RANDOM ACCESS IN WiMAX

Random access in IEEE 802.16 involves the request portion of the request–grant process for network users. A portion of each UL frame is allocated to the contention-based initial access. This contention interval (channel) is divided into ranging (RNG) and BW request regions. These regions are used for initial network entry, ranging, power adjustments, and BW requests for UL transmission. In addition, best-effort data may be sent on the contention channel, but this is only suitable for the transmission of small amounts of data. This data may also include additional requests for resources.

The major tasks that employ the contention channel are initial ranging (IR) and BW requests. IR comprises channel synchronization and ranging procedures during network entry, such as closed loop time–frequency and power adjustments. The contention channel is often termed the Ranging Channel in the IEEE802.16e-2005 standard because BW requests are not necessarily performed on a contention basis, e.g., unsolicited granted service (UGS) and real-time polling services (rtPS) can be employed. In this case, the contention interval is mainly used for IR. A user enters the network by sending a request to the BS to allocate UL resources for data transmission.

The BS evaluates an SS request in the context of the SS service level agreement, and grants resources accordingly. The granted resources are in the form of variable-sized time by frequency bursts in the UL subframe. A burst is a group of subchannels over some PSs. These bursts constitute the major part of the WiMAX frame structure. Burst allocation information is included in the UL-MAP MAC message of the broadcast downlink subframe, specifically in the information element (IE) field. The IE is a data structure that contains complete information about a burst, such as the dimensions in time and frequency, start and end of each burst, and the corresponding physical channel information.

We explain the contention mechanisms and scheduling services for the UL contention channel as defined in the IEEE 802.16d-2004 and IEEE 802.16e-2005 standards.

1 WIMAX QOS AND SCHEDULING SERVICES

The scheduling service employed plays a key role in defining the QoS in WiMAX. It determines the data-handling mechanisms supported by the MAC scheduler for data transport on a connection. Each connection is characterized by a connection identifier (CID) and a set of QoS parameters. The scheduling service determines the number and quality of the UL and DL transmission opportunities, in addition to BW allocation mechanisms.

The five QoS categories in WiMAX are:

• UGS: This class is designed to support delay-intolerant and real-time services with fixed-size data packets such as VOIP. UGS allocates fixed grants to the specified SSs on a periodic real-time basis. Thus an SS using the UGS class does not need to request resources because the size and amount of resources granted are defined at connection setup.
  
  
• rtPS: The rtPS class is designed to support variable-size data packets such as Motion Picture Expert Group (MPEG) video traffic. This service offers real-time unicast request opportunities on a periodic basis as with UGS, but with more request overhead than UGS. rtPS supports variable grant sizes. The BS provides periodic unicast request opportunities during the connection setup phase. This allows the SS to also use unicast request opportunities to obtain UL transmission opportunities. The unicast polling 
  opportunities are typically frequent enough to meet the latency and real-time services requirements.
  
  
• Nonreal-time polling service (nrtPS): nrtPS is similar to rtPS except that the SS uses contention-based polling in the UL to request BW. The BS provides timely unicast opportunities, but the interarrival time of adjacent opportunities is large compared to rtPS. The SSs in a polled group contend for resource request opportunities. The contention environment can result in collisions which require a resolution strategy.
  
  
• Best-effort service (BE): This service class is provided for applications not requiring QoS. Transmission is contention-based so users compete for transmission opportunities and only send data when resources are available.
  
  
• Extended real-time polling service (ertPS): ertPS combines UGS and rtPS so the SS can use UL allocations for both data transmission and resource requests. This allows the SS to accommodate time-varying BW requirements. The SS is only allowed to use this service on non-UGS-related connections (ertPS was introduced only recently in the IEEE802.16e standard for mobile WiMAX).

From the above categories, we see that only the nrtPS and BE scheduling services involve random access and contention-based mechanisms for BW requests. Performance evaluation on the different QoS categories.

2 UPLINK RESOURCE REQUESTS AND GRANT MECHANISMS

During initial network entry, the BS assigns up to three dedicated CIDs to the SS for transmitting and receiving MAC control messages. Connection begins using the basic CID. As mentioned previously, WiMAX DAMA services are given resources on a demand assignment basis (as the need arises). The downlink and UL request–grant mechanisms are distinct. In the downlink, allocation of resources to an SS is done on a CID basis. The BS scheduler allocates BW according to predetermined QoS levels. As MAC PDUs arrive for a CID, the BS determines the resources based on the corresponding QoS and the scheduling algorithm. The BS indicates these allocations in the DL-MAP control message of the downlink subframe. In the UL, the SS is controlled by the overall demand for resources.

Requests can be either standalone (occupying a dedicated MAC PDU for request purposes), or piggybacked on a generic MAC PDU. The UL/BW requests are either incremental or aggregate. When the BS receives an incremental request, it increases the BW granted according to the BW requested. The BW request type field of the MAC PDU indicates whether the request is incremental or aggregate. Because piggybacked requests are on a generic PDU with no type field, they are always incremental. Due to the possibility of collisions, BW requests sent in broadcast or multicast modes must be aggregate. The standard defines three BW request–grant mechanisms. These mechanisms are defined below. Evaluation and performance for various BW request–grant techniques based on QoS mechanisms.

• Unsolicited BW grants: In the UGS mechanism, BW requests are primarily allocated in dedicated slots in the UL subframe. The requests can also be piggybacked on a generic PDU.
  
  
• Unicast polling: Unicast polling or simply polling is the process where dedicated resources are provided to an individual SS in the UL to make BW requests. The BS indicates to the SS the request-allocated slots in the UL-MAP MAC message in the DL subframe to send standalone request PDUs. The BS assigns the polling allocation for requests to the polled SS using the primary CID (one of the three assigned during network entry and initialization). An SS being polled should not remain silent if no BW is needed during polling. Instead, the SS can send a dummy request PDU using padding (all zeros), to fill the allocation field in the current CID. A data grant information element (IE) is associated with the basic CID. Note that implicit UGS users will not be polled unless the Poll-Me bit is set in the header of the packet in the UGS connection.
  
  
• Multicast/broadcast polling—If the SS does not acquire sufficient BW when polled individually, multicast or broadcast polling can be used to poll a group of SSs. As with individual polling, the SS can join a group of SSs in multicast polling to obtain additional BW, but uses the BW allocated in the UL-MAP using a multicast/broadcast CID. All SSs in a polling group contend for request opportunities during the multicast/broadcast polling interval. Intuitively, multicast polling saves BW compared with unicast polling by grouping SSs. To reduce the likelihood of collisions, only SSs with a BW request will reply. However, if several SSs are requesting BW resources, collisions may occur and therefore contention resolution is necessary.

Contention-based requests are allocated in the random access channel. The size of the contention slot is assigned by the BS as a request IE. SSs belonging to a multicast polling group contend for the BW request slots. A collision occurs if two or more SSs simultaneously make requests in the same slot. Therefore, the requesting SSs must employ a contention resolution algorithm to acquire slots to send BW requests. The WiMAX standard does not define an explicit algorithm or strategy by which the SS knows the status of a request sent. In fact, if the SS does not receive the resources sought in the very next DL subframe, the request may be deemed lost or collided. Several strategies have been proposed to acknowledge the status of a request and also for resolution if a collision occurs. To better understand the contention phase and contention resolution in WiMAX, in the next section we define the WiMAX frame allocations and MAC management messages involved with contention and contention resolution mechanisms.