Design and measurement-based evaluations of coherent JT CoMP: a study of precoding, user grouping and resource allocation using predicted CSI

Abstract

Coordinated multipoint (CoMP) transmission provides high theoretic gains in spectral efficiency with coherent joint transmission (JT) to multiple users. However, this requires accurate channel state information at the transmitter (CSIT) and also user groups with spatially compatible users. The aim of this paper is to use measured channels to investigate if significant CoMP gains can still be obtained with channel estimation errors. This turns out to be the case, but requires the combination of several techniques. We here focus on coherent downlink JT CoMP to multiple users within a cluster of cooperating base stations. The use of Kalman predictors is investigated to estimate the complex channel gains at the moment of transmission. It is shown that this can provide sufficient CSIT quality for JT CoMP even for long (>20 ms) system delays at 2.66 GHz at pedestrian velocities or, for lower delays, at 500 MHz, at vehicular velocities. A user grouping and resource allocation scheme that provides appropriate groups for CoMP is also suggested. It provides performance close to that obtained by exhaustive search at very low complexity, low feedback cost and very low backhaul cost. Finally, a robust linear precoder that takes channel uncertainties into account when designing the precoding matrix is considered. We show that, in challenging scenarios, this provides large gains compared with zero-forcing precoding. Evaluations of these design elements are based on measured channels with realistic noise and intercluster interference assumptions. These show that high JT CoMP gains can be expected, on average over large sets of user positions, when the above techniques are combined -especially in severely intracluster interference limited scenarios.

Hierarchical precoding for ultra-dense heterogeneous networks

“…equal per-stream power constrained, which works perfectly for symmetric basestation-to-user constellations. The work in [12] introduces a so called demand matrix D and indicate that it will play an important role for non-symmetric cases which dominantly appear in the scenario of ultra-dense networks with joint signal processing among heterogeneous base stations. Again each base station equally distributes the transmit power P m among all antenna elements.…”

Robust linear precoder for coordinated multipoint joint transmission under limited backhaul with imperfect CSI

“…These penalty matrices can and will be used as design parameters to optimize an arbitrary criterion, see Section III-A. The desired target matrix, D, can be designed in various ways, see [8].…”

“…Note that the special case when the channel matrix H in (1) is invertible and perfectly know, i.e. ∆H CU = 0 in (6), the target matrix D = I in (9) and the penalty matrices in (8) are set to V = I and S = I where → 0 then the precoder in (11) coincides with the zero forcing solution (Ĥ −1 ). In [8] the proposed precoder was compared with the zero forcing solution where the power balancing was set throughĤ −1 D in the case of imperfect CSI.…”

“…∆H CU = 0 in (6), the target matrix D = I in (9) and the penalty matrices in (8) are set to V = I and S = I where → 0 then the precoder in (11) coincides with the zero forcing solution (Ĥ −1 ). In [8] the proposed precoder was compared with the zero forcing solution where the power balancing was set throughĤ −1 D in the case of imperfect CSI. The proposed precoder then performed similar to zero forcing for "good" user groups and significantly outperformed zero forcing for spatially difficult user groups.…”

“…The robust linear precoding solution of [8], [9] is here extended to also handle backhaul limitations without loosing optimality with respect to a robust MSE criterion, which averages over the CSI uncertainty. The extension is obtained by introducing penalties on specific elements of the precoder matrix.…”