What is WiMAX?

Worldwide Interoperability for Microwave Access (WiMAX) is a broadband access technology that enables low-cost mobile Internet applications and facilitates the convergence of mobile and fixed broadband access in one air interface and network architecture.

WiMAX initially offered 30 to 40 megabit-per-second data rates. With the 2011 update, it provides up to 1 Gbps for fixed stations. It combines OFDMA and advanced MIMO schemes with flexible bandwidth and fast link adaptation to create a very efficient air interface. It is built on all-IP network architecture for plug-and-play network deployments and can support a mix of different usage and service models.

The IEEE 802.16 Working Group was established to develop air interface standards for wireless metropolitan area networks (WMANs). The standards define the structure of the PHY and link layer operations between subscriber stations and base stations.

The network reference model (NRM) identifies key functional entities and reference points over which the network interoperability specifications are defined. The WiMAX NRM differentiates between network access providers (NAPs) and network service providers (NSPs).

NAP is a business entity that provides WiMAX radio access infrastructure. NSP is the business entity that provides IP connectivity and WiMAX services to WiMAX subscribers according to the service level agreement (SLA). One NSP may have a relationship with multiple NAPs in one or different geographical locations. One NAP may be shared by multiple NSPs. In some cases, NSP may be the same business entity as NAP.

WiMAX Network Reference Model (Etemad, 2008)

The WiMAX NRM consists of:

1. Mobile station (MS) is a generalized user equipment set providing wireless connectivity between one or more multiple hosts and the WiMAX network. It may be used generically to refer to both mobile and fixed device terminals.
2. Access service network (ASN) represents a complete set of network functions required to provide radio access to the MS. The functions include layer 2 connectivity with the MS and WiMAX system profile, transfer of authentication, authorization and accounting (AAA), messages to the home NSP, preferred NSP discovery and selection, relay functionality for establishing layer 3 (L3) connectivity with MS (IP address allocation) and radio resource management. To enable mobility, the ASN may also support ASN and CSN anchored mobility paging and location management and ASN-CSN tunneling.
3. Connectivity service network (CSN) is a set of network functions that provide IP connectivity services to WiMAX subscribers. It may further comprise of network elements such as routers, AAA proxy/servers, home agent, user databases and interworking gateways or enhanced network servers to support multicast and broadcast services and location-based services.
4. Base station (BS) consists of the radio related functions of ASN interfacing with an MS over-the-air link according to the MAC and PHY specifications in IEEE 802.16.
5. ASN gateway (ASN-GW) represents an aggregation of centralized functions related to QoS, security and mobility management for all the data connections serviced by its association with BSs through R6. It also hosts functions related to IP layer interactions with the CSN through R3 and interactions with other ASNs through R4 in support of mobility.

Multiple BSs may be logically associated with an ASN. A BS may be logically connected to more than one ASN-GW to allow load balancing and redundancy options.

IEEE 802.16m Protocol Stack

The following MAC/PHY structure is built on a simple OFDMA-based PHY and MAC layer, and is composed of two sublayers: a convergence sublayer and MAC common part sublayer (CPS).

IEEE 802.16m Protocol Stack (Ahmadi, 2009)

The MAC and PHY functions are classified into:

1. Data plane — comprised of functions in data processing, such as header compression, MAC and PHY data packet processing functions.
2. Control plane — consists of layer 2 control functions required to support radio resource configuration, coordination, signaling and management.
3. Management plane — handles external management and system configuration. All management entities fall into this plane.

The MAC layer is composed of two sublayers: Convergence sublayer (CS) and MAC common-part sublayer (MAC CPS). The MAC CPS is divided into two groups, i.e., upper class (resource control and management functional group) and lower class (MAC functional group).

The radio resource control and management functional group comprises of the following functional blocks:

• Radio resource management — adjusts radio network parameters related to the traffic load and includes the functions of load balancing, admission control and interference control.
• Mobility management — scans neighbour BSs and decides whether an MS should perform a handover operation.
• Network entry management — controls initialization and access procedures and generates management messages during initialization and access procedures.
• Location management — supports location-based service (LBS), generates messages (including LBS information) and manages the location update operation during idle mode.
• Idle mode management — controls idle mode operation and generates the paging advertisement message, based on a paging message from the paging controller in the core network.
• Security management — performs key management for secure communication. It uses a managed key to perform traffic encryption/decryption and authentication.
• System configuration management — manages system configuration parameters and generates broadcast control messages.
• Multicast and broadcast service (MBS) — controls and generates management messages and data associated with the MBS.
• Connection management — allocates connection identifiers (CIDs) during initialization/handover service flow creation procedures. It interacts with the convergence sublayer to classify MAC service data unit (MSDU) from upper layers and maps MSDUs into a particular transport connection.

The MAC functional group consists of functional blocks that are related to physical layer and link controls:

• PHY control — performs PHY signaling, such as ranging, measurement / feed back (CQI) and hybrid automatic repeat request (HARQ) (ACK and NAK). The control signaling block generates resource allocation messages.
• Control signaling — generates resource allocation messages (e.g., DL/UL MAP), specific control signaling messages and other signaling messages not in the form of general MAC messages (e.g., DL frame control header).
• Sleep mode management — handles sleep mode operation and generates management messages related to sleep operation and can communicate with the scheduler bock to operate according to the sleep period.
• QoS — performs rate control based on QoS input parameters from the connection management function for each connection
• Scheduling and resource multiplexing — schedules and multiplexes packets based on the properties of the connections.
• ARQ — performs the MAC ARQ functions. For ARQ-enabled connections, the ARQ block splits MSDUs logically and sequences logical ARQ blocks
• Fragmenting/packing — performs the fragmentation or packing of MSDUs based on input from the scheduler blocks.
• MAC PDU formation — constructs MAC protocol data units (PDUs) so that as BS/MS can transmit user traffic or management messages via PHY channels.

The IEEE802.16m protocol structure includes:

• Relay functions — handles relay functionality and packet routing in relay networks.
• Self-organization and self-optimization functions — a plug-and-play operation for an indoor BS (femtocell)
• Multi-carrier functions — controls operation of a number of adjacent or non-adjacent RF carriers, where the RF carriers can be assigned to unicast and/or multicast and broadcast services.
• Multi-radio coexistence functions — protocols for multi-radio coexistence, where the MS generates management messages to report the information about its co-located radio activities obtained from the inter-radio interface. The BS responds with the corresponding messages to support multi-radio coexistence operation.

The IEEE 802.16m defines permanent and temporary addresses for MS to identity the user and its connections during operations. The MS is identified by a unique 48-bit identifier and the following temporary identifiers:

• A station identifier during network entry/re-entry that uniquely identifies the MS within the cell,
• A flow identifier that uniquely identifies the management and transport connection with the MS.

Network entry is the procedure through which an MS detects a cellular network and establishes a connection with the network. An MS is in one of the following states:

1. Initialization State — MS performs the scanning and synchronization based on the BS preamble. As soon as it acquires the system configuration information, it is easy to perform the ranging process and transition to the Access State.

Because each MS has a unique distance from the BS, it is critical that the uplink synchronizes the symbols and equalize the received power levels among the various active MSs — this process is known as ranging.

2. Access State — messages are exchanged in order to initiate the ranging process and UL synchronization. For security purposes, Authentication and Authorization is performed before MS registration to the target BS. During the registration, the MS is associated with a MAC Connection Identifier (CID) and an IP address.
3. Connected State — consists of three modes. Traffic is transmitted and handover is performed when on Active Mode. The MS also needs to scan for other available BS, and to do so, it moves into the Scanning Mode.

   Sleep Mode, which was defined to conserve energy and prolong MS's battery life time, consists of two intervals. The sleep interval is the amount of time when the radio interfaces are periodically shut down. It is followed by a listen interval, during which a MS synchronizes with the serving BS and receives data or traffic indication message. Because packets are buffered when the radio is shut down multiple times, an optional Idle state was introduced.

4. Idle State — the MS can monitor the link channels, transition to the Active Mode and perform handover. The advantage of this implementation is that battery life is prolonged while maintaining delay below the agreed level.

Neighbour search is based on the same DL signals as initial network search except that some information is provided through neighbour advertisement messages by the serving BS.

A connection is identified using the station identifier and flow identifier. There are two types of connections:

1. A management connection is used to carry MAC management messages. It is bidirectional and the predefined values of the flow identifier are reserved for unicast management connections. It is established automatically after the station identifier is assigned to an MS during the initial network entry.
2. A transport connection is used to carry user data including upper-layer signaling messages and data-plane signaling, such as ARQ feedback. It also handles fragmentation and augmentation of the MSDUs. It is unidirectional and is established when a unique flow identifier is assigned during the service-flow establishment procedure. Each active service flow is uniquely mapped to a transport connection.

MAC assigns specific QoS requirements to a unidirectional flow of packets with a service flow. A service flow is mapped to a transport connection with a flow identifier. The BS and MS negotiate the QoS parameter set during the service flow set up / change procedure. The QoS parameters are used to schedule traffic and allocate radio resources. IEEE 802.16m supports adaptation of service-flow QoS parameters.

Quality of Service (QoS)
There are five QoS service classes:

1. Unsolicited Grant Scheme (UGS) provides a fixed period bandwidth allocation. Once the connection is set up, there is no need to send any other requests. It is designed for constant bit rate (CBR) real-time traffic such as E1/T1. The main QoS parameters are maximum sustained rate, maximum latency and tolerated jitter.
2. Extended Real Time Polling Service (ertPS) is designed to support VoIP with silence suppression — no traffic is sent during silent periods. It is similar to UGS in that the BS allocates the maximum sustained rate in active mode, but no bandwidth is allocation during the silent period. BS need not poll the MS during the silent period. The QoS parameters are the same as those in UGS.
3. Real Time Polling Service (rtPS) is for variable bit rate (VBR) real-time traffic, such as MPEG compressed video. Its bandwidth requirements vary, therefore, the BS has to regularly poll each MS to determine what allocations need to be made. The QoS parameters are similar to UGS but minimum reserved traffic rate and maximum sustained traffic rate has to be specified separately. For UGS and ertPS, these two parameters are the same, if present.
4. Non Real Time Polling Service (nrtPS) is for non-real-time VBR traffic with no guarantee delay. Only a minimum rate is guaranteed. An example of application is FTP.
5. Best-Effort Service (BE) — most data traffic falls into this category. It guarantees neither delay nor throughput. Bandwidth is allocated to the MS if and only if there is a leftover bandwidths from other classes. In practice, most implementations allow specifying minimum reserved traffic rate and maximum sustained traffic rate.

The PHY Layer
The PHY layer supports both TDD and FDD. The FDD mode also defines a half duplex FDD mode to support lower complexity terminals where one radio front unit is time-shared between UL and DL. In the OFDM frame structure for TDD mode, each 5 ms radio frame is divided into DL and UL sub-frames. These sub-frames are separated by small transmit/receive and receive/transmit transition gaps to prevent DL and UL transmission collisions. The frame structure defines the following physical channels:

• Preamble — broadcast in the first OFDMA symbol of the frame in DL and used by the MS initial and handover related scanning, and PHY synchronization with the BS.
• Frame control header (FCH) — follows the preamble and provides the frame configuration information, such as media access protocol (MAP) message length and coding scheme and usable sub-channels.
• DL-MAP and UL-MAP — provides resource allocation and other control information for the DL an UL sub-frames, respectively. The MAP is broadcast across the cell using a robust modulation and coding scheme (MCS).
• UL ranging — this sub-channel is allocated for an MS to perform closed-loop time, frequency and power adjustment as well as bandwidth requests.
• UL CQICH — this channel is allocated for the MS to feedback channel state information.
• UL ACK — is allocated for the MS to feedback DL HARQ ACKs.

Handover (HO)Procedure

The HO procedure is divided into three stages: HO initialization, HO preparation and HO execution. Upon completing HO execution, the MS is ready to perform network re-entry with the target BS. The HO cancellation procedure is defined to allow an MS to cancel the HO procedure.

The HO preparation is completed when the serving BS informs the MS of its HO decision through the HO control command. The control signalling includes an action time for the MS to start network re-entry with the target BS and indicates whether the MS should maintain communication with the serving BS during network re-entry. If the communication cannot be maintained, the serving BS stops allocating resources to the MS in action time.

If directed by the serving BS through the HO command, the MS performs network re-entry with the target BS during action time while continuously communicating with the serving BS. The MS cannot exchange data with the target BS before completing network re-entry.

Power Saving Modes

The MS negotiates periods of absence from the serving BS using the sleep mode state, which allows the MS to alternate between listening and sleep windows. The listening window is the time interval during which the MS is available to transmit/receive control signaling and data. The MS can dynamically adjust the duration of sleep and listening windows within a sleep cycle based on changing traffic patterns and HARQ operations.

When the MS is in active mode, it negotiates the sleep parameters with the BS. The BS instructs the MS to enter the sleep mode. MAC management messages are used for sleep mode request/response. The sleep cycle period is measured in units of frames or superframes and is the sum of the sleep and listening windows. During the MS listening window, the BS can transmit the traffic indication message intended for one or multiple MSs.

The idle mode allows the MS to become periodically available for DL broadcast-traffic messaging (e.g., paging) without registering with the network. The network assigns MSs in idle mode to a paging group during idle mode entry or location update. The MS monitors the paging message during the listening interval. The start of the paging-listening interval is calculated based on the paging cycle and the paging offset.

The serving BS transmits the list of paging group identifiers at a predetermined location at the beginning of the paging-available interval. The paging mechanism involves two step, i.e., a paging indication followed by a full paging message. The paging message contains the identification of the MSs to be notified of pending traffic or a location update.

Security

Security functions provide subscribers with privacy, authentication and confidentiality across the network. The MAC packet data units are encrypted over the connections between the MS and BS. The security architecture is divided into:

• Security management — handles overall security management and control, Extensible Authentication Protocol (EAP), encapsulation/decapsulation, privacy, key management control, security association management and identity/location privacy.
• Encryption and integrity logical entities — handles user data encryption and authentication, management message authentication and message confidentiality protection.

Source

• IEEE 802.16: Broadband Wireless Metropolitan Area Networks (MANs)
• K. Etemad, "Overview of Mobile WiMAX Technology and Evolution", IEEE Communications Magazine, Volume 46 Issue 10, 2008
• S. Ahmadi, "An Overview of Next-Generation Mobile WiMAX Technology", IEEE Communications Magazine, Volume 47, Issue: 6, 2009
• What is WiMAX?