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The link budget assumes a QPSK rate 1/2 modulation and coding operating at a 10 percent block error rate (BLER) for subscribers at the edge of the cell. This corresponds to a cell edge physical-layer throughput of about 150kbps in the downlink and 35kbps on the uplink, assuming a 3:1 downlink-to-uplink ratio. Table 2.8 shows that the system offers a link margin in excess of 140 dB at this data rate. Assuming 2,300MHz carrier frequency, a base station antenna height of 30 m, and a mobile station height of 1 m, this translates to a coverage range of about 1 km using the COST-231 Hata model discussed in Chapter 12. Table 2.8 shows results for both the urban and suburban models. The pathloss for the urban model is 3 dB higher than for the suburban model.

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It should be noted, however, that the high spectral efficiency obtained through the use of (1,1,) frequency reuse does entail an increased outage probability. As discussed in Chapter 12, the outage can be higher than 10 percent in many cases unless a 4 x 2 closed-loop MIMO scheme is used.

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IP-Based WiMAX Network Architecture

The network reference model developed by the WiMAX Forum NWG defines a number of functional entities and interfaces between those entities. (The interfaces are referred to as reference points.) Figure 2.3 shows some of the more important functional entities.

Base station (BS): The BS is responsible for providing the air interface to the MS. Additional functions that may be part of the BS are micromobility management functions, such as handoff triggering and tunnel establishment, radio resource management, QoS policy enforcement, traffic classification, DHCP (Dynamic Host Control Protocol) proxy, key management, session management, and multicast group management.

Access service network gateway (ASN-GW): The ASN gateway typically acts as a layer 2 traffic aggregation point within an ASN. Additional functions that may be part of the ASN gateway include intra-ASN location management and paging, radio resource management and admission control, caching of subscriber profiles and encryption keys, AAA client functionality, establishment and management of mobility tunnel with base stations, QoS and policy enforcement, foreign agent functionality for mobile IP, and routing to the selected CSN.

Connectivity service network (CSN): The CSN provides connectivity to the Internet, ASP, other public networks, and corporate networks. The CSN is owned by the NSP and includes AAA servers that support authentication for the devices, users, and specific services. The CSN also provides per user policy management of QoS and security. The CSN is also responsible for IP address management, support for roaming between different NSPs, location management between ASNs, and mobility and roaming between ASNs. Further, CSN can also provide gateways and interworking with other networks, such as PSTN (public switched telephone network), 3GPP, and 3GPP2.

The WiMAX architecture framework allows for the flexible decomposition and/or combination of functional entities when building the physical entities. For example, the ASN may be decomposed into base station transceivers (BST), base station controllers (BSC), and an ASN-GW analogous to the GSM model of BTS, BSC, and Serving GPRS Support Node (SGSN). It is also possible to collapse the BS and ASN-GW into a single unit, which could be thought of as a WiMAX router. Such a design is often referred to as a distributed, or flat, architecture. By not mandating a single physical ASN or CSN topology, the reference architecture allows for vendor/operator differentiation.

In addition to functional entities, the reference architecture defines interfaces, called reference points, between function entities. The interfaces carry control and management protocols— mostly IETF-developed network and transport-layer protocols—in support of several functions, such as mobility, security, and QoS, in addition to bearer data. Figure 2.4 shows an example.

Figure 2.4

Functions performed across reference points

The WiMAX network reference model defines reference points between: (1) MS and the ASN, called R1, which in addition to the air interface includes protocols in the management plane, (2) MS and CSN, called R2, which provides authentication, service authorization, IP configuration, and mobility management, (3) ASN and CSN, called R3, to support policy enforcement and mobility management, (4) ASN and ASN, called R4, to support inter-ASN mobility, (5) CSN and CSN, called R5, to support roaming across multiple NSPs, (6) BS and ASN-GW, called R6, which consists of intra-ASN bearer paths and IP tunnels for mobility events, and (7) BS to BS, called R7, to facilitate fast, seamless handover.

A more detailed description of the WiMAX network architecture is provided in Chapter 10.

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The WiMAX NWG has developed a network reference model to serve as an architecture framework for WiMAX deployments and to ensure interoperability among various WiMAX equipment and operators. The network reference model envisions a unified network architecture for supporting fixed, nomadic, and mobile deployments and is based on an IP service model. Figure 2.3 shows a simplified illustration of an IP-based WiMAX network architecture. The overall network may be logically divided into three parts: (1) mobile stations used by the end user to access the network, (2) the access service network (ASN), which comprises one or more base stations and one or more ASN gateways that form the radio access network at the edge, and (3) the connectivity service network (CSN), which provides IP connectivity and all the IP core network functions.

The architecture framework is defined such that the multiple players can be part of the WiMAX service value chain. More specifically, the architecture allows for three separate business entities: (1) network access provider (NAP), which owns and operates the ASN; (2) network services provider (NSP), which provides IP connectivity and WiMAX services to subscribers using the ASN infrastructure provided by one or more NAPs; and (3) application service provider (ASP), which can provide value-added services such as multimedia applications using IMS (IP multimedia subsystem) and corporate VPN (virtual private networks) that run on top of IP. This separation between NAP, NSP, and ASP is designed to enable a richer ecosystem for WiMAX service business, leading to more competition and hence better services.

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To further improve the reliability of retransmission, WiMAX also optionally supports type II H-ARQ, which is also called incremental redundancy. Here, unlike in type I H-ARQ, each (re)transmission is coded differently to gain improved performance. Typically, the code rate is effectively decreased every retransmission. That is, additional parity bits are sent every iteration, equivalent to coding across retransmissions.

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2.5.1 Advanced Antenna Systems

The WiMAX standard provides extensive support for implementing advanced multiantenna solutions to improve system performance. Significant gains in overall system capacity and spectral efficiency can be achieved by deploying the optional advanced antenna systems (AAS) defined in WiMAX. AAS includes support for a variety of multiantenna solutions, including transmit diversity, beamforming, and spatial multiplexing.

Transmit diversity: WiMAX defines a number of space-time block coding schemes that can be used to provide transmit diversity in the downlink. For transmit diversity, there could be two or more transmit antennas and one or more receive antennas. The space-time block code (STBC) used for the 2 x1 antenna case is the Alamouti codes, which are orthogonal and amenable to maximum likelihood detection. The Alamouti STBC is quite easy to implement and offers the same diversity gain as a 1 x 2 receiver diversity with maximum ratio combining, albeit with a 3 dB penalty owing to redundant transmissions. But transmit diversity offers the advantage that the complexity is shifted to the base station, which helps to keep the MS cost low. In addition to the 2 x 1 case, WiMAX also defines STBCs for the three- and four-antenna cases.

Beamforming: Multiple antennas in WiMAX may also be used to transmit the same signal appropriately weighted for each antenna element such that the effect is to focus the transmitted beam in the direction of the receiver and away from interference, thereby improving the received SINR. Beamforming can provide significant improvement in the coverage range, capacity, and reliability. To perform transmit beamforming, the transmitter needs to have accurate knowledge of the channel, which in the case of TDD is easily available owing to channel reciprocity but for FDD requires a feedback channel to learn the channel characteristics. WiMAX supports beamforming in both the uplink and the downlink. For the uplink, this often takes the form of receive beamforming.

Spatial multiplexing: WiMAX also supports spatial multiplexing, where multiple independent streams are transmitted across multiple antennas. If the receiver also has multiple antennas, the streams can be separated out using space-time processing. Instead of increasing diversity, multiple antennas in this case are used to increase the data rate or capacity of the system. Assuming a rich multipath environment, the capacity of the system can be increased linearly with the number of antennas when performing spatial multiplexing. A 2 x 2 MIMO system therefore doubles the peak throughput capability of WiMAX. If the mobile station has only one antenna, WiMAX can still support spatial multiplexing by coding across multiple users in the uplink. This is called multiuser collaborative spatial multiplexing. Unlike transmit diversity and beamforming, spatial multiplexing works only under good SINR conditions.

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• Signaling mechanisms for MS to request and establish MBS

• Subscriber station access to MBS over a single or multiple BS, depending on its capability and desire

• MBS associated QoS and encryption using a globally defined traffic encryption key

• A separate zone within the MAC frame with its own MAP information for MBS traffic

• Methods for delivering MBS traffic to idle-mode subscriber stations

• Support for macro diversity to enhance the delivery performance of MBS traffic

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Both FBSS and MDHO offer superior performance to HHO, but they require that the base stations in the active or diversity set be synchronized, use the same carrier frequency, and share network entry–related information. Support for FBHH and MDHO in WiMAX networks is not fully developed yet and is not part of WiMAX Forum Release 1 network specifications.

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Idle mode allows even greater power savings, and support for it is optional in WiMAX. Idle mode allows the MS to completely turn off and to not be registered with any BS and yet receive downlink broadcast traffic. When downlink traffic arrives for the idle-mode MS, the MS is paged by a collection of base stations that form a paging group. The MS is assigned to a paging group by the BS before going into idle mode, and the MS periodically wakes up to update its paging group. Idle mode saves more power than sleep mode, since the MS does not even have to register or do handoffs. Idle mode also benefits the network and BS by eliminating handover traffic from inactive MSs.

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Although it does not define the scheduler per se, WiMAX does define several parameters and features that facilitate the implementation of an effective scheduler:

• Support for a detailed parametric definition of QoS requirements and a variety of mechanisms to effectively signal traffic conditions and detailed QoS requirements in the uplink.

• Support for three-dimensional dynamic resource allocation in the MAC layer. Resources can be allocated in time (time slots), frequency (subcarriers), and space (multiple antennas) on a frame-by-frame basis.

• Support for fast channel-quality information feedback to enable the scheduler to select the appropriate coding and modulation (burst profile) for each allocation.

• Support for contiguous subcarrier permutations, such as AMC, that allow the scheduler to exploit multiuser diversity by allocating each subscriber to its corresponding strongest subchannel.

It should be noted that the implementation of an effective scheduler is critical to the overall capacity and performance of a WiMAX system.

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