The capability to perform adaptive transmit beamforming is provided in the 802.11n standard. With transmit beamforming (TxBF), we apply weights to the transmitted signal to improve reception. The weights are adapted from knowledge of the propagation environment or channel state information (CSI). Since by definition transmit beamforming weights are derived from channel information, spatial expansion as defined in Section 6.2 is not considered transmit beamforming.

The key advantage with transmit beamforming is the ability to significantly improve link performance to a low cost, low complexity device. This advantage is illustrated in Figure 12.1, which depicts a beamforming device with four antennas. Such a device could be an AP or a home media gateway. The device at the other end of the link has only two antennas, typical of a small client device. Such a system would benefit from 4 × 2 transmit beamforming gain from device A to device B. However, when transmitting from device B to device A, the system gain would be matched with 2 × 4 SDM with MRC as described in Section 6.1. Therefore link performance would be balanced in both directions.

In Figure 12.2, the generic MIMO system is modified to illustrate the application of beamformer weights to the transmitted signal. To simplify notation, the system description is given in terms of the frequency domain for a single subcarrier. Transmit beamforming as described is applied to each subcarrier in the frequency band.

What is the IP-based Multimedia Subsystem?

The IP-based Multimedia Subsystem (IMS) is the Next Generation Network (NGN) architecture, set of components, and interface specifications that allow convergence of wired and wireless networks. Convergence/integration of fixed and mobile wireless networks, the subject of this book, is part of the broader convergence just defined. The IMS convergent network will emerge from an Internet Protocol (IP)-based network infrastructure and a common service platform, allowing the development of a large array of telecommunications and multimedia applications. IMS is a user/operator-centric architectural framework that shifts much of its intelligence to the network periphery. Users of IMS services will benefit because of full mobility and service transparency across all networks.

IMS development was initiated in 1999 by a group of leading mobile service providers in conjunction with the promotion of future generations of mobile networks. This work was taken over by the 3G Partnership Project (3GPP) and presented for the first time in 3GPP Release 5 (3GPP R5) specifications. Release 5 was augmented by the addition, in Release 6 (3GPP R6), of the Internet Engineering Task Force (IETF) Session Initiation Protocol (SIP). In 3GPP Release 7, IMS incorporated the NGN concepts promoted by the European Telecommunications Standard Institute's (ETSI) Telecommunications and Internet Converged Services and Protocols for Advanced Networking (TISPAN) division. Also, the mobile networks support for IMS was extended from GSM/UMTS to CDMA2000 through 3GPP2 working specifications.

This chapter introduces some of the advanced channel access techniques in the 802.11 standard. In addition to, and built upon, the distributed channel access techniques described in Chapter 7, the 802.11 standard includes two centrally coordinated channel access techniques. The PCF was introduced in the original 802.11 specification and HCCA was introduced in the 802.11e amendment to support parameterized QoS and to fix some of the deficiencies in the PCF. The chapter then introduces new channel access techniques in the 802.11n amendment.

A very simple technique called the reverse direction protocol was introduced with one bit of additional signaling and some simple changes to the rules for operating a TXOP. This technique is particularly effective under EDCA for improving throughput for certain traffic patterns.

During the development of the 802.11n amendment, a strong interest emerged among many participants for improving the power efficiency of the MAC protocol. While outside the scope of the 802.11n PAR this resulted in the power-save multi-poll (PSMP).

PCF

Infrastructure network configurations may optionally include the point coordination function (PCF). With PCF, the point coordinator (PC), which resides in the AP, establishes a periodic contention free period (CFP) during which contention free access to the wireless medium is coordinated by the PC. During the CFP the NAV of all nearby stations is set to the maximum expected duration of the CFP. In addition, all frame transfers during the CFP use an inter-frame spacing that is less than that used to access the medium under DCF, preventing stations from gaining access to the medium using contention-based mechanisms.

Communications Networks

Communications by voice and physical signaling are common means of interaction between human beings. In the simplest forms, there is an emitting entity of information and a receiving entity of information. As the sources move apart the need for telecommunications appears self-evident. This simple model of communications becomes more complex when the information transmitted is not just sound, speech, or music, but full motion video images or various forms of data such as text, shared files, facsimile, graphics, still images, computer animation or instrumentation measurements. This information can be transmitted using electrical or optical signals, the native analog information undergoing numerous conversions and switching to accommodate various communications technologies. Telecommunications can take place over various media be that twisted copper pairs, coaxial cable, fiber optic, or wireless radio, microwave, satellite, and infrared links. Communication can be limited to a small group of people or extended to departments, compounds, or campuses and covering whole metropolitan areas, regions, countries, or continents. Hence, a shared infrastructure is needed, i.e., a communications network.

Communications networks can be classified in many ways, as there are distinct technologies and network equipment needed for voice communications, data/computer communications, and video communications. Among these types of communications, data communications, by the virtue of digitization of any type of information, has become the convergent system.

This section provides details on the MAC frame formats. The information provided here is sufficiently detailed to act as a reference for the topics discussed in this book, but it does not provide an exhaustive list of all field elements, particularly in the management frames. For a detailed treatment of the frame formats refer to the actual specification (IEEE 2007a, 2007b).

General frame format

Each MAC frame consists of the following:

• a MAC header

• a variable length frame body that contains information specific to the frame type or subtype

• a frame check sequence or FCS that contains a 32-bit CRC.

This frame format consists of a set of fields that occur in a fixed order as illustrated in Figure 11.1. Not all fields are present in all frame types.

Frame Control field

The Frame Control field is shown in Figure 11.2 and is composed of a number of subfields described below.

Protocol Version field

This field is 2 bits in length and is set to 0. The protocol version will only be changed when a fundamental incompatibility exists between a new revision and the prior edition of the standard, which to date has not happened.

Type and Subtype fields

The Type and Subtype fields together identify the function of the frame. There are three frame types defined: control, data, and management. Each frame type has several subtypes defined and the combinations are listed in Table 11.1.

The first version of the 802.11 standard was ratified in 1997 after seven long years of development. However, initial adoption of this new technology was slow, partly because of the low penetration of devices that needed the “freedom of wireless.”

The real opportunity for 802.11 came with the increased popularity of laptop computers just a few years later. This popularity brought a rapidly growing user base wanting network connectivity not only while connected to an Ethernet cable at home or at work, but also in between: in hotels, airports, conference centers, restaurants, parks, etc. 802.11 provided a cheap and easy way to make laptop mobility a reality for anyone who wanted it.

However, technology by itself is rarely sufficient, particularly in the networking space, where interoperability of devices from multiple vendors is almost always the key to market success. Having been formed as WECA in 1999, the Wi-Fi Alliance was ready to provide certification of multi-vendor interoperability.

With the right technology from the IEEE 802.11 Working Group, certified interoperability from the Wi-Fi Alliance, and a real market need based on a growing installed base of laptops, the conditions were ripe for the Wi-Fi market to take off, and indeed it did. By 2007 virtually every new laptop contains Wi-Fi as standard equipment. More importantly, and unlike some other “successful” wireless technologies, many of these devices are used regularly. With this wide use came a growing understanding of the power of cheap, easy-to-deploy, and easy-to-manage interoperable Wi-Fi networks.

Early on in the 802.11n standardization process it was recognized that even with significantly higher data rates in the PHY the fixed overhead in the MAC protocol was such that little of that gain would be experienced above the MAC. It was clear, as this chapter will show, that without throughput enhancements in the MAC the end user would benefit little from the improved PHY performance.

Reasons for change

Since the original 802.11 specification was completed, a number of amendments have introduced new PHY capabilities and with them enhanced performance. In addition, the 802.11e amendment which primarily added QoS features also enhanced MAC performance with the introduction of the TXOP concept and block acknowledgment. However, these MAC performance improvements were only slight, and with the potential for significantly higher PHY performance it was soon realized that the existing MAC protocol did not scale well with PHY data rate.

Throughput without MAC changes

The poor scaling of throughput above the MAC with PHY data rate is illustrated in Figure 8.1 where the theoretical throughput is given for unicast data sent from one station to another assuming a 3 ms TXOP limit, block ack protocol, and a 10% packet error rate (PER). As the PHY data rate is increased beyond the 54 Mbps peak data rate of 802.11a/g, throughput begins to level off.

One of the functional requirements in the development of the 802.11n standard was that some modes of operation must be backward compatible with 802.11a (and 802.11g if 2.4 GHz was supported) as described in Stephens (2005). Furthermore, the 802.11n standard development group also decided that interoperability should occur at the physical layer. This led to the definition of a mandatory mixed format (MF) preamble in 802.11n. In this chapter, we first review the 802.11a packet structure, transmit procedures, and receive procedures to fully understand the issues in creating a preamble that is interoperable between 802.11a and 802.11n devices. For further details regarding 802.11a beyond this review, refer to clause 17 in IEEE (2007a). Following this overview, the mixed format preamble, which is part of 802.11n, is discussed.

11a packet structure review

The 802.11a packet structure is illustrated in Figure 4.1.

The Short Training field (STF) is used for start-of-packet detection and automatic gain control (AGC) setting. In addition, the STF is also used for initial frequency offset estimation and initial time synchronization. This is followed by the Long Training field (LTF), which is used for channel estimation and for more accurate frequency offset estimation and time synchronization. Following the LTF is the Signal field (SIG), which contains the rate and length information for the packet. Example rates are BPSK, rate ½ encoding and 64-QAM, rate ¾ encoding. Following this is the Data field. The first 16 bits of the Data field contain the Service field. An example of an 802.11a transmit waveform is given in Figure 4.2.

WiMAX Mobile Convergent Applications

In Chapter 8 we listed the applications carried over WiMAX networks but didn't separate out those that are facilitated by convergence of WiMAX with other fixed or mobile wired and wireless networks. A short list of WiMAX convergent applications follows. Some of these applications will be detailed when we analyze WiMAX convergent case studies in subsequent sections of this chapter.

• WiMAX and cellular mobile convergence (extension of reach and connectivity of cellular mobile networks to remote, rural areas);

• WiMAX and WLAN networks convergence;

• WiMAX and WLAN metro mesh networks convergence;

• WiMAX, WLAN, and WPAN networks convergence;

• Backhaul connectivity for cellular mobile networks and traffic;

• Backhaul channel for wired Internet Service Providers;

• Disaster recovery using WiMAX links as wireless channel alternatives;

• WiMAX and EPON (as a backhaul link) networks convergence;

• WiMAX-based broadband location/navigation and customer search; and

• Internet Protocol Television (IPTV) and High Definition Television (HDTV).

WiMAX and Internet Protocol Television

IPTV, one of the applications listed above, is considered the killer application in both the fixed wired public Internet and cellular mobile networks. WiMAX, as a broadband wireless access network in metropolitan areas, is a good candidate to support IPTV. This support will include video on demand, live content, and multicast video communications, either as managed (paid TV) or unmanaged service (You Tube, for example).

WiMAX technology is strengthened by the degree of mobility conferred by the newly adopted IEEE 802.16e standard.

What is the Session Initiation Protocol-SIP?

The Session Initiation Protocol (SIP) is an Internet application layer protocol designed to control voice calls or multimedia session setups. SIP is a family of IETF-recommendations and draft standards that provide signaling specifications for Internet conferencing, VoIP, event notifications, and instant messaging over IP-based networks.

SIP-based signaling serves as multimedia call control between calling parties, and is used to set up calling sessions such as audio/video conferences, peer-to-peer communications, or to just indicate the caller's presence. SIP works in tandem with another application layer protocol, Session Description Protocol (SDP), hence the tandem abbreviation of SIP/SDP.

As indicated in Figure 16.1, there are three major components that make up a SIP network/system: Endpoints SIP User Agents (SIP-UA), the callers and beneficiaries of services; SIP Proxies (SIP-P), acting as relays of SIP messages; and SIP Registrars (SIP-R), acting as SIP directories.

As indicated with broken lines, communications between SIP UA and SIP Proxies as well as between SIP Proxies is limited to signaling. The actual transport service is delivered by specialized Applications Servers (AS) such as VoIP Servers, Presence Servers or Push-to-Talk over Cellular (PoC) Servers. The user information, data or digitized voice/video multimedia, is carried in separate frames and packets, using different transport protocols.

SIP System Architecture

A high-level SIP-based functional model is indicated in Figure 16.2.

Communications between SIP User Agents may use a series of SIP proxies that relay SIP/SDP call setup messages.

Economic Drivers of Fixed-Mobile Convergence

Although the title of this section includes the word “economics”, it is not the intent here to provide an economic analysis of fixed-mobile convergence implementations beyond some general considerations. It is not a secret that cost considerations can make or break any technical solution. The convenience of having one mobile phone that works transparently across all wireless networks is very appealing. However there are cost considerations that make such services economically justifiable. Three economics terms are used that need short explanation: Return on Investment (ROI), Compound Annual Growth Rate (CAGR), and Average Return Per Unit (ARPU).

Return on Investment (ROI), or rate of return, is the ratio of money gained relative to the money invested. It is usually calculated on an annual basis. ROI is also used to give a general qualification for a sound investment that brings profit as opposed to losing money. Compound Annual Growth Rate (CAGR) is the cumulative annualized growth rate that takes into account the cumulative effect of investment or growth over a period of time; five years for example. Average Return Per Unit (ARPU), as used in mobile telephony, indicates the revenue generated per unit, on a monthly basis, as the result of subscription and usage of services. Addition of a new service increases the ARPU.

If we consider a working network infrastructure, the cost of building a Fixed-Mobile Convergent network can be quantified by calculating the value of the following main components:

• Cost of the initial design of the FMC network;

• […]