Cooperative Interference Alignment for the Multiple Access Channel

Interference alignment (IA) has emerged as a promising technique for the interference channel that guarantees a constant degree-of-freedom (DoF) gain for each user. Although successive interference cancellation (SIC) is known to achieve the sum capacity of the MAC channel, the DoF allocation to the users depends on the decoding order, e.g., the first few users get zero DoF, while the last users being decoded see full DoF gain. For a large number of users K, randomization between decoding order introduces significant latency in the system. One the other hand, orthogonal access schemes, e.g., TDMA/FDMA, waste significant resources for large number of users. To combat these problems, we propose a novel technique for the K-user multiple access (MAC) channel with N receivers, called Cooperative Interference Alignment (C-IA) and derive bounds on the DoF for each user. Assuming K 2 ≤ N ≤ K, we show that C-IA is sum-DoF optimal, i.e, the sum rate scaling is as RΣ(SNR) = N log2(SNR) + o(log2(SNR)) and simultaneously achieves a constant DoF-peruser, i.e., Rk(SNR) = dk log2(SNR) + o(log2(SNR)) where dk ∈ {1/2, 1}, without randomization between different decoding strategies and without the use of orthogonal access schemes.