In 2006 the IEEE started to work on “Mobile Multihop Relay (MMR)” under 802.16j framework. IEEE 802.16j focuses enhancements to OFDMA physical layer and MAC layer to enable operation of a wireless relay station in order to enhance coverage, throughput, and system capacity with multihop relay capabilities and functionalities of interoperable relay stations.
One of the main operational expenditure drivers in mobile networks today is the requirement to connect each base station directly to the network. Typically, this is fulfilled by installing a cable or microwave connection at each base station. Currently, UMTS/HSPA base stations use E-1 or T-1 links, each capable of transmitting about 2Mbps. However, this backhaul link capacity will increase with OFDMA air link. Also consider the situations where the demand for capacity is ad hoc, then installing a base station would be wasteful. The basic idea behind MMR is to allow WiMAX base stations that do not have a backhaul connection to communicate with base stations that do with some portion of the air link bandwidth. It is a simple and elegant way of extending network. This operation requires following modifications:
• A new frame structure to support in-band relaying
• Extension to security sublayer
• Enhanced MAC layer to handle bandwidth requests, handover, and packet delivery, etc.
The proposed modifications apply to new BS and RS (Relay Station) and work with existing 802.16e MS. IEEE 802.16j considers two types of relay modes as seen in the illustration below: transparent relay and nontransparent relay.
Transparent Relay Stations (T-RS) are typically considered for the situations where base stations’ control information can reach the MS but data can be relayed through the transparent relay nodes. Control information such as preamble, FCH, MAP, and DCD/UCD messages are sent with the robust modulation and also can be fortified with multiple antenna systems. But, downlink coverage does not incur the same uplink coverage. As a result, MS can talk to BS through Transparent RS in the uplink.
Nontransparent Relay Stations (NT-RS) work like a base station and they are allowed to transmit downlink control information. This type of operation is suitable when BS cannot reach the MS.
