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.

ADMISSION CONTROL AND BANDWIDTH ALLOCATION

In general, admission control is a network’s QoS mechanism that determines whether a new session (or connection), with given bandwidth and delay requirements, can be established or not. For providing QoS, this procedure has been applied to both wireline and wireless networks. In the case of WiMAX, whenever a new session wants to make use of the wireless network, an admission control request is sent by the SS to the BS. This admission control request will be accepted by the BS if there is enough available bandwidth, QoS guarantees for bandwidth and delay can be met and the QoS of existing connections is not disturbed. An admission control scheme for WiMAX has been proposed together with the derivation of rules for each of the four classes of WiMAX. In addition, a token bucket based admission control for rtPS flows has been proposed. Omitting any further discussion involving admission control, we now present a brief overview of bandwidth allocation mechanisms in WiMAX.

The BS allocates bandwidth on a per SS basis, known as the grant per subscriber station (GPSS); further, each SS distributes this bandwidth among all of its active connections. The SS can efficiently distribute the allocated bandwidth as it has up-to-date information about the queue status of each connection. Thus, GPSS requires a packet scheduler at each SS, which may increase the complexity and the cost of an SS. However, GPSS is scalable to a large number of SSs and is, therefore, the only bandwidth allocation mechanism being employed in the current WiMAX.^[*]

With GPSS, each SS treats various connections separately at its own level and these are then pooled together as one entity for bandwidth allocation at the BS. Thus, the scheduler at the BS will only need a small amount of information about the overall bandwidth required by a particular SS. This approach has the additional advantage of avoiding the time lag in receiving updated information about individual connections at the SS. Once a lump of bandwidth has been granted to a particular SS, then it is responsible for the appropriate scheduling, according to priorities and the QoS for each active connection. This process greatly reduces the workload on the BS. For instance, suppose that an urgent packet arrives at the SS, then the BS does not need to have information about it and it is the duty of the scheduler at the SS to provide the required throughput and delay.

BS and SS communicate with each other by using a bidirectional path, viz., Uplink (UL: SS to BS) and Downlink (DL: BS to SS); whereas the bandwidth requirements are made by the UL and grants are made by the DL. WiMAX supports both Frequency Division Duplex (FDD) and Time Division Duplex (TDD) modes as shown in Figures 1 and 2, respectively.

Figure 1: An example of Burst FDD bandwidth allocation. (From IEEE-802.16-2004, IEEE standard for local and metropolitan area networks—Part 16: An interface for fixed and mobile broadband wireless access systems, October 2004. With permission.)

Figure 2: Frame structure of TDD. (From IEEE-802.16-2004, IEEE standard for local and metropolitan area networks—Part 16: An interface for fixed and mobile broadband wireless access systems, October 2004. With permission.)

In FDD mode, both the UL and DL are operating at separate frequencies and DL data can be sent in bursts. To facilitate various types of modulation, a fixed duration frame is used for both the DL and UL transmissions. Also, it allows the simultaneous use of both full and half duplex SSs; a full duplex SS can transmit and receive data at the same time whereas a half duplex SS can either transmit or receive data at any given time. If half duplex SSs are used, then bandwidth controller will not allocate UL bandwidth at the same time that it is expecting to receive data on DL channel. It should also take into account the allowance for propagation delay, SS transmit/receive transition gap, and SS receive/transmit transition gap. In FDD mode, the use of a fixed duration frame, for both the DL and UL channels, also helps in simplifying the design of algorithms for bandwidth allocation. It can be noted that a full duplex SS can listen to a DL channel continuously, whereas the half duplex SS can only listen to a DL when it is not transmitting on the UL channel.

In the case of TDD, the UL and DL transmissions occur at different time intervals, while usually employing the same frequency. A TDD frame is also of fixed duration and is composed of one DL and one UL subframe. For easy partitioning of bandwidth, a TDD frame is divided into an integer number of physical slots. Also, TDD framing is adaptive and the bandwidth allocated to the UL and DL parts can vary and is controlled by the higher layers. The DL-MAP and UL-MAP messages define the usage of the corresponding transmission intervals. The BS also regularly transmits DL and UL channel descriptors, DCD and UCD, for the physical description of the corresponding channel. The complete list of MAC management messages. It should be noted that a WiMAX network can be planned with either FDD or TDD, but the former mode has been discussed more frequently in the literature.