| As time goes by, the4th generation mobile communication system (4G) has almost been put into commercial uses in China. The key technologies in4G have been already developed perfectly. As an important branch of communication technology, the technology of wireless communication has widely drawn our attention. The research and development of communication technology make the distance between people shorter and lead to the progress and advance of the whole society. Higher data rate and better quality are always the goals of wireless communication. However, in fact, wireless communication system faces with many complicated problems, such as multi-path, multi-access influence, and limited bandwidth, which greatly influence the quality of communication links and also become the key obstructive factors during the development of wireless communications. Multi-input-multi-output (MIMO) technologies introducing useful space resources have received wide attention because it can make full use of space to increase the data rate and improve the power efficiency. Space-time signal processing technologies can improve the capability to against the fading in MIMO channels by take advantage of space resources.Unitary space-time modulation (USTM) is suitable for rapid fading scenarios at high signal-to-noise ratio (SNR), where channel state information (CSI) is not available for both transmitters and receivers. It has been widely considered in modern wireless communications systems where the channel changes fast due to the high-speed mobility. However, long distance or "shadow effect" may lead to a pretty low SNR. which makes USTM lose its advantages. Relay-based wireless communications can keep longer distance transmission and higher SNR by means of supplying power again at relays while there are few relaying schemes for USTM. Moreover, in order to reduce the influence of error propagation at the receiver caused by hard decision at the relays, we focus on emerging soft forwarding technique. Soft forwarding technique regenerates the soft signal by many different ways and then forwards the soft signal to the destination, which can improve the reliability of the whole relaying transmission. Therefore, research on the soft forwarding technique for USTM is practically meaningful for high transmission quality in fast fading channels.In this paper, we propose two soft forwarding schemes for USTM, that is, the EF scheme and the MIF scheme, which well fit for the fast fading channels with no CSI at all nodes. To our best knowledge, the approach of soft forwarding technique is not generalized to the unitary space-time modulation for rapid fading channels. In our proposed EF scheme for USTM, the relay node regenerates the forwarding signal via the conditional expectation instead of hard information. The MIF scheme employs the instantaneous mutual information between the source unitary space-time (UST) signal and the received signal at the relay node as a measurement for maximum likelihood (ML) detection. For the two proposed soft forwarding schemes, we provide the relay functions, the normalization factors, and the detection methods firstly. Furthermore, for our unitary space-time modulated relaying systems, the generalized signal-to-noise ratio (GSNR) at the receiver is described and analyzed. For a parallel relay network equipped with two relays, the proposed EF scheme achieves2dB and3dB SNR gain over amplify-and-forward (AF) and decode-and-forward (DF) schemes, respectively. Moreover, the MIF scheme can further give about1.5dB SNR gain. Besides, for our unitary space-time modulated systems, we also consider the influences of some systematic factors on BER performances for the proposed schemes, such as the number of parallel relays, the number of antennas, and the coherence time. |
