Impact of Channel Asymmetry on Performance of Channel Estimation and Precoding for Downlink Base Station Cooperative Transmission

Base station (BS) cooperative transmission can improve the spectrum efficiency of cellular systems, whereas using which the channels will become asymmetry. In this paper, we study the impact of the asymmetry on the performance of channel estimation and precoding in downlink BS cooperative multiple-antenna multiple-carrier systems. We first present three linear estimators which jointly estimate the channel coefficients from users in different cells with minimum mean square error, robust design and least square criterion, and then study the impact of uplink channel asymmetry on their performance. It is shown that when the large scale channel information is exploited for channel estimation, using non-orthogonal training sequences among users in different cells leads to minor performance loss. Next, we analyze the impact of downlink channel asymmetry on the performance of precoding with channel estimation errors. Our analysis shows that although the estimation errors of weak cross links are large, the resulting rate loss is minor because their contributions are weighted by the receive signal to noise ratio. The simulation results verify our analysis and show that the rate loss per user is almost constant no matter where the user is located, when the channel estimators exploiting the large scale fading gains.Comment: Submitted to the Transactions on Communication

Interference Rejection for Parametric Channel Estimation in Reuse-1 Cellular OFDM Systems

Abstract—This paper proposes a joint channel-estimation method for the desired and interfering channels for cell edge users in reuse-1 orthogonal frequency-division multiplexing (OFDM) cellular systems. The following assumptions are made in proposing the algorithm: 1) The desired and interferer’s channel multipath delays do not overlap; 2) the same pilot sequence is sent from the desired and interfering base stations (BSs); and 3) reuse-3 preamble symbols, as in the preamble structure in IEEE 802.16d/e (Wimax) systems, are used to obtain initial channel estimates without interference. If we make these assumptions, then it is possible to estimate the desired and interfering channels, even with reuse-1 pilots. The proposed pilot-based channel estimation technique exploits the delay subspace structure to reduce the impact of cochannel interference (CCI) on channel estimation. Delay subspace refers to the set of basis vectors spanning the frequency response of the desired and interfering multipath channels. This enables us to jointly estimate and track the desired and interfering channels with a lower mean-squared error (MSE), when compared with the conventional modified least-squares (mLS) technique, which ignores the structure of interference. The significance of accurate channel estimates in symbol detection schemes is demonstrated for systems employing two or more receiver antennas. The proposed channel estimator significantly improves the performance of symbol-detection schemes based on either interference nulling combiner (INC) or minimum MSE diversity combiner (MMSE-DC), when compared with detection schemes using mLS-based channel estimates. Analytical expressions are derived for the MSE of the estimated multipath delays. Simulation results are also provided to show the improved performance for the proposed channel-estimation method compared with the mLS-based channel estimation method, when used in conjunction with INC and MMSE-DC detectors. Index Terms—Cellular orthogonal frequency-division multiplexing (OFDM), cochannel interference (CCI), estimation of signal parameters using rotational invariance method (ESPRIT), IEEE 802.16d/e, joint channel estimation, parametric channel estimation