Relaying

While a variety of MIMO techniques can be utilized at the BS side, the MS options are limited. In fact, it is physically challenging for a MS to support multiple antennas. Consequently, the network designer needs to find a solution to bring the MIMO virtues to MSs especially at the cell-edge. Such a solution has been found in cooperative communications techniques, also known as cooperative diversity, [8, 9]. Using these techniques, single-antenna MSs can enjoy the MIMO advantages through mutually relaying their signals to the BS. However, this cooperation overcomplicates the processing at the BS side as well as the pricing policies. In addition, it will cause a significant reduction in the MSs battery-lives, which is a critical issue for the users. Consequently, dedicated Relay Stations (RSs) have been proposed to replace the user-cooperation. These stations are not given high-processing or decision-making capabilities; hence they are much cheaper than BSs. As a result, they can be used to increase the coverage area, reduce the transmission range from and to the MSs, hence increasing their achievable throughputs by increasing their Signal to Noise Ratios (SNRs). Unlike BSs, RSs access the network backbone through the BSs. Hence, careful resource allocation strategies are needed. For all these reasons, RSs provide a lower Operational Expenditure (OPEX) and Capital Expenditure (CAPEX) option that allows faster roll out and a flexible configuration.

A variety of RS deployment options can be considered, ranging from the low complexity repeaters to more sophisticated relaying. These variations can help the operator choose the scenario that suits operational needs. The utilization of traditional Amplify-and-Forward (AF) RSs precedes the introduction of IMT-advanced technologies. The role of AF, however, will become increasingly important as it is the most basic and cost-effective form of enhancing communication experience, especially for cell-edge users. AF relays operate in a continuous and nonselective or non-discriminate mode, that is, their operation is not controlled by the BS. On the other hand, Layer 1 (L1) relaying can be selective. Similar to basic repeaters, L1 relays are transparent to MSs. However, the BS controls the RSs transmission power as well as the identity of the RS-served MSs. This also extends to scheduling and retransmission, that is ARQ control.

Both standardization bodies classify RSs based on the deployment objective into two main types, transparent tRSs (Figure 2.8), and non-transparent ntRSs (Figure 2.9). tRS operate within a BS's cell coverage where MSs fully recognize the BS's control message, but have their UL transmission go through the RS. Hence, tRSs aim at expanding the cell's capacity. On the other hand, ntRS are utilized in instances in which MSs are beyond a BS's coverage, and rely fully on the RS for both DL and UL signaling and data transfer. Thus, ntRSs aim at expanding the cell's coverage area.

Note that frequency reuse can also be applied in relay assisted networks, despite being more challenge. The use of a directional antenna at the BS and both omnidirectional and directional antenna at the RS can help to reduce ICI and improve the system capacity. Such a setup would increase the complexity at the BS and RS. Also, in the case of antenna radiation diagrams overlap regions, some frequency channels subsets can be allocated to those MSs in the overlap areas, while other subsets can be assigned to those MSs in the non overlap areas. This mechanism will reduce the ICI in the overlap regions and improve the fairness among MSs in the cell.

Figure 2.8 Network where transparent RSs are used for capacity enhancement.