Achieving high throughputs with low outage probability in a large cell is a challenge, but this drives effective deployment with MIMO and multihop relay techniques, which have been intensively explored in this chapter. We have analyzed the effective relay efficiency. For relay systems without radio resource sharing, a higher relay SNR gain was required. This implies that the system requires a higher transmit power at the RS. In contrast, radio resource sharing offers high potential for a mobile WiMAX network when relays are deployed. Radio resource sharing is very applicable to MIMO relaying. In addition to multiuser transmission, the relay with radio resource sharing has the potential to achieve optimal multiuser relay gain.
To achieve the practical application of radio resource sharing, a relay system must be based on a topology that fully exploits effective resource assignment based on the spatial separation of nodes. Directional distributed relay topologies were introduced for highly efficient relay deployment. This scheme is fully backward compatible with the current mobile WiMAX standard.
Greater efficiencies in spectrum use can be achieved by coupling channel-quality information in the resource-allocation process. However, this study is heavily dependent upon realistic application environments. Channel modeling becomes critical to reflect application environments. With this purpose, a multihop relay measurement campaign was performed and is presented in this chapter. The measurement results have been analyzed and compared with several statistical and standardized models. It was found that the 802.16d model only matched well to the BS–RS link in terms of path loss and antenna-height correction factor. For the BS–MS link, the WINNER models were a good match to the shadowing, and the COST 231 Hata and WI models were useful for predicting path loss. The RS–MS link was also well matched by the WINNER shadowing values, but none of the path-loss models considered were a good match to any of individual measurement locations.
To demonstrate the potential of the directional distributed relay architecture in a realistic outdoor environment, a site-specific ray-tracing propagation model is used. Results show that, compared to a relay system without resource sharing, the implementation of resource sharing improves capacity significantly. For the OFDMA multiuser transmission, it is ideal to adopt flexible channelization and MIMO LA on different users, and multihop relay with efficient radio resource management.
