Application driven joint uplink-downlink optimization in wireless communications

Abstract

This paper introduces a new mathematical framework which is used to derive joint uplink/downlink achievable rate regions for multi-user spatial multiplexing between one base station and multiple terminals. The framework consists of two models: the first one is a simple transmission model for uplink (UL) and downlink (DL), which is capable to give a lower bound on the capacity for the case that the transmission is subject to imperfect channel state information (CSI). A detailed model for concrete channel estimation and feedback schemes provides the parameter input to the former model and covers the most important aspects such as pilot design optimization, linear channel estimation, feedback delay, and feedback quantization.We apply this framework to determine optimal pilot densities and CSI feedback quantity, given that a weighted sum of UL and DL throughput is to be maximized for a certain user velocity. We show that for low speed, and if DL throughput is of particular importance, a significant portion of the UL should be invested into CSI feedback. At higher velocity, however, DL performance becomes mainly affected by CSI feedback delay, and hence CSI feedback brings little gain considering the inherent sacrifice of UL capacity. We further show that for high velocities, it becomes beneficial to use no CSI feedback at all, but apply random beamforming in the DL and operate in time-division multiplex.

Uplink CoMP under a Constrained Backhaul and Imperfect Channel Knowledge

“…Note that σ 2 pilots can differ from σ 2 if multi-cell (quasi-)orthogonal pilot sequences are employed. In the sequel, we assume unit-power pilots (p pilots = 1), and choose N p = 2, which has been motivated through the observation of a concrete channel estimation scheme in a frequencyselective OFDMA system for a channel of average coherence time and bandwidth in [42]. Let us now state the following theorem:…”

“…Note that σ 2 pilots can differ from σ 2 if multi-cell (quasi-)orthogonal pilot sequences are employed. In the sequel, we assume unit-power pilots (p pilots = 1), and choose N p = 2, which has been motivated through the observation of a concrete channel estimation scheme in a frequencyselective OFDMA system for a channel of average coherence time and bandwidth in [42]. Let us now state the following theorem: Theorem 1 (Modified transmission equation under imperfect CSI): An inner bound for the capacity region (considering average rates over many estimation errors) of the transmission in (1) under imperfect CSI can be found by observing the capacity region connected to the transmission…”

Achievable Net-Rates in Multi-User OFDMA with Partial CSI and Finite Channel Coherence

“…The estimation of the actual channel h k f,t is given byh k f,t = h k f,t + e k f,t . This model has also been used in [9], [10] and considers an error independent of the channel realization. By contrast, [11], [12] …”

“…This expression follows from the effective transmission equation given in [9] and equals the lower bound given in [11, eq. (17)].…”