Virtual Maximum Ratio Transmission for Downlink OFDMA Relay-based Networks

Abstract

In this paper, a novel and practical transmission scheme for a Multiple Input Multiple Output-Orthogonal Frequency Division Multiple Access (MIMO-OFDMA) relaybased network is proposed and evaluated. We coin the term Virtual Maximum Ratio Transmission (VMRT) for the scheme which obtains diversity and array gain at both links in a two-hop transmission, while keeping the complexity low. Besides, the requirements of Channel State Information needed at the different network elements such as the Base Station, Relay Station or User Terminals are also limited, and so the feedback. The obtained global performance by using proposed VMRT outperforms other schemes. Diversity gains larger than 6 can be easily obtained with a reduced number of relays. For this reason, VMRT is a good candidate to achieve high speed transmission or increase coverage and reliability in slow varying channels for relay-based networks.

Effects of Channel Estimation on Multiuser Virtual MIMO-OFDMA Relay-Based Networks

“…Only user 1 results are presented because similar performance is obtained by the different users, as explained before. In [25], it is shown that we can obtain diversity and array gain on both hops, and this gain increases as the number of RS does. Since this paper is focused on the performance of channel estimation and LDPC codes, we fixed the number of transmit antennas at the BS to 4, the number of relays to 8 (and 8, 16, 32 for LDPC codes), and the number of users to 4.…”

“…(4) All RSs receive this value from each UT, and according to the minimax BER criterion, the one with the minimum maximum BER is scheduled to transmit. As was shown in [25], this metric is the one which obtains the performance closest to the optimum. If qualities are sorted out in ascending order so that q 1 > q 2 > • • • > q Nu , the UT with q 1 is selected.…”

“…In order to obtain diversity in both links with reduced complexity and CSI in all the elements in the network, in [25], the following scheme is proposed, denoted as Virtual Maximum Ratio Transmission, because the relays are used as a cooperative virtual beamformer. In this scheme, the BS uses STBC (2, 4, or 8 scheme) to transmit to relays as in the 2h-STBC scheme.…”

“…By using this scheme, diversity is exploited in both links, especially on the second one, since usually the number of RS is higher than the number of transmit antennas. The reader is referred to [25] for more details.…”

“…In [25], the authors propose and analyze a practical transmission scheme with the DF strategy taking the relays as a Virtual Multiple-Input Transceiver (VMIT). However, perfect and instantaneous CSI is assumed and no channel code is used.…”