IEEE 802.16e-2005 Specification

The IEEE 802.16e-2005 specification defines the mobile WiMAX network protocols and the related message sequences. The specification consists of the PHY and the MAC layers, as shown in Figure 1. The MAC layer is composed of a security sublayer, a MAC common part sublayer, and a convergence sublayer.

Figure 1: IEEE 802.16e-2005 protocol layers.

Data transmission between a BS and a SS at the PHY layer relies on the resource allocation of data burst through the OFDMA scheme. A BS can transmit to multiple SSs concurrently in the downlink (DL) direction in separate subchannels or separate symbols; similarly, multiple SSs can transmit to the same BS concurrently in the uplink (UL) direction in separate subchannels or separate symbols. Each channel width is from 1.25 to 20 MHz, which spreads to different frequency bands. Therefore, the PHY layer supports orthogonal subchannels for multiple accesses. This design can reduce interference and improve capacity. Moreover, it has the advantages of flexible subchannelization and bandwidth allocation.

The security sublayer is to ensure the privacy of subscribers across the WiMAX network by encrypting connections between a SS and a BS. In addition, a BS can protect against unauthorized access to data transport services by enforcing encryption of the associated service flows across the network. Privacy employs an authenticated client/server key management protocol in which a BS, the server, controls distribution of keying material to a SS, the client. Additionally, the basic privacy mechanisms are strengthened by adding digital-certificate-based SS authentication to its key management protocol.

Over the security sublayer, there are the MAC common part sublayer and the convergence sublayer. The MAC common part sublayer utilizes a shared medium mechanism to efficiently handle the resource of communication links. On top of the MAC common part sublayer is the convergence sublayer, which includes MAC service access points (APs). The MAC layer functionalities in IEEE 802.16e-2005 specification are illustrated in Figure 2.

Figure 2: Medium Access Control (MAC) layer functionalities in the IEEE 802.16e-2005 specification.

In summary, the IEEE 802.16e-2005 specification offers improvements over the technology specified by the original fixed WiMAX standard. These significant improvements can cost-effectively deliver broadband services to end-users, offering increased performance in NLOS environments for mobility and fixed indoor applications. These improvements can be categorized as follows.

§  Mobility: The support for mobility is the major feature of mobile WiMAX, which introduces new MAC for handover and allows a SS to maintain a connection when moving from one BS to another. Mobile WiMAX is designed to support mobility applications up to 160 km/h.

§  High availability: High connection availability in NLOS environments can be supported in mobile WiMAX by using advanced antenna, channel coding, subchannelization, and dynamic modulation technologies to increase link budget.

§  NLOS performance: New technologies have been introduced in mobile WiMAX. These include support for intelligent antenna technology, such as Multiple-Input Multiple-Output (MIMO) and adaptive antenna system (AAS), high-performance coding, such as turbo coding (TC), and a Hybrid Automatic Repeat reQuest (HARQ) mechanism for increasing NLOS performance.

§  Security: Based on the security features of the fixed WiMAX standard, the mobile WiMAX specification introduces a number of enhancements. For example, the AES as well as 3DES are now a mandatory feature. New high-performance coding schemes, such as TC and low-density parity check (LDPC), are included. These features enhance the security of the mobile WiMAX air interface.

§  QoS: Both the connection and service-type-based QoS are designed to meet the requirements of mobile broadband services. These two QoS mechanisms manage both UL and DL directions and support two-way traffic, such as VoIP. The mobile WiMAX QoS has the features of service multiplexing, low data latency, and varying granularity to support real-time broadband multimedia applications.

Overview of the Mobile WiMAX Specification

IEEE 802.16 Standard

The IEEE 802.16 standard, which includes Medium Access Control (MAC) and physical (PHY) layer specifications, aims at supporting Internet services over wireless metropolitan area networks (WMAN). It is also an alternative to traditional wired networks, such as Digital Subscriber Line (DSL) and cable-modem. There are two modes defined in WiMAX networks: point-to-multiple-points (PMP) mode and mesh mode.

In the PHY layer, the IEEE 802.16 standard adopts the orthogonal frequency division multiplexing (OFDM), which is a multicarrier modulation scheme. The IEEE 802.16 standard has two OFDM-based modes: OFDM and orthogonal frequency division multiplexing access (OFDMA). Both of these technologies allow subcarriers to be adaptively modulated (e.g., QPSK, 16-QAM, and 64-QAM), depending on transmission distance and noise. Moreover, OFDMA has scalability to provide efficient use of bandwidth.

The MAC layer of IEEE 802.16 standard was originally designed for the PMP mode. On the later amendments of the IEEE 802.16a and the IEEE 802.16d, the mesh mode was included. The IEEE 802.16a adopts OFDM to provide greater spectral efficiency and to mitigate interference. IEEE 802.16b covers most of the quality of service (QoS) aspects. The IEEE 802.16e introduces scalable OFDMA into the standard, and supports mobile communications. With handover mechanisms, WiMAX is thus able to support mobile communications at vehicular speeds. We summarize the history of the evolution of the IEEE 802.16 standard in Figure 1.

Figure 1: Evolution of the IEEE 802.16 standard.

The IEEE 802.16 working groups on broadband access standards developed the IEEE 802.16 WirelessMAN standard for WMANs. On the other hand, the WiMAX forum was formed in June of 2001 to ensure interoperability among 802.16 products from different vendors. These groups and their activities may help popularizing WiMAX networks and systems by bringing vendors together and improving the specifications.

According to the IEEE 802.16 specification, the non-line-of-sight (NLOS) transmission range is 4 miles. With the combination of soft-switch technologies, a WiMAX network can work as a wireless "last mile" and make a viable alternative to the Public switched telephone network (PSTN) for VoIP services. In addition, a WiMAX network can work as a point-to-point backhaul trunk with a transmission capability of 72 Mbps at a transmission distance over 30 miles. With its technological advantages of throughput, power, transmission range, and versatility, WiMAX might be a strong competitor of other technologies, such as WiFi and 3G. Therefore, from both economical and technical points of view, WiMAX could be an appealing choice for broadband wireless services.

WiMAX Network Architecture

WiMAX has an IP-based wireless access architecture, which contains three parts: user terminal devices, access service network (ASN), and core service network (CSN). A user terminal device can be a fixed or portable/mobile terminal device, which supports the fixed/nomadic/mobile usage scenarios. Each device can establish a connection link to a WiMAX Base Station (BS), and perform authentication and registration through an access gateway in the CSN. The system architecture is illustrated in Figure 2.

Figure 2: Internet Protocol (IP)-based wireless access architecture of WiMAX.

A mobile WiMAX network has a similar architecture as a cellular network, where PMP links are between each BS and multiple Subscriber Stations (SSs). Each BS provides frequency and timing reference to SSs for synchronization purpose. The detailed MAC layer protocols and message sequences will be described later.

Related Work on Queueing Analysis for Wireless Transmission Systems

In general, in a wireless transmission system, arriving packets are buffered into radio link level queue before transmission to the target mobile (in downlink) or to the base station (in uplink). This buffering process causes delay in wireless transmission. Again, the transmission rate of a wireless channel varies due to variations in channel quality and channel errors. Therefore, a queueing analysis which can capture the channel and the radio link level buffer dynamics becomes a useful mathematical tool to investigate the QoS performances of a wireless transmission system. By using a queueing model, the performance results can be obtained more efficiently when compared with simulations. In addition, a queueing analytical model can be used not only to analyze the system's behavior under different parameter settings, but also to optimize the system performances in which several standard techniques in optimization (e.g., Markov decision process) can be applied.

Different queueing models for wireless transmission systems were proposed in the literature. Queueing analyses for a polling system and a system with cyclic service queues with Bernoulli scheduler were presented. These works considered only single rate transmission at the physical layer in which the modulation and coding scheme is fixed. On the other hand, multirate transmission based on adaptive modulation and coding (AMC) has been proposed in most of the current wireless standards to archive higher system capacity. Queueing analysis for radio link level scheduling with adaptive modulation in time division multiple access (TDMA) system was proposed. Also, a queueing model was used for optimizing the radio link level system parameters.

Multimedia services will be common in next generation wireless systems. A queueing model developed specifically for video source (e.g., MPEG traffic) was presented. In addition, the authors demonstrated how the video source coding parameters can be optimally adjusted to achieve the best performance. Again, in a multiservice traffic scenario, prioritization of real-time traffic over best-effort traffic is required. An analytical model for priority queueing was presented.

Combatting transmission errors due to interference and noise is one of the challenging issues in wireless transmission systems. Automatic Repeat reQuest (ARQ) is one of these methods to recover erroneous transmissions. When a transmission fails (i.e., the receiver cannot decode the transmitted information correctly), the receiver requests the transmitter to retransmit. Different variants of ARQ can be used (e.g., probabilistic retransmission, finite and infinite retransmission). Queueing models with ARQ mechanism were proposed. For example, queueing models for go-back-N and selective repeat ARQ were presented. Since with AMC multiple packets can be transmitted in one time slot, if an error occurs, N packets up to the last transmission will be retransmitted for go-back-N ARQ and only the erroneous packets will be retransmitted in case of selective repeat ARQ. Due to the time and space-dependent wireless channel errors and the burstiness of the errors, some connections could experience inferior performance compared with the others. Therefore, a compensation mechanism was introduced to maintain fairness (by allowing more transmissions in the current time frame to compensate errors in the previous time frame) and guarantee the target QoS performance. Also, a queueing model for this compensation mechanism was proposed.

In a multi-user wireless system, packet scheduling is required to allocate the available transmission resources to the ongoing users in a fair and efficient manner. Various packet scheduling policies were proposed in the literature. The most common policy is fair scheduling in which the ongoing users receive services based on their preassigned weights. A queueing model for this scheduling policy in a wireless transmission environment was presented. On the other hand, opportunistic scheduling was developed specifically for wireless transmission systems. This scheduling policy takes advantage of multi-user diversity to improve the throughput of the entire system. In particular, the user who has the best channel quality in the current time slot will be selected to transmit. A queueing model of this scheduling policy was proposed. In addition, queueing analyses for different resource-sharing schemes (i.e., max-min fairness, proportional fairness, and balanced fairness) were presented. Also, the stability condition for each of the schemes was studied.

While most of the queueing models in the literature considered only the variation of wireless channel on system performance, a few works considered the impacts of resource allocation and admission control on queueing performance. For example, impacts of resource allocation and admission were considered in the queueing model for a TDMA-based cellular wireless system  using adaptive modulation and coding, and a code division multiple access (CDMA)-based system  with rate adaptation. These investigations showed that resource reservation for handoff connections (i.e., through guard channel) as well as the transmission rate adaptation can impact the queueing performance of the mobile users significantly.

Queueing analysis can be also used for the development of admission control mechanisms. This approach was presented in the literature. In particular, given the traffic parameters and the estimated wireless channel quality, for a given medium access control mechanism, information on queueing delay and packet loss can be obtained and used by the admission controller to decide whether a new connection can be accepted or not. The decision on acceptance or rejection of a new connection is based on whether the QoS performances of both the ongoing connections and the new connection can be maintained at the target level or not. A queueing model was presented which could be used for admission control in Bluetooth-based wireless personal area networks (PANs). A queueing model for IEEE 802.11-based wireless local area networks (WLANs) was proposed. In these works, the MAC protocol was assumed to be carrier sense multiple access/collision avoidance (CSMA/CA).

In addition to the traditional wireless systems which rely on the Single-Input Single-Output (SISO) transmission, Multiple-Input Multiple-Output (MIMO) system has been developed to provide better performance in terms of error or transmission rate. A queueing model for MIMO system was proposed in which the advantages of spatial diversity in terms of smaller queueing delay and packet loss were demonstrated.

Multihop communication will be a significant component in the next generation wireless systems. In a multihop network, the transmission range of a transmitter (e.g., in the base station) can be extended by relaying the traffic through multiple intermediate nodes. This multihop transmission is a common feature in wireless ad hoc, mesh, and sensor networks. Queueing models for this multihop wireless communication were proposed in the literature. For example, end-to-end performances of a multihop wireless network in terms of latency, reliability, and throughput were studied through a queueing model considering adaptive modulation and coding at the physical layer. A tandem queueing model for multihop transmission was presented in. For sensor networks, energy conservation is one of the challenging issues. Since the amount of energy available at a sensor node is limited (e.g., due to battery size or energy-harvesting technology such as a solar cell), an energy saving mechanism is required and it can impact the wireless transmission performance significantly. A queueing model for sensor networks with energy conservation feature through sleep and wake-up mechanism was presented. A vacation queueing model was used to investigate the inter-relationship between the transmission performance and the energy consumption.