Comparing Relay Adoption

Wireless multihop relaying entails delivering an MT's connection to the BS through dedicated RSs. Both candidate technologies show interest in introducing and enhancing relayed transmission. LTE-Advanced defines two types of relay: Type-I and Type-II, while IEEE 802.16j defines transparent and non-transparent relaying. The main objective of relaying in IMT-Advanced systems is to extend the cell coverage through the RS and enhance the overall cell throughput. Type I and non-transparent RSs extend the BS's coverage to include MTs that cannot connect directly to the BS. Such RSs are required to broadcast control information to the MTs as the MTs cannot receive the BS's own control transmission. Meanwhile, Type II RSs in LTE-Advanced and transparent RSs in IEEE 802.16m are not used for extending a BS's coverage. Rather, they are used to enhance the service quality and the link capacity within the cell coverage area. These RSs do not need dedicated control messages as the MTs can identify the BS's own control messages.

Relaying aids in meeting the user requirements from three perspectives: increased coverage, higher throughput and improved reliability. Through relaying, a user can easily roam over considerably longer distances with the support of the same network technology. In areas with strong fading, a RS enhances network connectivity and reliability, and extends its coverage. However, Type II and transparent relaying are used to realize higher throughput and data rates, supporting multimedia applications and providing for their QoS requirements.

Little difference can be noted between the Type II RSs and the transparent RSs. There are, however, apparent differences between the Type I RSs and the non-transparent RSs. Type I RSs are limited to two hops, while non-transparent RSs are unlimited in the number of hops. This limitation, reflected in the relevant designs of frame structures and signaling, gives an advantage to IEEE 802.16m (which is based on IEEE 802.16j for multihop communication) over LTE-Advanced. IEEE 802.16m can widen the coverage area in a cost-efficient manner. However, it should be remarked that multiple hops results in substantial increase in the delay.