Front Inform Technol Electron Eng

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1 Interference coordination in full-duplex HetNet with large-scale antenna arrays Zhao-yang ZHANG, Wei LYU Zhejiang University Key words: Massive MIMO; Full-duplex; Small cell; Wireless backhaul; Distributed algorithm Contact: Zhao-yang ZHANG ning ORCID: Zhao-yang Zhang, Wei Lyu, Interference coordination in full-duplex HetNet with large-scale antenna arrays. Frontiers of Information Technology & Electronic Engineering, 18(6): Zhao-yang ZHANG, Wei LYU (ZJU) Full-Duplex HetNet with Massive MIMO 1 / 45

2 Overview 1 Motivation 2 System model 3 Problem formulation 4 Interference coordination with centralized algorithms 5 Interference coordination with distributed algorithm 6 Performance evaluation Zhao-yang ZHANG, Wei LYU (ZJU) Full-Duplex HetNet with Massive MIMO 2 / 45

3 Overview 1 Motivation 2 System model 3 Problem formulation 4 Interference coordination with centralized algorithms 5 Interference coordination with distributed algorithm 6 Performance evaluation Zhao-yang ZHANG, Wei LYU (ZJU) Full-Duplex HetNet with Massive MIMO 3 / 45

4 Heterogeneous Network Heterogeneous Network (HetNet): A network that consists of a mix of high-power macro cells and low-power nodes, e.g., Pico, Femto cells. Zhao-yang ZHANG, Wei LYU (ZJU) Full-Duplex HetNet with Massive MIMO 4 / 45

5 Heterogeneous Network Benefits: Achieve higher spectrum efficiency. Meet the tremendous mobile data traffic growth of 5G. Challenges: Complicated cross-tier interference and co-tier interference. Small cell BSs are not always in easy-to-reach locations, which makes conventional fiber backhaul in cellular network impractical. Zhao-yang ZHANG, Wei LYU (ZJU) Full-Duplex HetNet with Massive MIMO 5 / 45

6 Massive MIMO and Full-Duplex Massive MIMO and Full-Duplex are also very promising techniques for future 5G communication systems. Massive MIMO scales up the conventional MIMO by orders of magnitude, thus providing aggressive spatial multiplexing capabilities and significant array gains. Full-Duplex helps BSs efficiently reuse the radio access network spectrum by transmitting and receiving signals on the same frequency at the same time. Motivation: When combining massive MIMO and Full-Duplex in HetNet, how to effectively suppress the co-tier interference and cross-tier interference in such scenarios? Zhao-yang ZHANG, Wei LYU (ZJU) Full-Duplex HetNet with Massive MIMO 6 / 45

7 Overview 1 Motivation 2 System model 3 Problem formulation 4 Interference coordination with centralized algorithms 5 Interference coordination with distributed algorithm 6 Performance evaluation Zhao-yang ZHANG, Wei LYU (ZJU) Full-Duplex HetNet with Massive MIMO 7 / 45

System Model We consider the downlink of a two-tier HetNet: A MBS is equipped with N antennas and serves K MUEs, while each of the S SBS employs a single antenna to serve its associated SUE using

8 System Model We consider the downlink of a two-tier HetNet: A MBS is equipped with N antennas and serves K MUEs, while each of the S SBS employs a single antenna to serve its associated SUE using full-duplex techniques; the MBS provides wireless backhaul connections to these SBSs. Fig. A two-tier HetNet system with one massive MIMO macro-cell base station and S fullduplex small-cell base stations Zhao-yang ZHANG, Wei LYU (ZJU) Full-Duplex HetNet with Massive MIMO 8 / 45

9 Assumption Massive MIMO in MBS, N K and N S. TDD. Linear zero-forcing beamforming (LZFBF) precoder. Channel model: Path loss, Shadow fading, Rayleigh fading. Zhao-yang ZHANG, Wei LYU (ZJU) Full-Duplex HetNet with Massive MIMO 9 / 45

10 Full-Duplex Modes of SBS Out-of-band Full-duplex Mode (OBFD): access link and backhaul link employ different frequency bands. In-band Full-duplex Mode (IBFD): access link and backhaul link are conducted at the same frequency band. Zhao-yang ZHANG, Wei LYU (ZJU) Full-Duplex HetNet with Massive MIMO 10 / 45

11 Interference Mode Fig. 2 Graphical illustration of the different interference in OBFD (a) and IBFD (b) modes. Downlink cross- and co-tier interferences experienced at MUE, SBS, and SUE are illustrated Zhao-yang ZHANG, Wei LYU (ZJU) Full-Duplex HetNet with Massive MIMO 11 / 45

12 Macro-cell Transmission Model Access link from the MBS to MUEs: y MUE,k (t)=h b2mu,k Wx b2mu (t) + P MK h b2mu,k W 2 γ MUE,k = P S i S O h s2mu,ik 2 + σ 2. i S O h s2mu,ik x s2su,i (t) + n(t). Zhao-yang ZHANG, Wei LYU (ZJU) Full-Duplex HetNet with Massive MIMO 12 / 45

13 Small-cell Transmission Model SBS in OBFD Mode: Backhaul link from the MBS to SBSs: y O SBS,i(t) = h b2s,i Gx b2s (t) + l S I h s2s,il x s2su,l (t) + n(t). P MS γsbs,i O h b2s,i G 2 = P S l S I h s2s,il 2 + σ. 2 Access link from SBS to SUE: y O SUE,i(t) = h s2su,ii x s2su,i (t) + p S O\i + h b2su,i Wx b2mu (t) + n(t). h s2su,ip x s2su,p (t) Zhao-yang ZHANG, Wei LYU (ZJU) Full-Duplex HetNet with Massive MIMO 13 / 45

14 Modified ZF Precoder To mitigate the cross-tier interference from the MBS, we exploit the modified zero-forcing precoder proposed in Li et al. (2015) 1 : W = R(H b2mu R) + Γ 1 2 W, where R is a matrix that satisfies H b2su R = 0, and Γ 1 2 W is a diagonal matrix that normalizes R(H b2mu R) +. The SINR of the i-th SUE: γsue,i O P S h s2su,ii 2 = P S p S O \i h s2su,ip 2 + σ. 2 1 Li, B., Zhu, D., Liang, P., Small cell in-band wireless backhaul in massive MIMO systems: a cooperation of next generation techniques. IEEE Transactions on Wireless Communications, 14(12): Zhao-yang ZHANG, Wei LYU (ZJU) Full-Duplex HetNet with Massive MIMO 14 / 45

15 Small-cell Transmission Model SBS in IBFD Mode: Backhaul link from the MBS to SBSs: y I SBS,j(t) = h b2s,j Gx b2s (t) + q S I\j P MS γsbs,j I h b2s,j G 2 = P S q S h I\j s2s,jq 2 + I SI + σ. 2 h s2s,jq x s2su,q (t) + I SI w(t) + n(t). Zhao-yang ZHANG, Wei LYU (ZJU) Full-Duplex HetNet with Massive MIMO 15 / 45

16 Small-cell Transmission Model SBS in IBFD Mode: Access link from SBS to SUE (G is also a modified ZF precoder): y I SUE,j(t)=h s2su,jj x s2su,j (t)+ q S I\j γsue,j I P S h s2su,jj 2 = P S q S h I\j s2su,jq 2 + σ. 2 h s2su,jq x s2su,q (t)+h b2su,j Gx b2s (t)+n(t). Zhao-yang ZHANG, Wei LYU (ZJU) Full-Duplex HetNet with Massive MIMO 16 / 45

17 Overview 1 Motivation 2 System model 3 Problem formulation 4 Interference coordination with centralized algorithms 5 Interference coordination with distributed algorithm 6 Performance evaluation Zhao-yang ZHANG, Wei LYU (ZJU) Full-Duplex HetNet with Massive MIMO 17 / 45

18 Total Throughput The total throughput of this two-tier HetNet C = C M + CS O + CS I = 1 2 log 2 (1 + γ MUE,k ) k K + 2 log 2 (1 + min (γsbs,i, O γsue,i)) O i S O log 2 (1 + min (γsbs,j, I γsue,j)). I j S I Zhao-yang ZHANG, Wei LYU (ZJU) Full-Duplex HetNet with Massive MIMO 18 / 45

19 Throughput Optimization We note x = [x 1, x 2,..., x S ] is the set of all SBSs, and set as follows { xi = 1, if the i-th SBS is in IBFD mode x i = 0, if the i-th SBS is in OBFD mode. The throughput maximization problem max C = ) 1 log x 2 (1 + k K i S (1 x i)α k,i + β k + ) 1 1 (1 x i )log 2 (1+min ( i S j S x, ja i,j +b i j S (1 x ) j)c i,j +d i + ) 1 1 x i log 2 (1 + min ( i S j S x, je i,j + f i j S x ) jg i,j + h i s.t. x i {0, 1}, i S. Zhao-yang ZHANG, Wei LYU (ZJU) Full-Duplex HetNet with Massive MIMO 19 / 45

20 Throughput Optimization The parameters α k,i = KP S h s2mu,ik 2 Kσ, β P M h b2mu,k W 2 k = 2, P M h b2mu,k W 2 a i,j = SP S h s2s,ij 2 Sσ, b P M h b2s,i G 2 i = 2, P M h b2s,i G 2 h s2su,ij 2, j i c i,j = h s2su,ii 2 σ, d i = 2, P 0, j = i S h s2su,ii 2 SP S h s2s,ij 2, j i e i,j = P M h b2s,i G 2, f i = S(I SI+σ 2 ), P 0, j = i M h b2s,i G 2 h s2su,ij 2, j i g i,j = h s2su,ii 2 σ, h i = 2. P 0, j = i S h s2su,ii 2 Zhao-yang ZHANG, Wei LYU (ZJU) Full-Duplex HetNet with Massive MIMO 20 / 45

21 Problem Formulation A combinatorial optimization problem. The complexity is Θ(2 S ) using brute force algorithm. To solve this problem more efficiently, we assume that the throughput of backhaul link is always larger than that of small-cell access link. The assumption is practical due to the performance gain provided by massive MIMO and advanced self-interference cancellation techniques in full-duplex systems. Zhao-yang ZHANG, Wei LYU (ZJU) Full-Duplex HetNet with Massive MIMO 21 / 45

22 Problem Formulation Final optimization problem max x C = 1 log 2 (1 + k K i S (1 x ) i)α k,i + β k + 1 (1 x i ) log 2 (1 + i S j S (1 x ) j)c i,j + d i + 1 x i log 2 (1 + i S j S x ) jc i,j + d i s.t. x i {0, 1}, i S. Zhao-yang ZHANG, Wei LYU (ZJU) Full-Duplex HetNet with Massive MIMO 22 / 45

23 Overview 1 Motivation 2 System model 3 Problem formulation 4 Interference coordination with centralized algorithms 5 Interference coordination with distributed algorithm 6 Performance evaluation Zhao-yang ZHANG, Wei LYU (ZJU) Full-Duplex HetNet with Massive MIMO 23 / 45

24 Centralized Algorithms The objective function is non-convex. The optimal vector x can hardly be obtained even if we relax the constraint x i {0, 1}. To find a near-optimal solution, we propose two centralized algorithms, i.e., a genetic algorithm and a greedy algorithm. Zhao-yang ZHANG, Wei LYU (ZJU) Full-Duplex HetNet with Massive MIMO 24 / 45

25 Genetic Algorithm Algorithm 1 Genetic algorithm 1: set IndividualLength = S, PopulationSize = S + 10, MaxIteration = 100, CrossoverPro = 0.6, MutationPro = : Generate initial population randomly, i.e., x i = randint(1), i S. 3: for i = 1 to MaxIteration do 4: Calculate the fitness of each individual, fitness is the value of the objective function. 5: Roulette wheel selection. 6: if rand(1) < CrossoverPro then 7: Crossover selection. 8: end if 9: if rand(1) < MutationPro then 10: Mutation selection. 11: end if 12: Generate new population. 13: end for Zhao-yang ZHANG, Wei LYU (ZJU) Full-Duplex HetNet with Massive MIMO 25 / 45

26 Genetic Algorithm An adaptive heuristic search algorithm based on the evolutionary ideas of natural selection and genetics. The overall complexity of the GEA is O(S 3 ). Zhao-yang ZHANG, Wei LYU (ZJU) Full-Duplex HetNet with Massive MIMO 26 / 45

27 Greedy Algorithm Algorithm 2 Greedy algorithm 1: Generate initial solution randomly, i.e., x i = randint(1), i S 2: loop 3: Max = 0 4: for i = 1 to S do 5: y = x 6: y i = 1 y i 7: Delta = C(y) C(x) 8: if Delta > Max then 9: Max = Delta 10: MaxIndex = i 11: end if 12: end for 13: if Delta > 0 then 14: x i = 1 x i 15: else 16: break 17: end if Zhao-yang ZHANG, Wei LYU (ZJU) Full-Duplex HetNet with Massive MIMO 27 / 45

28 Greedy Algorithm At each iteration we switch the mode of an SBS when this switch can bring the largest throughput gain; the iteration will terminate if the largest throughput gain becomes negative. The complexity of one iteration is O(S 3 ), the overall complexity of GRA will be no less than O(S 3 ). Zhao-yang ZHANG, Wei LYU (ZJU) Full-Duplex HetNet with Massive MIMO 28 / 45

29 Greedy Algorithm For the iterative process in the proposed GRA, we prove its convergence in the following proposition. Proposition 1 For any initial solution, the greedy algorithm can converge. Proof: We note the number of iterations as t = 1, 2,, and the iterative procedure of GRA is in fact a sequence of C t. We always promise that the Delta > 0, i.e., C t+1 > C t, thus C t is monotonous increasing with t. Also, we can prove that the C t has an upper bound. Zhao-yang ZHANG, Wei LYU (ZJU) Full-Duplex HetNet with Massive MIMO 29 / 45

30 Greedy Algorithm C = 1 log 2 (1 + k K i S (1 x ) i)α k,i +β k + 1 (1 x i ) log 2 (1 + i S j S (1 x ) j)c i,j +d i + i S 1 x i log 2 (1 + j S x ) jc i,j +d i < log 2 (1 + 1 ) + log β 2 (1 + 1 ) = U, k d i k K i S where U is a constant that only depends on β k and d i which are limited by CSI, noise variance, and transmit power. Because C t is monotonous increasing with t and has an upper bound U, Proposition 1 holds based on the monotone convergence theorem. Zhao-yang ZHANG, Wei LYU (ZJU) Full-Duplex HetNet with Massive MIMO 30 / 45

31 Overview 1 Motivation 2 System model 3 Problem formulation 4 Interference coordination with centralized algorithms 5 Interference coordination with distributed algorithm 6 Performance evaluation Zhao-yang ZHANG, Wei LYU (ZJU) Full-Duplex HetNet with Massive MIMO 31 / 45

32 Distributed Graph Color Algorithm The co-tier interference is only incurred when SBSs are in the same full-duplex modes, thus we can determine that there is an intuitive relationship between our problem and the graph coloring problem if we regard these two full-duplex modes as two distinct colors. To find a near-optimal solution, we propose a distributed graph color algorithm (DGCA), which could sufficiently reduces the computation overhead of the MBS by having each SBS choose the full-duplex mode by itself. Zhao-yang ZHANG, Wei LYU (ZJU) Full-Duplex HetNet with Massive MIMO 32 / 45

33 Adjacent Graph Definition 1 The i-th SBS and the j-th SBS are adjacent if and only if η i,j = 1 { 1, if c i,j > Γ th or c j,i > Γ th η i,j 0, otherwise Applying this definition to all SBS pairs, an adjacent graph is generated. If we regard these two full-duplex modes as two distinct colors, we should assign adjacent SBSs with different colors as far as possible to minimize the co-tier interference. The incurred cross-tier interference when a SBS is colored with OBFD mode makes our contribution different from the conventional graph color algorithms. Zhao-yang ZHANG, Wei LYU (ZJU) Full-Duplex HetNet with Massive MIMO 33 / 45

34 Price Definition 2 Let C I denote the throughput when all SBSs are in IBFD mode, and let C i,o be the throughput when only the i-th operates in OBFD mode. Then the price of the i-th SBS is defined as ξ i C i,o C I. It is needed to determine which type of interference is more predominate for each SBS. ξ i represents the balance between co-tier interference and cross-tier interference induced by the i-th SBS. If ξ i is negative and low, it means SBS i induces severe cross-tier interference and tends to choose the IBFD mode. If ξ i IS positive and high, it shows that SBS i results in serious co-tier interference, and is better to operate in a different mode with its adjacent SBSs. Zhao-yang ZHANG, Wei LYU (ZJU) Full-Duplex HetNet with Massive MIMO 34 / 45

35 Combined Price for adjacent SBSs x I IBFD IBFD IBFD i-th x O OBFD OBFD OBFD List all the adjacent SBSs of SBS i and divide them into two categories based on their full-duplex modes. Zhao-yang ZHANG, Wei LYU (ZJU) Full-Duplex HetNet with Massive MIMO 35 / 45

36 Combined Price for adjacent SBSs Definition 3 For SBS i and its adjacent SBS j, their combined price is ξ i + ξ j, if x i = x j = 1 ω 2 ξ i, if x i = 0, x j = 1 ξ i,j ω 2 ξ j, if x i = 1, x j = 0 ξ i + ξ j + ω 2 ξ i + ω 2 ξ j, if x i = x j = 0 If SBS i and SBS j both operate in IBFD mode, it is trivial that their combined price is the summation of the individual prices. If SBS i and SBS j are in different modes, co-tier interference no longer exists but one of them is bound to create cross-tier interference. if SBS i and SBS j both work in OBFD mode, the individual prices as well as the extra price induced by cross-tier interference need to be considered. Zhao-yang ZHANG, Wei LYU (ZJU) Full-Duplex HetNet with Massive MIMO 36 / 45

37 Sum Price of SBS Proposition 2 The sum price of SBS i and all its adjacent SBSs is ξ I + ξ i + l S adj ω 2 ξ l, if x i =1 and ξ I 0 O,i l S adj ω 2 ξ l, if x i =1 and ξ I =0 O,i ξ sum,i = ξ O + ξ i + l S adj ω 2 ξ l l S adj O,i O,i + ω 2 ξ i, if x i =0 and ξ O 0 ω 2 ξ l + ω 2 ξ i, if x i =0 and ξ O =0 Zhao-yang ZHANG, Wei LYU (ZJU) Full-Duplex HetNet with Massive MIMO 37 / 45

38 Full-Duplex Selection Rule Proposition 3 The i-th SBS should select the full-duplex mode with the following rules: Case I: If ξ I = 0, then x i = 1. Case II: If ξ I 0, ξ O 0, then { 1, if ξ I < ξ O + ω 2 ξ i x i = 0, otherwise Case III: If ξ I 0, ξ O = 0, then { 1, if ξ I + ξ i < ω 2 ξ i x i = 0, otherwise Zhao-yang ZHANG, Wei LYU (ZJU) Full-Duplex HetNet with Massive MIMO 38 / 45

39 Algorithm Procedure Algorithm 3 Distributed graph coloring algorithm 1: Obtain adjacent graph B = [η ij ] S S, i, j S. 2: Obtain the price of each SBS. Then sort these prices in ascending order, and denote the sorted sequence of SBSs as SBS 1, SBS 2,, SBS S. 3: for i = 1 to S do 4: SBS i collects the full-duplex modes and prices informations of its adjacent nodes. 5: SBS i colors itself using the rules in Proposition 3. 6: end for Zhao-yang ZHANG, Wei LYU (ZJU) Full-Duplex HetNet with Massive MIMO 39 / 45

40 Overview 1 Motivation 2 System model 3 Problem formulation 4 Interference coordination with centralized algorithms 5 Interference coordination with distributed algorithm 6 Performance evaluation Zhao-yang ZHANG, Wei LYU (ZJU) Full-Duplex HetNet with Massive MIMO 40 / 45

41 Simulation Parameters Parameter Value Macro-cell radius, R M 1000 m Small-cell radius, R S 40 m Carrier frequency, f c 2 GHz System bandwidth, B 20 MHz Number of MUEs, K 20 Transmit power of MBS, P M 46 dbm Transmit power of SBS, P S 24 dbm Noise power, σ dbm/hz Zhao-yang ZHANG, Wei LYU (ZJU) Full-Duplex HetNet with Massive MIMO 41 / 45

42 Simulation Results Zhao-yang ZHANG, Wei LYU (ZJU) Full-Duplex HetNet with Massive MIMO 42 / 45

43 Simulation Results Zhao-yang ZHANG, Wei LYU (ZJU) Full-Duplex HetNet with Massive MIMO 43 / 45

44 Simulation Results Zhao-yang ZHANG, Wei LYU (ZJU) Full-Duplex HetNet with Massive MIMO 44 / 45

45 Thank You! Zhao-yang ZHANG, Wei LYU (ZJU) Full-Duplex HetNet with Massive MIMO 45 / 45

PERFORMANCE COMPARISON OF DATA-SHARING AND COMPRESSION STRATEGIES FOR CLOUD RADIO ACCESS NETWORKS. Pratik Patil, Binbin Dai, and Wei Yu

PERFORMANCE COMPARISON OF DATA-SHARING AND COMPRESSION STRATEGIES FOR CLOUD RADIO ACCESS NETWORKS Pratik Patil, Binbin Dai, and Wei Yu Department of Electrical and Computer Engineering University of Toronto,