UL/DL Mode Selection and Transceiver Design for Dynamic TDD Systems 1 UL/DL Mode Selection and Transceiver Design for Dynamic TDD Systems Antti Tölli with Ganesh Venkatraman, Jarkko Kaleva and David Gesbert e-mail: atolli@ee.oulu.fi Centre for Wireless Communications, University of Oulu, Finland 2016 Tyrrhenian International Workshop on Digital Communications, Livorno, Italy 14 Sept, 2016 A. Tölli, J. Kaleva, G. Venkatraman & D. Gesbert, Joint UL/DL Mode Selection and Transceiver Design for Dynamic TDD Systems, in Proc. 2016 IEEE Global Conference on Signal and Information Processing (GLOBALSIP), Washington, D.C., USA, Dec. 7 9, 2016
UL/DL Mode Selection and Transceiver Design for Dynamic TDD Systems 2 frequency UL and DL control channels Dynamic TDD UL or DL data channels time Figure: Flexible TDD frame structure 1 Significant load variation between adjacent cells Flexible UL/DL allocation provides large potential gains in spectral efficiency 2 More challenging interference management 1 Nokia Networks, 5G radio access system design aspects, Nokia white paper, Aug. 2015. Available: http://networks.nokia.com/file/37611/5g-radio-access 2 3GPP TSG RAN WG1, Study on scenarios and requirements for next generation access technologies TR 38.913, 3rd Generation Partnership Project 3GPP, www.3gpp.org, 2016
UL/DL Mode Selection and Transceiver Design for Dynamic TDD Systems 3 Dynamic TDD Figure: UL-DL/DL-UL interference in Dynamic TDD Additional UL-to-DL and DL-to-UL interference associated with the dynamic TDD Interference mitigated by coordinated beamforming. More measurements and info exchange also at the terminal side Similar interference scenarios in underlay D2D transmission 3 3 A. Tölli, J. Kaleva & P. Komulainen, Mode Selection and Transceiver Design for Rate Maximization in Underlay D2D MIMO Systems, in Proc. IEEE ICC 2015, London, UK, June, 2015
UL/DL Mode Selection and Transceiver Design for Dynamic TDD Systems 4 System Model & Problem Formulation OFDM system with N sub-channels and N B BSs, N T TX antennas per BS K users each with N R antennas Goal: minimize the number of packets in BS/user queues via joint uplink (UL) / downlink (DL) cell mode selection, TX/RX design and resource allocation over spatial and frequency resources
UL/DL Mode Selection and Transceiver Design for Dynamic TDD Systems 5 Queueing Model Each user is associated with backlogged packets of size Q k. Queued DL (UL) packets Q k ( Q k ) of each user follows dynamic equation at the ith instant as Q k (i + 1) = [ Q k (i) t k (i)] + + λk (i) (1) where t k = N n=1 L l=1 t l,k,n denotes the total number of transmitted packets corresponding to user k λ k represents the fresh arrivals of user k at BS b k Separate user specific queues for UL and DL traffic
UL/DL Mode Selection and Transceiver Design for Dynamic TDD Systems 6 Objective Minimize the total number of backlogged packets in DL and UL 4 minimize t k, t k where α k, β k are arbitrary priority weights and v k = Q k t k = Q k N L u k = Q k t k = Q k N k U α k v k q + β k u k q (2) n=1 L n=1 l=1 log 2(1 + γ l,k,n ) (3) l=1 log 2(1 + γ l,k,n ) (4) q = 1, 2,..., plays different role based on the value it assumes Inherent maximum rate constraint: N L n=1 l=1 t l,k,n Q k Special cases (when Q k > N L n=1 l=1 t l,k,n k): q = 1: Sum rate maximization q = 2: Queue-Weighted Sum Rate Maximization (Q-WSRM) 4 G. Venkatraman, A. Tölli, L-N. Tran & M. Juntti, Traffic Aware Resource Allocation Schemes for Multi-Cell MIMO-OFDM Systems, IEEE Transactions on Signal Processing, vol. 64, no. 11, pp. 2730 2745, June 2016.
UL/DL Mode Selection and Transceiver Design for Dynamic TDD Systems 7 DL SINR 5 Γ l,k,n = UL SINR Γ l,k,n = Ǹ 0 + Ǹ 0 + Spatial Overloading in SINR w H l,k,n H b k,k,n m l,k,n 2 L wl,k,n H H b i,k,nm j,i,n 2 + i U\{k} j=1 }{{} DL-DL interference w l,k,n H HT b k,k,n m l,k,n 2 L w l,k,n H HT b k,i,n m j,i,n 2 + i U\{k} j=1 }{{} UL-UL interference L wl,k,n H H i,k,n m j,i,n 2 i U\U bk j=1 }{{} UL-DL interference (5) L w l,k,nĥb H i,b k,nm j,i,n 2 i U\U bk j=1 }{{} DL-UL interference (6) 5 Note that UL-DL and DL-UL interference terms in (5), and (6), respectively, include potential interference from all other-cell users. UL/DL mode selection per BS/user is handled separately via (relaxed) binary selection.
UL/DL Mode Selection and Transceiver Design for Dynamic TDD Systems 8 Queue Minimization with UL/DL Mode Selection min. ṽ q + ũ q (7a) s. t. γ l,k,n Γ l,k,n l, k, n (7b) γ l,k,n Γ l,k,n l, k, n N n=1 N L k U b L l=1 m l,k,n 2 x b P max b (7c) (7d) m l,k,n 2 x bk P UE n=1 l=1 max k (7e) x b + x b = 1 b, x b {0, 1}, x b {0, 1} (7f) 1 q where ṽ k ak (Q k N L n=1 l=1 t l,k,n) and t l,k,n = log(1 + γ l,k,n ) Nonconvex (difference of convex) SINR constraints, and integer UL/DL selection constraints
UL/DL Mode Selection and Transceiver Design for Dynamic TDD Systems 9 Approximation of the SINR Constraints The DL SINR constraints in (7b) are relaxed as 6 (UL similarly) w H l,k,n H 2 b k,k,n m l,k,n γ l,k,n = p2 l,k,n + q2 l,k,n β l,k,n β l,k,n β l,k,n Ǹ0 + i U\{k} L j=1 wh l,k,n H b i,k,nm j,i,n 2 + i U\U bk L j=1 wh l,k,n H i,k,n m j,i,n 2 (8) (9) where p l,k,n R(w H l,k,n H b k,k,n m l,k,n ), q l,k,n I(w H l,k,n H b k,k,n m l,k,n ) Difference of convex constraint solved via successive convex (linear) approximation (SCA) 6 G. Venkatraman, A. Tölli, L-N. Tran & M. Juntti, Traffic Aware Resource Allocation Schemes for Multi-Cell MIMO-OFDM Systems, IEEE Transactions on Signal Processing, vol. 64, no. 11, pp. 2730 2745, June 2016.
UL/DL Mode Selection and Transceiver Design for Dynamic TDD Systems 10 Binary Relaxation Binary variables x b, x b {0, 1} are replaced by continuous variables x b, x b [0, 1], b Problem (7) becomes convex (for fixed receivers, at any given linearization point of the SINR constraints) Sparsity must be enforced! Use a regularization function 7 minimize successively linearized as minimize N B ṽ q + ũ q + ψ (log(x b + ɛ) + log( x b + ɛ)). (10) t=1 ṽ q + ũ q + ψ } {{ } Concave N B b=1 ( xb x (i) b x (i) b + ɛ + x b x (i) b x (i) b + ɛ ) (11) 7 E. J. Candes, M. B Wakin, and S. Boyd, Enhancing Sparsity by Reweighted l1 Minimization, Journal of Fourier analysis and applications, vol. 14, no. 5-6, pp. 877 905, 2008.
UL/DL Mode Selection and Transceiver Design for Dynamic TDD Systems 11 Simulation Setup Figure: Final UL/DL allocation for a random drop of users and traffic states
UL/DL Mode Selection and Transceiver Design for Dynamic TDD Systems 12 2.5 Numerical Example Average queue backlog (bits). 2 1.5 1 0.5 50/50 Fixed ( α = 0.4, β = 10) Relaxed Binary ( α = 0.4, β = 10) 50/50 Fixed ( α = 0.2, β = 10) Relaxed Binary ( α = 0.2, β = 10) 50/50 Fixed ( α = 0.2, β = 5) Relaxed Binary ( α = 0.2, β = 5) 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Arrival rate (A). Figure: Average number of queued bits per user with varying packet arrival rates. The mean arrival rate across all low and high rate demand users is (1 α)a + αβa.
UL/DL Mode Selection and Transceiver Design for Dynamic TDD Systems 13 Next Steps Decentralization, decoupling the problem Inter-carrier, inter-sector UL-DL interference Signalling, CSI acquisition Time-scale of changing UL/DL allocation? Impact of more practical traffic models