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In last couple of years, wireless communications has grown from an obscure, unknown service to a ubiquitous technology that serves almost half of the people in Earth. As the technological development is leaping into the next generation, demand for high speed internet services is growing exponentially. Traditional solutions for broadband access using wired connections, such as Ethernet, Cable connections, DSL lines and Fibre Optics provide high speed broadband access but these sorts of connections are hard to maintain as well as expensive especially in remote and rural areas; and for this reason, service providers are less interested to provide service in distance remote areas.
IEEE 802.16 standardizes the WiMAX which is an air interface standard suite that provides the wireless technology for mobile data access. Stationary transmission was declared by the IEEE 802.16-2004 standard and is designed for, and the 802.16e amendment deals with both stationary and mobile transmissions. WiMAX employs orthogonal frequency division multiplexing (OFDM), and supports adaptive modulation and coding depending on the channel conditions. WiMAX wireless systems cover large geographic areas without the need for a costly cable infrastructure to each service access point. WiMAX offers cost-effective and quickly deployable alternative to cabled networks such as fibre optic links, traditional cable or digital subscriber lines, or T1 networks. WiMAX offers fast deployment and a cost-effective solution to the last-mile wireless connection problem in metropolitan areas and underserved rural areas. For mass adoption and large-scale deployment of a broadband wireless access system, it must support quality of service (QoS) for real-time and high- bandwidth applications. The IEEE 802.16 standard is a QoS-rich platform. Different access methods are supported for different classes of traffic. Best Effort traffic is one of the most important of these classes as it represents the majority of the overall data traffic. WiMAX employs a reservation-based MAC technology. Reservation-based MAC protocols have been a primary access method for broadband access technologies. For example, the General Packet Radio System, Digital Subscriber Line DSL, and Hybrid Fibre Coaxial HFC cable technologies employ reservation based multiple access systems.
The IEEE 802.16 group was formed in 1998 to develop an air-interface standard for wireless broadband. The group's initial focus was the development of a LOS-based point-to-multipoint wireless broadband system for operation in the 10GHz-66GHz wave band. The resulting standard-the original 802.16 standard, completed in December 2001-was based on a single-carrier physical (PHY) layer with a burst time division multiplexed (TDM) MAC layer. Many of the concepts related to the MAC layer were adapted for wireless from the popular cable modem DOCSIS (data over cable service interface specification) standard. The IEEE 802.16 group subsequently produced 802.16a, an amendment to the standard, to include NLOS applications in the 2GHz-11GHz band, using an orthogonal frequency division multiplexing (OFDM)-based physical layer. Additions to the MAC layer, such as support for orthogonal frequency division multiple access (OFDMA), were also included. Further revisions resulted in a new standard in 2004, called IEEE 802.16-2004, which replaced all prior versions and formed the basis for the first WiMAX solution. These early WiMAX solutions based on IEEE 802.16-2004 targeted fixed applications, and we will refer to these as fixed WiMAX. In December 2005, the IEEE group completed and approved IFEEE 802.16e-2005, an amendment to the IEEE 802.16-2004 standard that added mobility support. The IEEE 802.16e-2005 forms the basis for the WiMAX solution for nomadic and mobile applications and is often referred to as mobile WiMAX. The basic characteristics of the various IEEE 802.16 standards are summarized in Table 2.1. Note that these standards offer a variety of fundamentally different design options. For example, there are multiple physical-layer choices: a single-carrier-based physical layer called Wireless-MAN-SCa, an OFDM-based physical layer called WirelessMAN-OFDM, and an OFDMAbased physical layer called Wireless-OFDMA. Similarly, there are multiple choices for MAC architecture, duplexing, frequency band of operation, etc. These standards were developed to suit a variety of applications and deployment scenarios, and hence offer a plethora of design choices for system developers. In fact, one could say that IEEE 802.16 is a collection of standards, not one single interoperable standard. For practical reasons of interoperability, the scope of the standard needs to be reduced, and a smaller set of design choices for implementation need to be defined. The WiMAX Forum does this by defining a limited number of system profiles and certification profiles. A system profile defines the subset of mandatory and optional physical- and MAC-layer features selected by the WiMAX Forum from the IEEE 802.16-2004 or IEEE 802.16e-2005 standard. It should be noted that the mandatory and optional status of a particular feature within a WiMAX system profile may be different from what it is in the original IEEE standard. Currently, the WiMAX Forum has two different system profiles: one based on IEEE 802.16-2004, OFDM PHY, called the fixed system profile; the other one based on IEEE 802.16e-2005 scalable OFDMA PHY, called the mobility system profile. A certification profile is defined as a particular instantiation of a system profile where the operating frequency, channel bandwidth, and duplexing mode are also specified. WiMAX equipment are certified for interoperability against a particular certification profile. The WiMAX Forum has thus far defined five fixed certification profiles and fourteen mobility certification profiles (see Table 2.2). To date, there are two fixed WiMAX profiles against which equipment have been certified. These are 3.5GHz systems operating over a 3.5MHz channel, using the fixed system profile based on the IEEE 802.16-2004 OFDM physical layer with a point-to-multipoint MAC. One of the profiles uses frequency division duplexing (FDD), and the other uses time division duplexing (TDD).
2.2 Salient Features of WiMAX
WiMAX is a wireless broadband solution that offers a rich set of features with a lot of flexibility in terms of deployment options and potential service offerings. Some of the more salient features that deserve highlighting are as follows:
OFDM-based physical layer: The WiMAX physical layer (PHY) is based on orthogonal frequency division multiplexing, a scheme that offers good resistance to multipath, and allows WiMAX to operate in NLOS conditions. OFDM is now widely recognized as the method of choice for mitigating multipath for broadband wireless. Chapter 4 provides a detailed overview of OFDM.
Very high peak data rates: WiMAX is capable of supporting very high peak data rates. In fact, the peak PHY data rate can be as high as 74Mbps when operating using a 20MHz2 wide spectrum. More typically, using a 10MHz spectrum operating using TDD scheme with a 3:1 downlink-to-uplink ratio, the peak PHY data rate is about 25Mbps and 6.7Mbps for the downlink and the uplink, respectively. These peak PHY data rates are achieved when using 64 QAM modulations with rate 5/6 error-correction coding. Under very good signal conditions, even higher peak rates may be achieved using multiple antennas and spatial multiplexing.
Scalable bandwidth and data rate support: WiMAX has a scalable physical-layer architecture that allows for the data rate to scale easily with available channel bandwidth. This scalability is supported in the OFDMA mode, where the FFT (fast fourier transform) size may be scaled based on the available channel bandwidth. For example, a WiMAX system may use 128-, 512-, or 1,048-bit FFTs based on whether the channel bandwidth is 1.25MHz, 5MHz, or 10MHz, respectively. This scaling may be done dynamically to support user roaming across different networks that may have different bandwidth allocations.
Adaptive modulation and coding (AMC): WiMAX supports a number of modulation and forward error correction (FEC) coding schemes and allows the scheme to be changed on a per user and per frame basis, based on channel conditions. AMC is an effective mechanism to maximize throughput in a time-varying channel. The adaptation algorithm typically calls for the use of the highest modulation and coding scheme that can be supported by the signal-to-noise and interference ratio at the receiver such that each user is provided with the highest possible data rate that can be supported in their respective links. AMC is discussed in Chapter 6.
Link-layer retransmissions: For connections that require enhanced reliability, WiMAX supports automatic retransmission requests (ARQ) at the link layer. ARQ-enabled connections require each transmitted packet to be acknowledged by the receiver; unacknowledged packets are assumed to be lost and are retransmitted. WiMAX also optionally supports hybrid-ARQ, which is an effective hybrid between FEC and ARQ.
Support for TDD and FDD: IEEE 802.16-2004 and IEEE 802.16e-2005 supports both time division duplexing and frequency division duplexing, as well as a half-duplex FDD, which allows for a low-cost system implementation. TDD is favored by a majority of implementations because of its advantages: (1) flexibility in choosing uplink-to-downlink data rate ratios, (2) ability to exploit channel reciprocity, (3) ability to implement in nonpaired spectrum, and (4) less complex transceiver design. All the initial WiMAX profiles are based on TDD, except for two fixed WiMAX profiles in 3.5GHz.
Orthogonal frequency division multiple access (OFDMA): Mobile WiMAX uses OFDM as a multiple-access technique, whereby different users can be allocated different subsets of the OFDM tones. As discussed in detail in Chapter 6, OFDMA facilitates the exploitation of frequency diversity and multiuser diversity to significantly improve the system capacity.
Flexible and dynamic per user resource allocation: Both uplink and downlink resource allocation are controlled by a scheduler in the base station. Capacity is shared among multiple users on a demand basis, using a burst TDM scheme. When using the OFDMA-PHY mode, multiplexing is additionally done in the frequency dimension, by allocating different subsets of OFDM subcarriers to different users. Resources may be allocated in the spatial domain as well when using the optional advanced antenna systems (AAS). The standard allows for bandwidth resources to be allocated in time, frequency, and space and has a flexible mechanism to convey the resource allocation information on a frame-by-frame basis.
Support for advanced antenna techniques: The WiMAX solution has a number of hooks built into the physical-layer design, which allows for the use of multiple-antenna techniques, such as beam forming, space-time coding, and spatial multiplexing. These schemes can be used to improve the overall system capacity and spectral efficiency by deploying multiple antennas at the transmitter and/or the receiver. Chapter 5 presents detailed overview of the various multiple antenna techniques.
Quality-of-service support: The WiMAX MAC layer has a connection-oriented architecture that is designed to support a variety of applications, including voice and multimedia services. The system offers support for constant bit rate, variable bit rate, real-time, and non-real-time traffic flows, in addition to best-effort data traffic. WiMAX MAC is designed to support a large number of users, with multiple connections per terminal, each with its own QoS requirement.
Robust security: WiMAX supports strong encryption, using Advanced Encryption Standard (AES), and has a robust privacy and key-management protocol. The system also offers a very flexible authentication architecture based on Extensible Authentication Protocol (EAP), which allows for a variety of user credentials, including username/password, digital certificates, and smart cards.
Support for mobility: The mobile WiMAX variant of the system has mechanisms to support secure seamless handovers for delay-tolerant full-mobility applications, such as VoIP. The system also has built-in support for power-saving mechanisms that extend the battery life of handheld subscriber devices. Physical-layer enhancements, such as more frequent channel estimation, uplink sub channelization, and power control, are also specified in support of mobile applications.
IP-based architecture: The WiMAX Forum has defined a reference network architecture that is based on an all-IP platform. All end-to-end services are delivered over an IP architecture lying on IP-based protocols for end-to-end transport, QoS, session management, security, and mobility. Reliance on IP allows WiMAX to ride the declining cost curves of IP processing, facilitate easy convergence with other networks, and exploit the rich ecosystem for application development that exists for IP.
Fixed broadband wireless access (FBWA) system is defined by the IEEE 802.16 standard. FBWA provides network access to buildings through exterior antennas communicating with central radio base stations (BSs). The IEEE standard 802.16-2004 specifies the air interface for FBWA systems supporting multimedia services. The MAC supports a point-to-multipoint (PMP) architecture with the optional mesh topology. It is structured to support multiple physical layer (PHY) specifications each suited to a particular operational environment. For operating frequencies of 10-66 GHz, the wireless MAN-SC based on single carrier modulation is specified. For frequencies below 11 GHz, where the propagation without direct line of sight must be accommodated, the wireless MAN-OFDM (orthogonal frequency division multiplexing), the wireless MAN-OFDMA (orthogonal frequency division multiplexed access), and the wireless MAN-SC (using single carrier modulation) are employed. The MAC of the IEEE 802.16 has three sub layers. The service-specific convergence sub layer (CS) provides mapping of external data, received through the CS service access point (SAP), into MAC service data units (SDUs) received by the MAC common part sub layer (CPS) through the MAC SAP. This includes classifying external network SDUs and associating them to the proper MAC service flows and connection identifier (CID). It may also include functions such as payload header suppression. Multiple CS specifications are provided for interfacing with various protocols. The MAC CPS provides the core MAC functionality of system access, bandwidth allocation, connection establishment, and maintenance. It receives data from the various CSs, through the MAC SAP and classifies to particular MAC connections. The IEEE 802.16 standard defines the QoS signalling framework and various types of service flows, but the actual QoS mechanisms such as packet scheduling and admission control algorithms for these service flows are unspecified in the standard.
A typical wireless communication system contains several signal processing steps. In addition to the radio front-end, radio systems commonly incorporate several digital components such as a digital baseband processor, a media access controller, and an application processor. An overview of such a system is illustrated in Figure 1.1. Most wireless systems contain two main computational paths, the transmit path and the receive path. In the transmit path, the baseband processor receives data from the media access control (MAC) processor and performs
• Channel coding
• Symbol shaping
before the data is sent to the radio front-end via a digital to analog converter (DAC). In the receive path, the RF signal is first down-converted to an analog baseband signal. The signal is then conditioned and filtered in the analog baseband circuitry. After this the signal is digitized by an analog to digital converter (ADC) and sent to the digital baseband processor that performs
• Filtering, synchronization, and gain control
• Demodulation, channel estimation, and compensation
• Forward error correction (FEC)
before the data is transferred to the MAC protocol layer.
WiMAX Network Architecture:
The PHY and MAC of the radio link alone are not sufficient to build an interoperable broadband wireless network. Rather, an interoperable network architecture framework that deals with the end-to-end service aspects such as IP connectivity and session management, security, QoS, and mobility management is needed. The WiMAX Forum has adopted a three-stage standards development process similar to that followed by 3GPP. In stage 1, the use case scenarios and service requirements are listed; in stage 2, the architecture that meets the service requirements is developed; and in stage 3, the details of the protocols associated with the architecture are specified. At the time of this writing, the WiMAX NWG is close to completing all three stages of its first version, referred to as Release 1, with ongoing work on the next version, referred to as Release 1.5.
We begin this chapter with an outline of the design tenets followed by the WiMAX Forum NWG. We then introduce the WiMAX network reference model and define the various functional entities and their interconnections. Next, we discuss the end-to-end protocol layering in a WiMAX network, network selection and discovery, and IP address allocation. Then, we describe in more detail the functional architecture and processes associated with security, QoS, and mobility management.
General Design Principles of the Architecture
Development of the WiMAX architecture followed several design tenets, most of which are akin to the general design principles of IP networks. The NWG was looking for greater architectural alignment with the wireline broadband access networks, such as DSL and cable, while at the same time supporting high-speed mobility. Some of the important design principles that guided the development of the WiMAX network systems architecture include the following:
The architecture shall be based on functional decomposition principles, where required features are decomposed into functional entities without specific implementation assumptions about physical network entities. The architecture shall specify open and well-defined reference points between various groups of network functional entities to ensure multivendor interoperability. The architecture does not preclude different vendor implementations based on different decompositions or combinations of functional entities as long as the exposed interfaces comply with the procedures and protocols applicable for the relevant reference point.
Deployment modularity and flexibility
The network architecture shall be modular and flexible enough to not preclude a broad range of implementation and deployment options. For example, a deployment could follow a centralized, fully distributed, or hybrid architecture. The access network may be decomposed in many ways, and multiple types of decomposition topologies may coexist within a single access network. The architecture shall scale from the trivial case of a single operator with a single base station to a large-scale deployment by multiple operators with roaming agreements.
Support for variety of usage models
The architecture shall support the coexistence of fixed, nomadic, portable, and mobile usage models. The architecture shall also allow an evolution path from fixed to nomadic to portability with simple mobility (i.e., no seamless handoff) and eventually to full mobility with end-to-end QoS and security support. Both Ethernet and IP services shall be supported by the architecture.
Decoupling of access and connectivity services
The architecture shall allow the decoupling of the access network and supported technologies from the IP connectivity network and services and consider network elements of the connectivity network agnostic to the IEEE 802.16e-2005 radio specifications. This allows for unbundling of access infrastructure from IP connectivity services.
Support for a variety of business models
The network architecture shall support network sharing and a variety of business models. The architecture shall allow for a logical separation between (1) the network access provider
(NAP)-the entity that owns and/or operates the access network, (2) the network service provider (NSP)-the entity that owns the subscriber and provides the broadband access service, and (3) the application service providers (ASP). The architecture shall support the concept of virtual network operator and not preclude the access networks being shared by multiple NSPs or NSPs using access networks from multiple NAPs. The architecture shall support the discovery and selection of one or more accessible NSPs by a subscriber.
Extensive use of IETF protocols
The network-layer procedures and protocols used across the reference points shall be based on appropriate IETF RFCs. End-to-end security, QoS, mobility, management, provisioning, and other functions shall rely as much as possible on existing IETF protocols. Extensions may be made to existing RFCs, if necessary.
Support for access to incumbent operator services
The architecture should provide access to incumbent operator services through interworking functions as needed. It shall support loosely coupled interworking with all existing wireless networks (3GPP, 3GPP2) or wireline networks, using IETF protocols.
Link Level Performance of WiMAX:
The goal of any communication system is to reliably deliver information bits from the transmitter to the receiver, using a given amount of spectrum and power. Since both spectrum and power are precious resources in a wireless network, it should come as no surprise that efficiency is determined by the maximum rate at which information can be delivered using the least amount of spectrum and power. Since each bit of information must reach the intended receiver with a certain amount of energy-over the noise level-a network's power efficiency and bandwidth efficiency cannot be maximized at the same time; there must be a trade-off between them. Thus, based on the nature of the intended application, each wireless network chooses an appropriate trade-off between bandwidth efficiency and power efficiency. Wireless networks intended for low-data-rate applications are usually designed to be more power efficient, whereas wireless networks intended for high-data-rate applications are usually designed to be more bandwidth efficient. Most current wireless standards, including WiMAX (IEEE 802.16e-2005), provide a wide range of modulation and coding techniques that allow the system to continuously adapt from being power efficient to bandwidth efficient, depending on the nature of the application. The amount of available spectrum for licensed operation is usually constrained by the allocations provided by the regulatory authority. Thus, in the given spectrum allocation, most cellular communication systems strive to maximize capacity while using the minimum amount of power. A complete PHY and MAC simulation of an entire wireless network consisting of multiple base stations (BSs) and multiple mobile stations (MSs) is prohibitive in terms of computational complexity. Thus, it is common practice to separate the simulation into two levels: link- level simulations and system-level simulations. Link-level simulations model the behavior of a single link over short time scales and usually involve modeling all aspects of the PHY layer and some relevant aspects of the MAC layer. These simulations are then used to arrive at abstraction models that capture the behavior of a single link under given radio conditions. Often, these abstraction models are represented in terms of bit error rate (BER) and block error rate (BLER) as a function of the signal-to-noise ratio (SNR). The abstracted model of a single link can then be used in a system-level simulator that models an entire network consisting of multiple BSs and MSs. Since in a system-level simulation, each link is statistically abstracted, it is sufficient to model only the higher protocol-layer entities, such as the MAC, radio resource management (RRM), and mobility management.
System Level Performance of WiMAX:
The link-level simulation and analysis results presented in Chapter 11 describe the performance of a single WiMAX link, depending on the choice of various physical-layer features and parameters. The results also provide insight into the benefits and the associated trade-offs of various signal-processing techniques that can be used in a WiMAX system. These results, however, do not offer much insight into the overall system-level performance of a WiMAX network as a whole. The overall system performance and its dependence on various network parameters, such as frequency-reuse pattern, cell radius, and antenna patterns, are critical to the design of a network and the viability of a business case. In this chapter, we provide some estimates of the system-level performance of a WiMAX network, based on simulations. In the first section of this chapter, we describe the broadband wireless channel and its impact on the design of a wireless network. Next, we describe the system-simulation methodology used to generate the system-level performance results of a WiMAX network. Finally, we discuss the system-level performance of a WIMAX network under various network configurations. These results illustrate the dependency of system-level performance on network parameters, such as frequency reuse; type of antenna used in the mobile station (MS);1 environmental parameters, such as the multipath power-delay profile; and the traffic model, such as VoIP, FTP, and HTTP. We also offer some results pertaining to system-level benefits of open-loop and closed-loop MIMO features that are part of the IEEE 802.16e-2005 standards.
The validity of simulation-based performance analysis of wireless systems depends crucially on having accurate and useful models of the wireless broadband channel. We therefore begin with a brief overview of how wireless broadband channels are modeled and used for the performance analysis presented in this chapter. For the purposes of modeling, it is instructive to characterize the radio channel at three levels of spatial scale. As discussed in Section 3.2, the first level of characterization is at the largest spatial scale, with a mathematical model used to describe the distance-dependent decay in power that the signal undergoes as it traverses the channel. These median pathloss models are useful for getting a rough estimate of the area that can be covered by a given radio transmitter. Since radio signal power tends to decay exponentially with distance, these models are typically linear on a logarithmic decibel scale with a slope and intercept that depend on the overall terrain and clutter environment, carrier frequency, and antenna heights. Median pathloss models are quite useful in doing preliminary system designs to determine the number of base stations (BSs) required covering a given area. Widely used median pathloss models derived from empirical measurements are the Okumura-Hata model, the COST-231-Hata model, the Erceg model, and the Walfisch-Ikegami model, which are discussed in the chapter appendix.
12.2 Methodology for System-Level Simulation
Link-level simulations usually model a single link and study the small-scale behavior of the system that is affected by instantaneous variations in the channel. Also, link-level simulations usually model the wireless channel only over a small area and/or over a small time duration. In order to determine the overall performance and capacity of a wireless network, such as WiMAX, system-level simulations that model the network with multiple BSs and MSs are required. System-level simulations usually model the wireless channel based on median propagation loss and shadow fading-channel variation over large scales-to the fullest extent. However, in order to increase the accuracy of the results by capturing small-scale variations in the channel, system level simulations can also model the multipath fading-channel variation over small scales. In the case of a WiMAX, it is imperative to model the behavior of the channel in the frequency domain over both short and long time scales to the multicarrier nature and its MIMO features.