Degrees of Freedom in Wireless Interference Networks with Cooperative Transmission and Backhaul Load Constraints

Degrees of freedom (DoF) gains are studied in wireless networks with cooperative transmission under a backhaul load constraint that limits the average number of messages that can be delivered from a centralized controller to the basestation transmitters. The backhaul load is defined as the sum of all the messages available at all the transmitters per channel use, normalized by the number of users. For Wyner’s linear interference network, where each transmitter is connected to the receiver having the same index as well as one succeeding receiver, the per user DoF is characterized and the optimal scheme is presented. When the backhaul load is constrained to an integer level B, the asymptotic per user DoF is shown to equal 4B−1 4B . Furthermore, it is shown that the optimal assignment of messages to transmitters is asymmetric and satisfies a local cooperation constraint, and that the optimal coding scheme relies only on one-shot zero-forcing transmit beamforming. The results are then extended to locally connected linear interference networks and the more practical hexagonal sectored cellular networks. For locally connected networks where each transmitter is connected to the receiver with the same index as well as L succeeding receivers, we show that an asymptotic per user DoF of 1 2 is achievable for L ≤ 6 with simple zero-forcing (interference avoidance) and without incurring overall load on the backhaul (B = 1). For hexagonal cellular networks, we also show that an asymptotic per user DoF of 1 2 is achievable with simple zero-forcing and the backhaul load constraint B = 1. Further, we show that 1 2 is the optimal value if each message can only be available at a single transmitter (no cooperative transmission), and that 3 7 is an upper bound on the asymptotic per user DoF achievable through interference avoidance if no cooperative transmission is allowed. Hence, our main conclusion for all the studied network models is that by allowing for cooperative transmission and a flexible message assignment that is constrained only by an average backhaul load, one can deliver the rate gains promised by information-theoretic upper bounds with practical one-shot schemes that incur little or no additional load on the backhaul. Parts of this work were presented at the International Symposium on Information Theory (ISIT) in 2014 [1] and 2015 [2]. A. El Gamal is with the Department of Electrical and Computer Engineering, Purdue University, IN 47907 USA (e-mail: [email protected]). M. Bande and V. V. Veeravalli are with the Coordinated Science Laboratory and the Department of Electrical and Computer Engineering, University of Illinois at Urbana-Champaign, Urbana, IL 61801 USA (e-mail: [email protected], [email protected]). This research was supported in part by the US NSF WIFiUS Program under grant number CNS 14-57168, through the University of Illinois at Urbana-Champaign, and in part by Huawei Innovation Research Program (HIRP) under grant number 301689, through Purdue University. May 24, 2017 DRAFT ar X iv :1 61 0. 09 45 3v 2 [ cs .I T ] 2 3 M ay 2 01 7