In this letter, we present the impact of carrier frequency offset (CFO) in dual-hop orthogonal frequency division multiplexing (OFDM) systems with a fixed relay for frequency-selective fading channels. Approximate expressions of the average signal-to-noise ratios (SNRs) for both downlink and uplink are obtained and validated by simulations. It is shown that dual-hop systems have slightly worse average SNR degradation than single-hop systems. We also show that the average SNR degradation due to the CFO varies according to the gap between average received SNRs for the first and the second hop.

In this letter, we propose a novel singular value decomposition zero-forcing beamforming (SVD-ZFBF) relaying scheme in the multiuser downlink MIMO broadcasting channel with fixed relays. Based on the processing scheme, we apply SUS [5] to select users at the relay station (RS) and develop a joint power allocation strategy at the base station (BS) and RS. By increasing the power at RS or selecting active users to obtain more multiuser diversity, SVD-ZFBF can approach an upper bound and outperform SVD-ZFDPC [1] with much lower complexity. Moreover, we show that the noise power ratio of RS to users significantly impacts the performance.

This paper investigates two issues of cellular engineering for cellular systems enhanced with two-hop fixed relay nodes (FRNs): spectrum partitioning and relay positioning, under the assumption of frequency reuse distance being equal to one. A channel-dependent spectrum partitioning scheme is proposed. According to this scheme, the ensemble mean of signal-to-interference-ratio on respective sets of links are taken into account to determine the bandwidths assigned to links connecting base station (BS) and FRNs, those connecting FRNs and mobile terminals (MTs) and those connecting BS and MTs. The proper FRN positioning is formulated as a constraint optimization problem, which tries to maximize the mean user data rate while at the same time ensures in probability 95% users being better served than in conventional cellular systems without relaying. It is demonstrated with computer simulations that FRN positioning has a strong impact on system performance. In addition, when FRNs can communicate with BS over line-of-sight channels the FRN enhanced cellular system with our proposed spectrum partitioning can remarkably outperform that with a known channel-borrowing based scheme and the conventional cellular systems without relaying. Simulation results also show that with proper FRN positioning the proposed spectrum partitioning scheme is robust against the unreliability of links connecting BS and FRNs.

Resource management for infrastructure-based two-hop fixed relay systems which are applicable to TDD-FH-OFDMA based cellular systems with sectorization is proposed in this paper. The severe interference problem caused by both inter-sector and inter-cell can be tackled by employing 6-sector directional antennas combined with the resource allocation. The simulation results demonstrate that at the outer-region of the cell, the high data rate service coverage can be extended.