Beach, Mark A

Lu, Shani

Nix, Andrew R

Tian, Yue

The non-orthogonal multiple access (NOMA) is regarded as a promising multiple access technique for 5G networks. In this research, we consider the joint transmission (JT) NOMA for a Coordinated Multi-Point (CoMP) system, and propose anovel opportunistic NOMA (ONOMA) scheme. This aims to improve the efficiency of the successive interference cancellation (SIC) decoding in conventional NOMA. Besides, by comparing with the JT-NOMA in CoMP system, the data rate of the ONOMA is shown in the numerical results under the ideal and non-ideal scenarios

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                          Tian, Y., Lu, S., Nix, A. R., & Beach, M. A. (2016). A Novel OpportunisticNOMA in Downlink Coordinated Multi-Point Networks. In 2015 IEEE 82ndVehicular Technology Conference (VTC Fall 2015): Proceedings of ameeting held 6-9 September 2015, Boston, MA, USA. (pp. 1-5). Institute ofElectrical and Electronics Engineers (IEEE). DOI:10.1109/VTCFall.2015.7390804Peer reviewed versionLink to published version (if available):10.1109/VTCFall.2015.7390804Link to publication record in Explore Bristol ResearchPDF-documentThis is the author accepted manuscript (AAM). The final published version (version of record) is available onlinevia IEEE at http://ieeexplore.ieee.org/xpl/articleDetails.jsp?arnumber=7390804. Please refer to any applicableterms of use of the publisher.University of Bristol - Explore Bristol ResearchGeneral rightsThis document is made available in accordance with publisher policies. Please cite only the publishedversion using the reference above. Full terms of use are available:http://www.bristol.ac.uk/pure/about/ebr-terms.htmlA Novel Opportunistic NOMA in DownlinkCoordinated Multi-Point NetworksYue Tian, Shani Lu, Andy Nix, Mark BeachCommunication System and Network Group,University of BristolEmail: {yue.tian, shani.lu, andy.nix, m.a.beach}@bristol.ac.ukAbstract—The non-orthogonal multiple access (NOMA) is re-garded as a promising multiple access technique for 5G networks.In this research, we consider the joint transmission (JT) NOMAfor a Coordinated Multi-Point (CoMP) system, and propose anovel opportunistic NOMA (ONOMA) scheme. This aims toimprove the efficiency of the successive interference cancellation(SIC) decoding in conventional NOMA. Besides, by comparingwith the JT-NOMA in CoMP system, the data rate of theONOMA is shown in the numerical results under the ideal andnon-ideal scenarios.I. INTRODUCTIONCoordinated Multi-Point (CoMP) transmission is currentlyan area of intensive research, as this is considered as one ofthe key technology enhancements for LTE-A. However, as thenumber of cell-edge users increases, the spectral efficiency ofthe CoMP system degrades [1], [2]. Recently, a promising non-orthogonal multiple access (NOMA) scheme was proposed forCoMP in an attempt to address this problem [3].The NOMA technique has been recognized as a promisingmultiple access techniques for 5G networks because of itssuperior spectral efficiency [4]. By using NOMA, whichcombines the superposition coding (SC) at transmitters withthe successive interference cancellation (SIC) at receivers [5],[6], all the users in CoMP system are able to decode theirdesired signal even though they are sharing a same frequencychannel at same time. This approach can offer significantlyenhanced spectrum efficiency.The complexity of SIC in NOMA and the number of usersin CoMP system is always directly proportional. Thus, thesuccessive manner of SIC becomes complicated and intensefor a large numbers of users. In order to reduce the complexityof SIC decoding and meanwhile to improve the capacityof the CoMP system, we propose an opportunistic NOMA(ONOMA) scheme. According to our numerical results, itis obvious that when comparing the JT-NOMA with theONOMA, we can reduce the complexity of SIC at receivers,and also significantly enhance the capacity of the CoMPsystem in the high signal-to-interference (SIR) scenarios.II. NOMA IN COMPWe consider a downlink CoMP network which includesB single-antenna access points (APs) and K single-antennausers. Let hk,b denote the channel from the bth AP to the kthuser, and hk,b is the i.i.d circular symmetric complex Gaussianrandom variable, representing Rayleigh fading; sk and nkdenote the kth user’s desired message and white Gaussiannoise with variance σ2z respectively. We assume that: (a) allAPs have the perfect channel state information (CSI) of allusers (b) each user can estimate the channel perfectly via thereference signal (c) the power of sk is normalised.A. Joint Transmission NOMA (JT-NOMA) in CoMPFigure 1. JT-NOMA in CoMPFor the JT-NOMA in CoMP, the APs will jointly transmitthe signal to the target users (as shown in Fig.1); morespecifically, the bth AP (1 ≤ b ≤ B) in CoMP broadcasts∑Ki=1√aiPbsi to the K users , where Pb denotes the nor-malised transmit power at bth AP and ai denotes the powerallocation coefficient (PAC) to si. According to the JT-CoMP[1], we assume that all APs will allocate the same transmitpower to si, where aiP1 = . . . = aiPB = aiP , then theobservation at the kth user (1 ≤ k ≤ K) is given byrk =K∑i=1h˙k√aiPsi + nk (1)where h˙k is the equivalent channel coefficient to the kth userand h˙k =∑Bb=1 hk,b. Without loss of generality, we assumethat:∣∣∣h˙1∣∣∣2 ≤ · · · ≤ ∣∣∣h˙K∣∣∣2. According to the SC in NOMA,the PACs to the K users are sorted as: a1 ≥ · · · ≥ aK ;then each receiver employes the SIC technique and is ableto perfectly decode the messages of the users in the weakerchannels [5], [6]. For example, the messages from s1 to sk−1can be detected by the kth user in sequence: when the kthuser is detecting the si (i < k), the messages from si+1 tosK will be treated as noise at the kth user; before detectingthe si+1, the si will be removed from its observation.978-1-4799-8091-8/15/$31.00 ©2015 IEEEAs a result, the required successive times of SIC (I) for theK users are sorted as: I1 ≤ · · · ≤ IK . At user k, the rate ofdetecting sk is given byRk = log[1 +∣∣h˙k∣∣2ak∑Km=k+1∣∣h˙k∣∣2am + σ2P−1](2)The K users’ sum-rate for conventional JT-NOMA networkcan be expressed asRsum =K−1∑k=1{log(1 +∣∣h˙K∣∣2aKσ2P−1)(3)+ log(1 +∣∣h˙k∣∣2ak∑Km=k+1∣∣h˙k∣∣2am + σ2P−1)}(4)where the term in (3) is the data rate of the user in the weakestchannel.B. Opportunistic NOMA Strategy in CoMPFigure 2. Ideal case and non-ideal case of ONOMA in CoMP systemLet Pk,b denote the transmit power allocated to sk on thebth AP, where Pk,b = akP and Pmin ≤ Pk,b ≤ Pmax. Theopportunistic NOMA transmission cells (ONOMA cell) willbe generated via the following steps:• Step 1. The B APs separately broadcast the referencesignal sr to the K users, with transmit power Pmax.• Step 2. Via the B reference signals, each usergenerates the first reference power matrix (FRPM).The kth user’s FRPM is expressed as ηk ={Pmax |hk,1|2 , · · · , Pmax |hk,B |2}, where the maximumelement is denoted as ηmaxk .• Step 3. The B APs separately broadcast the referencesignals again, with transmit power Pmin.• Step 4. Each user generates the second reference powermatrix (SRPM). The kth user’s SRPM is expressedas ηˆk = {Pmin |hk,1|2 , · · · , Pmin |h1,B |2}, where theminimum element is denoted as ηˆmink .• Step 5. Each user implements Algorithm 1 to generateits APs selection set (APSS), and feeds it back to the BAPs. The APSS of the kth user, which is denoted as λk,indicates the indexes of the APs that are preferred by thekth user.• Step 6. Based on the APSS-feedbacks, the bth AP addsthe indexes of the users that select it in their APSSs, intoits user schedule set (USS), ωb.• Step 7. The CoMP system generates ONOMA cells basedon the USSs and inform the kth user with {ωb, b ∈ λk}.Let Ψk denote the interference from the inter-NOMA cells,which do not have the overlapping area with the kth ONOMAcell, then the observation at kth user is given byrk =∑b∈λkhk,b∑i∈ωb√aiPsi + Ψk + nk (5)where Ψk =∑Bj=1,j /∈λk hk,j∑n∈ωj√anPsn.Remark 1. The bth ONOMA cell includes the bth AP and theusers in ωb.Algorithm 1 Opportunistic APSS Generation AlgorithmInitialization.Input: ηk , ηˆk , λk = null and n = 1.if ηˆmink > ηmaxk (e.g. Pmin∣∣hk,q∣∣2 > Pmax∣∣hk,p∣∣2) ,add the index of ηˆmink to λk (e.g. λk = {q});elsefor n = 1 : K,compare ηk{n} with ηˆminkif ηk{n} > ηˆminkadd n to λkelseλk = λkend foroutput: λk1) Ideal Case: For the ideal case, each UE only selectsone AP in APSS, so that there is no overlapping area betweenthe different NOMA cells, as the example shown in Fig. 2.In such case, the bth AP only implements NOMA strategy tothe users in ωb, and the USSs of the 2 APs are expressed asω1 = {1, 3} and ω2 = {2, 4, 5}, respectively. The transmittedsignal from the 1st ONOMA cell may cause the interferenceto the 2nd ONOMA cell, however, as the maximum power ofthe interference signal is smaller than the minimum powerof the desired signal (e.g. Pmin∣∣h1,1∣∣2 > Pmax∣∣h1,2∣∣2 orPmin∣∣h4,2∣∣2 > Pmax∣∣h4,1∣∣2), the desired signal is decodableby using SIC [5], [6]. If am<ak where m ∈ ∪b∈λkωb, the rateof detecting sk at user k can be expressed as follows:Rk = log⎛⎜⎝1 +∣∣hk,b∣∣2ak∑m∈ωb∣∣hk,b∣∣2am + Ψk + σ2P−1⎞⎟⎠ (6)where Ψk =∑Bj=1,j =b∣∣hk,j∣∣2 ∑n∈ωjan.Let ρ denote the transmit SNR, where ρ = Pσ−2 and defineGb as the user with maximum PAC in ωb , then the sum-ratefor the K users is given byRsum =B∑b=1{log(1 + ρ∣∣hGb,b∣∣2aGb) (7)+∑k∈ωb,k =Gblog[1 +ak∑m∈ωb am + Ψk +(ρ∣∣hk,b∣∣2)−1]}(8)2) Non-ideal Case : For the non-ideal case, two or moreNOMA cells may have the overlapping area (as shown inFig.2) and that is because some users select multiple APs. Herewe define Ob as the NOMA cells which are overlapping withthe bth ONOMA cell. After the CoMP obtaining the APSS-feedbacks, the ONOMA will be implemented via the followingsteps:• Step 1. Each AP computes the size of λ.• Step 2. Let L(Ω) denote the number of elements in setΩ. If L(λ1) ≥ · · · ≥ L(λK), then the PACs to theK users will be sorted as a1 ≥ · · · ≥ aK . For the gusers whose APSSs are the same, their PACs will besorted based on the equivalent channel coefficients, e.g.let [˚h1, . . . , h˚g] denote the g users’ equivalent channels,where h˚g =∑b∈λg hg,b, if∣∣h˚1∣∣2 ≤ · · · ≤ ∣∣h˚g∣∣2, thena1 ≥ · · · ≥ ag .• Step 3. The CoMP system computes the USS, and findthe user with the largest APSS size in each USS.• Step 4. For the bth ONOMA cell, compare the size ofeach APSS in Ob NOMA cells with the largest size ofAPSS in the bth ONOMA cell (L¯b). If a user in Ob hasa larger APSS size than L¯b, the index of the user will belisted into a set ωˆb.• Step 4. The bth AP broadcasts∑i∈{ωb∪ωˆb}√aiPsi tothe users in bth ONOMA cell.• Step 5. Each user in CoMP detects its observation. Byusing SIC, the message with larger PAC will get higherdetecting priority.Assume that am<ak, then m ∈ ∪b∈λkωb but m /∈ ∪b∈λkωˆb. Asa result, the rate of detecting sk at user k can be expressed asfollows:Rk = log⎧⎨⎩1 +∣∣h˚k∣∣2ak∑b∈λk hk,b∑m∈ωbam + Ψk + σ2P−1⎫⎬⎭ (9)where Ψk =∑Bj=1,j /∈λk∣∣hk,j∣∣2 ∑n∈{ωj∪ωˆj}an. The sum-ratefor the K users is given byRsum =B∑b=1{log(1 + ρ∣∣h˚Gb ∣∣2aGb)+ (10)∑k∈ωb,k =Gblog(1 +∣∣h˚k∣∣2ak∑b∈λk∑m∈ωb∣∣hk,b∣∣2am + Ψk + ρ−1)}(11)where the term in (10) is the data rate of the user in the weakestchannel of bth ONOMA cell.III. MINIMUM SUCCESSIVE TOPOLOGY IN ONOMAIn Fig.3, we use the branch network to indicate the suc-cessive process of SIC in ONOMA strategy. Note that for thenon-ideal cases, the user-node which has multi-braches, suchas node 1 in Fig.3 , means it is in the overlapping area. Theset of nodes {1...Nb} indicates the users in the bth ONOMAcell, and their PACs are sorted as a1 > ...>aNb .It is obvious in the branch network that from AP-node b tothe user-node Nb, it need to pass through Nb−1 user nodes, thatmeans by using SIC process, the user Nb requires to detect themessages of user 1 to user Nb−1 in sequence before detectingits own message. The successive times of SIC at user Nb isequal to the number of the user-nodes from 1 to Nb; in otherwords, there is INb = Nb. Note that the distance betweenany two nodes does not represent the real channel distance(or channel strength), it only indicates the successive times ofSIC, e.g. the successive times of SIC at user 3 are 3.Figure 3. Successive Process of SICCorollary 2. For the ONOMA network which has B APs andK users, if the B ONOMA cells are symmetrical with eachother, the sum successive times of SIC of the B ONOMA cellswill be the minimum.Proof: we first consider a CoMP system which has Bsymmetrical ONOMA cells. No matter the ideal or non-idealONOMA scenarios, the total SIC successive times in the bthONOMA cell can be expressed as I(b)sum = N2b+Nb2 . As the BONOMA cells are symmetrical , thenI(1)sum = ... = I(B)sum =N 2 +N2(12)Isum = I(1)sum + ... + I(B)sum =(N + 1)NB2(13)Now move part of the users from one ONOMA cell to another,for example, move Q users from the 1st ONOMA cell to the2nd ONOMA cell, so that the two ONOMA cells are no longersymmetrical with each other. In such case,I¯(1)sum =(N −Q)2 +N −Q2(14)I¯(2)sum =(N + Q)2 +N + Q2(15)Based on (12) to (15), we can achieve thatI¯sum − Isum =[I¯(1)sum + I¯(2)sum]−[I(1)sum + I(2)sum]= Q2 > 0(16)In the same way, it is easy to proof that if any of theB ONOMA cells are non-symmetrical with others, the sumsuccessive times of SIC will be larger than that in the totallysymmetrical B ONOMA cells.IV. NUMERICAL RESULTSWe consider a system which includes 2 APs and 5 users( B = 2 and K = 5). Comparing with the JT-NOMA, thenumerical result in Fig.4 shows the minimum successive ofSIC (Imin) by using ONOMA with Kmax = 5. The “o” inFig.4 denotes the number of the users that in the overlappingarea of the two NOMA cells.Figure 4. Minimum successive times of SICAccording to (6) and (9), it is obvious that the transmitrate is related to the topology of the NOMA cells. Here, weconsider a non-ideal scenario that 1 user is in the overlappingarea, and other 4 users are symmetrically distributed in the2 NOMA cells, where ω1 = {1, 3, 5} and ω2 = {1, 2, 4},as shown in Fig. 5. The Rayleigh fading channel gain fromthe bth AP to the kth user is expressed as: hk,b = gγk,b,where g is the normalised Rayleigh fading channel gain. TheRayleigh channel parameter γk,b =[√1 + (dk,b)α]−1, wheredk,b denotes the distance from the bth AP to the kth user, andα is the path loss factor.Figure 5. Minimum successive topologyThe Fig.6 shows the tranmit rate of each user and thesum rate in the considered system. The fixed channel isconsidered in the numerical results, the Rayleigh channelparameters in the inter-cells are set as: {γ1,1, γ3,1, γ5,1} ={γ1,2, γ2,2, γ4,2} = {0.5, 0.6, 0.8}; the Rayleigh channelparameters in the intra-cell are set as: {γ2,1, γ4,1} ={γ3,2, γ5,2} = {0.1, 0.05}. Let ai = K−i+1μ , where μ is aparameter to ensure∑i=Ki=1 ak = 1, then {a1, a2, a3, a4, a5} ={ 115 , 215 , 315 , 415 , 515}. The results in Fig.7 show the comparisonof the data rate performance between JT-NOMA and ONOMAstrategy. The numerical results are considered under the infi-nite SIR scenarios. It is obvious in Fig.7 that the ONOMA canachieve much better sum-rate performance than the JT-NOMAwhen the inter-cell interference is small.Figure 6. Data rate performance of ONOMA (the topology is shown in Fig.5)Figure 7. Comparison of the data rate performance between JT-NOMA andONOMA under high SIR scenariosV. CONCLUSIONIn this paper, we introduce a novel opportunistic NOMAstrategy in multiuser CoMP system which aims to reducethe complexity of signal processing and improve the capacityof the system. The relationship between the topology of theONOMA cell and the complexity of SIC decoding processis analysed. Comparing with the conventional JT-NOMA inCoMP, the numerical results show that the proposed ONOMAcan reduce the complexity of SIC and improve the systemcapacity under the high SIR scenarios.REFERENCES[1] M. Sawahashi, Y.Kishiyama, A. Morimoto, D. Nishikawa, M. Tanno, "Co-ordinated multipoint transmission/reception techniques for LTE-advanced[Coordinated and Distributed MIMO]," Wireless Communications, IEEE, vol.17, no.3, pp.26,34, June 2010.[2] V. Jungnickel, L.Thiele, T. Wirth, T. Haustein, S. Schiffermuller, A.Forck,S.Wahls, S.Jaeckel, S. Schubert, H. Gabler, C. Juchems, F. Luhn, R.Zavrtak, H. Droste, G. Kadel, W. Kreher, J. Mueller, W. Stoermer,G. Wannemacher, "Coordinated Multipoint Trials in the Downlink,"GLOBECOM Workshops, 2009 IEEE , vol., no., pp.1,7, Nov. 30 2009-Dec. 4 2009[3] Choi Jinho, "Non-Orthogonal Multiple Access in Downlink CoordinatedTwo-Point Systems," Communications Letters, IEEE , vol.18, no.2,pp.313,316, February 2014.[4] Yuya. 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A Novel Opportunistic NOMA in Downlink Coordinated Multi-Point Networks

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A Novel Opportunistic NOMA in Downlink Coordinated Multi-Point Networks

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