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.
