Chair of IEEE GLOBECOM 2005 Symposium onAutonomous Networks, TPC Co-Chair of IEEE GLOBECOM 2007 GeneralSymposium, TPC Co-Chair of IEEE VTC 2010 Spring and TPC Co-Chairof IEEE ICC 2010 Next Generation Networking and Internet Symposium.The papers she co-authored with her students won the Best Paper Awards ofIEEE ICME 2000 and IEEE GLOBECOM 2011, and IEEE ComSoc 2011Multimedia Communications Best Paper Award. She was a recipient of theRepublic of China (R.O.C.) Distinguished Women Medal in 2000 and waselected as a Distinguished Lecturer of IEEE Communications Society (2011-2012). She is a Fellow of IEEE.

cooperative multicasting for wireless scalable video transmissions

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