Unsolicited Grant Service

The UGS algorithm is designed to support real-time service flows, such as Voice over Internet Protocol (VoIP), that generate fixed size data packets periodically. BS periodically assigns fixed-size grants to voice users. These grants are sufficient to send voice data packets generated by the maximum data rate of enhanced variable rate “voice” codec (EVRC). The grant period are negotiated during the initialization process of the connection. Thus, MAC overhead and UL access delay caused by bandwidth request process are minimized. The drawback of the UGS algorithm is the following. Generally, voice users do not always have voice data packets to send throughout the duration of a connection, because voice users have frequent silence periods. A typical voice codec switches intermittently between “on” and “off” states as illustrated in Figure 1. While in “on” state, popular voice codecs like the EVRC also have variable data rates. For example, the EVRC operates at 1/8 of the full data rate during the off state, while the device has three different rates during the on state (rates 1, 1/2, and 1/4). Therefore for a UGS algorithm that reserves a flat amount of resources capable of sending data at the maximum rate of EVRC periodically, a significant amount of UL resources is wasted when the codec is in silence (or off) mode as well as when the codec is on but not operating at the full rate. This is illustrated in Figure 2. A number of other algorithms have thus been designed to adaptively determine the actual UL needs of each connection during frame periods, so as to minimize these resources wastage.

 
Figure 1: Voice codec status.

 
Figure 2: UL resource allocation using UGS algorithm.

Scheduling Algorithms

In the WiMAX standard (802.16), four scheduling classes have been defined: Unsolicited grant service (UGS), real-time polling service (rtPS), nonreal-time polling service (nrtPS), and best effort (BE). As illustrated in Figure 1, each traffic connection is associated with one of the four scheduling services, and the SS scheduler selects packets to be transmitted from each queue based on the scheduling policy employed. Usually, the scheduler selects packets to be transmitted from the highest priority queue that is not empty. Transmission of packets from lower priority queues are postponed until there is no packet available to send from a higher priority queue. Since UL traffic is generated at SS, the SS scheduler is able to arrange the transmission based on the up-to-date information on the current numbers/status of UL connections, which help to improve QoS performance. In the following, we review the various scheduling algorithms provided in the standard for handling the transmissions of packets belonging to these various services.

 
Figure 1: UL scheduling at the SS.

QOS SCHEDULING AND ADAPTIVE RESOURCE ALLOCATIONS

IEEE 802.16 supports fixed-length frame, with flexible (adaptive) DL/UL resource usage ratios. The BS adaptively adjusts DL and UL subframe lengths on a frame-by-frame basis depending on the DL/UL traffics and channel conditions. Typically, the DL:UL resources can be varied from 3:1 to 1:1 in a PMP WiMAX network. Figure 1 illustrates the fixed-length frame in the PMP WiMAX network, and the flexible DL/UL subframes. The figure also depicts the network entry process for subscriber stations (SS) and the scheduling periods for assigning transmission opportunities to SS already initiated into the network. For access (PMP) mode, new SS detects preamble and frame control header (FCH), and identifies the number of DL burst transmissions from the DL MAP in the FCH. At the end of the last DL burst (Figure 1), new SS uses a contention period to exchange network entry request signal with the BS. If successful, the BS process the request and sends entry instruction (assigned DL/UL transmission opportunities, power, etc.) in the DL/UL MAPS of the next frame, and the SS gets initiated into the network. For the mesh mode, new SS waits for network entry signal broadcast at the beginning of a frame, to which they can respond within a specified period. Scheduling process is used for initiating new SS into the network. SS transmits on the scheduled slots.

 
Figure 1: Adaptive DL/UL subframes in WiMAX standard.

In the WiMAX standard (802.16e), UL and DL assignments are based on time division multiple access (TDMA). In each frame, the BS scheduler assigns UL and DL transmission opportunities to SS until their negotiated data periods expire. The resources given to an SS for its data transmission are both in the frequency and time domain. WiMAX MAC thus supports frequency-time resource allocation in both DL and UL on a per-frame basis. The resource allocation is delivered in media access protocol (MAP) messages at the beginning of each frame. Therefore, the resource allocation can be dynamically changed frame-by-frame in response to traffic and channel conditions. Additionally, the amount of resource in each allocation can range from one slot to the entire frame in the time domain, and from one subchannel to the entire subchannels in an OFDM symbol, in frequency domain. Also WiMAX employs fast scheduling both in the DL and UL to respond to fast variations in channel conditions. This fast and fine granular resource allocation allows superior QoS for data traffic in a bursty traffic and rapidly changing channel condition. The fundamental premise of the IEEE 802.16 MAC architecture is QoS. It defines services flows which can map to Diffserve code points or MPLS flow labels that enable end-to-end IP-based QoS. Additionally, subchannelization and MAP-based signaling schemes provide a flexible mechanism for optimal scheduling of space, frequency, and time resources over the air interface on a frame-by-frame basis. This flexible scheduling allows QoS to be better enforced and enable support for guaranteed service levels including committed and peak information rates, latency, and jitter for various types of traffic on a customer-by-customer basis.