Downlink Noncoherent Cooperation without Transmitter Phase Alignment

Abstract

Multicell joint processing can mitigate inter-cell interference and thereby increase the spectral efficiency of cellular systems. Most previous work has assumed phase-aligned (coherent) transmissions from different base transceiver stations (BTSs), which is difficult to achieve in practice. In this work, a noncoherent cooperative transmission scheme for the downlink is studied, which does not require phase alignment. The focus is on jointly serving two users in adjacent cells sharing the same resource block. The two BTSs partially share their messages through a backhaul link, and each BTS transmits a superposition of two codewords, one for each receiver. Each receiver decodes its own message, and treats the signals for the other receiver as background noise. With narrowband transmissions the achievable rate region and maximum achievable weighted sum rate are characterized by optimizing the power allocation (and the beamforming vectors in the case of multiple transmit antennas) at each BTS between its two codewords. For a wideband (multicarrier) system, a dual formulation of the optimal power allocation problem across sub-carriers is presented, which can be efficiently solved by numerical methods. Results show that the proposed cooperation scheme can improve the sum rate substantially in the low to moderate signal-to-noise ratio (SNR) range.Comment: 30 pages, 6 figures, submitted to IEEE Transactions on Wireless Communication

Time-Asynchronous Robust Cooperative Transmission for the Downlink of C-RAN

“…We emphasize that n R,i = N U /2 is the smallest number of RRH antennas that are able to serve the N U UEs without creating inter-UE interference in the presence of perfect synchronization. Note that the achievable rates can be easily computed in the presence of a phase offset by using (1) in (7) as done in the proof of Proposition 1.…”

“…This approach, referred to as noncooperative transmission in Sec. V, includes the transmitter selection scheme as a special case, and the optimization can be addressed by Algorithm 1 with the additional constraints Ω = 0 (see also [7] for the optimization).…”

“…Owing to the time offset, the BBU cannot control the correlation of the signals transmitted by the RRHs, which, in turn, makes it impossible to ensure the constructive superposition of the RRHs' signals at the desired receivers. In fact, when the synchronization is imperfect, it may be beneficial to restrict transmission to non-cooperative strategies, as studied in [7].…”

The Non-Coherent Ultra-Dense C-RAN Is Capable of Outperforming Its Coherent Counterpart at a Limited Fronthaul Capacity

“…Since different RRHs send different signals to each UE, each UE needs to detect all the signals from all its serving RRHs. One low-complexity detection algorithm named successive interference cancellation [26]- [29] can be adopted, where each UE sequentially detects its signals from different RRHs. In particular, each UE first detects its signal from the first RRH in I k , while regarding the other desired signals as interference.…”