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CDMA, GSM, TDMA and PCS Satellite - all in one complete guidebook!
Four digital technologies -- CDMA, GSM, TDMA and PCS Satellite -- will dominate the marketplace for second-generation mobile and wireless networks. In this book, internationally respected telecommunications expert Uyless Black examines and compares all four, from the standpoint of the communications engineer and manager. You'll learn how each technology seeks to deliver improved clarity, reliability and speed. You'll also review every critical issue faced by engineers, network managers, and others involved in deploying these systems:
Black clarifies the often-confusing terminology surrounding digital cellular and wireless services, and reviews the key differences between first- and second-generation networks. He presents detailed, up-to-the-minute coverage of the new GSM standard for North America; introduces new high-performance vocoders for digitizing and compressing voice signals; compares leading approaches to deploying Wireless Local Loop (WLL) technology; and previews third-generation mobile systems already on the horizon. Whatever your role in implementing wireless technology, you'll refer to this book constantly, for years to come.
This chapter reviews the basic concepts of a mobile wireless sYstem. A classification scheme is provided that will be useful for tracking the second generation systems in later chapters. We take a look in a general way at the systems covered in this book, and examine some market projections on the deployment of second generation systems.
One of the fastest growth areas in the telecommunications industry is the mobile cellular phone market. Since the late 1980s and early 1990s, the market has increased its growth rate each year. The recent growth in this industry is depicted in Figure 1-1, as well as the prediction for growth through the year 2001 (as compiled by Dataquest Inc. and published by the Wall Street Journal, September 11, 1997).
Figure 1-la shows the worldwide cellular market, tabulated by the number of subscribers. Figure 1-lb shows the service revenue; that is, the income the cellular service providers receive from the cellular customers. Of course, the predictions on the future subscriber base and the service revenues, are just that-predictions. But many of the marketing forecasts on the growth of mobile, cellular technology have been inaccurate-they have been too conservative.
Nonetheless, if these predictions hold true, in the next few years the mobile phone will become a common appliance in the homes of many families. Moreover, due to the relatively modest capital investments required to deploy a wireless system (in comparison to a wire-based alter-native), much of this high growth is occurring in so-called third-world countries that do not have an abundance of capital in which to invest in their telecommunications infrastructure. Simply stated, we can look to the future and see that the mobile phone will play an increasing role in our professional and personal lives.
It is assumed the reader of this book has a basic understanding of mobile wireless networks and the concepts of cells. Notwithstanding, this section provides an overview of some of these ideas to make sure several key concepts are understood. This diversion is to review air interfaces, the channels on these interfaces, and channel utilization techniques. We then use this information to compare first and second generation mobile wireless networks. Figure 1-2 will be a helpful reference during this discussion.
Frequency Division Multiple Access (FDMA)
First, frequency-division multiplexing divides the bandwidth of the air interface between the mobile station (MS) and the base station (BS) into multiple analog channels; each radio frequency (RF) channel occupies one part of a larger frequency spectrum (see Figure 1-2a). For example, some second generation systems divide a larger frequency spectrum of 15 MHz into multiple 200 kHz channels. This technique is called frequency division multiple access (FDMA). And as another example, some second generation systems divide a larger frequency spectrum of 10 MHz into multiple 30 kHz channels.
If two separate FDMA channels are available, one for each direction of transmission, the system is said to be operating in a frequency division duplex (FDD) mode, also known as full-full duplex (FFD).
Time Division Multiple Access (TDMA)
Time-division multiple access (TDMA) operates on an RF channel (for example, a 200 kHz RF channel), but divides this analog channel into time slots, which contain digital traffic (see Figure 2-2b).
With TDMA, a user is given a digital time slot, and the slots are rotated among the users on a periodic basis. For example, user A might be assigned time slot 1 on a specific 200 kHz RF channel, user B could be assigned time slot 4 on the same RF channel, and so on. Each user is assured of having these slots available at a known time, which means the user's mobile station knows the exact time to send (and stop sending) traffic.
Bits, bytes, or blocks of voice or data from the user application are multiplexed together and interleaved into the slots. The slots are combined together to form TDMA frames, and it is these frames that are sent onto the single radio frequency (RF) carrier.
As stated earlier, the digital TDMA signals are modulated onto an analog carrier (an FDMA channel). Therefore, a TDMA system is a combination of FDMA and TDMA. In contrast, a pure FDMA interface does not divide the RF channel into slots, but dedicates each RF channel to one user. FDMA is strictly an analog-based air interface.
In some TDMA systems, a channel-sharing technique called time division duplex (TDD) is used on the air interface. With this approach, one frequency is used for both directions between two stations and each station takes turns using the channel-first the mobile station uses the RF channel, then the base station uses it, and so on.
TDD is also known by two other rather self-descriptive names: flipflop and ping-gong. Essentially, TDD is a half-duplex system that gives the illusion of full-duplex operations, because the channel is flip-flopped quite rapidly. For example, one system flips the direction of transmission every 5 ms-so quickly that two persons engaged in a normal conversation cannot discern that they are not given a dedicated channel in each direction.
Extended TDMA (E-TDMA)
A conventional TDM system wastes the bandwidth of the communications link for certain applications because the time slots are often unused. Vacant slots occur when an idle user terminal has nothing to transmit in its slot; for example, during a pause in a conversation on the phone. To address this problem, a different technique called statistical TDM (STDM) multiplexing dynamically allocates the time slots among active user terminals. Dedicated time slots (TDMs) are not provided for each mobile phone on the channel. Consequently, an idle terminal time does not waste the RF channel capacity, because its slots can be borrowed by another active user (someone who is talking, or perhaps a data application that is sending e-mail).
This technique has been around for a number of years, and is now being employed in the new versions of second generation systems. It is known as extended TDMA (E-TDMA).
Code Division Multiple Access (CDMA)
The third major channel sharing technique is called code division multiple access (CDMA), depicted in Figure 1-2c. This technology does not divide the time spectrum nor the frequency spectrum into pieces. Rather, CDMA places all users onto the same frequency spectrum at the same time. Thus, the conventional concepts of TDMA and FDMA are not used in CDMA...
|Ch. 2||First Generation Systems||39|
|Ch. 3||Digital AMPS (D-AMPS): IS-54-B||55|
|Ch. 4||The Global System for Mobile Communications (GSM)||77|
|Ch. 7||IS-41-C and IS-634||219|
|Ch. 8||Satellite PCS||261|
|Ch. 9||Data Operations||280|
|Ch. 10||The Wireless Local Loop (WLL)||323|
|Ch. 11||Third Generation Mobile Systems (TGMSs)||337|