Data-Plane and Control-Plane Functions in Relay Stations

Multi-hop relay is an optional entity that may be deployed in conjunction with base stations to provide additional coverage or performance improvements in a radio access network. In relay-enabled networks, the BS may be replaced by a multi-hop relay BS (i.e., a BS that supports relay capability over the relay links) and one or more relay stations (RS). The traffic and signaling between the mobile station and relay-enabled BS are relayed by the RS, thus extending the coverage and performance of the system in areas where the relay stations are deployed. Each RS is under the control of a relay-enabled BS. 

In a multi-hop relay system, the traffic and signaling between an access RS and the BS may also be relayed through intermediate relay stations. The RS may either be fixed in location or it may be mobile. The mobile station may also communicate directly with the serving BS. The various relay-enabled BS features defined in the IEEE 802.16j-2009 standard allow a multi-hop relay system to be configured in several modes. The air interface protocols, including the mobility features on the access link (i.e., RS-MS link), remain unchanged.

The IEEE 802.16j-2009 standard specified a set of new functionalities on the relay link to support the RS–BS communication. Two different modes; i.e., centralized and distributed scheduling modes, were specified for controlling the allocation of bandwidths for an MS or an RS. In centralized scheduling mode the bandwidth allocation for subordinate mobile stations of an RS is determined at the serving BS. On the other hand, in distributed scheduling mode the bandwidth allocation of the subordinate stations is determined by the RS, in cooperation with the BS. Two different types of RS are defined, namely transparent and non-transparent. A non-transparent RS can operate in both centralized and distributed scheduling mode, while a transparent RS can only operate in centralized scheduling mode. A transparent RS communicates with the base station and subordinate mobile stations using the same carrier frequency. A non-transparent RS may communicate with the base station and the subordinate mobile stations via the same or different carrier frequencies. 

Relaying in the IEEE 802.16m system is performed using a decode-and-forward paradigm and supports TDD and FDD duplex modes. In TDD deployments, the relay stations operate in time-division transmit and receive (TTR) mode,xii whereby the access and relay link communications are multiplexed using time division multiplexing over a single RF carrier. In the IEEE 802.16m system, the relay stations operate in non-transparent mode, which essentially means that the relay stations compose and transmit the synchronization channels, system information, and the control channels for the subordinate stations. In any IEEE 802.16m deployment supporting relay functionality, a distributed scheduling model is used where each infrastructure station (BS or RS) schedules the radio resources on its subordinate links. In the case of a relay station, the scheduling of the resources is within the radio resources assigned by the BS. The BS notifies the relay and mobile stations of the frame structure configuration. The radio frame is divided into access and relay zones. In the access zone, the BS and the RS transmit to, or receive from, the mobile stations. In the relay zone, the BS transmits to the relay and the mobile stations, or receives from the relay and mobile stations. The start times of the frame structures of the BS and relay stations are aligned in time. The BS and relay stations transmit synchronization channels, system information, and the control channels to the mobile stations at the same time.

The MAC layer of a relay station includes signaling extensions to support functions such as network entry of an RS and of an MS through an RS, bandwidth request, forwarding of PDUs, connection management, and handover. Two different security modes are defined in the IEEE 802.16j-2009 standard: (1) a centralized security mode that is based on key management between the BS and an MS; and (2) a distributed security mode which incorporates authentication and key management between the BS and a non-transparent access RS, and between the access-RS and an MS. An RS may be configured to operate either in normal CID allocation mode, where the primary management, secondary, and basic CIDs are allocated by the BS, or in local CID allocation mode where the primary management and basic CID are allocated by the RS. 

The IEEE 802.16m RS uses the same security architecture and procedures as an MS to establish privacy, authentication, and confidentiality between itself and the BS on the relay link. The IEEE 802.16m relay stations use a distributed security model. The security association is established between an MS and an RS during the key exchange similar to a macro BS. The RS uses a set of active keys shared with the MS to perform encryption/decryption and integrity protection on the access link. The RS runs a secure encapsulation protocol with the BS based on the primary security association. The access RS uses a set of active keys shared with the BS to perform encryption/decryption and integrity protection on the relay link. The MAC PDUs are encapsulated within one relay MAC PDU and are encrypted or decrypted by primary security association, which is established between the RS and the BS. The security contexts used for the relay link (between a BS and an RS) and the access links (between an RS and an MS) are different and are maintained independently. The key management is the same as that performed by a macro BS.