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Digital Wireless Communications Technology


Source Encoding  |  Channel Encoding  |  Modulation  |  Physical Channel  |  Channel Models
Digital Filters  |  Demodulation  |  Synchronization/Carrier Recovery  |  Detection  |  Data Processing

The primitive state of mobile data transmission will be transformed over the next three years as operators construct third-generation networks. For all the hype about the wireless Internet, today's mobile data networks are rather primitive. Usually bolted onto cell phone systems built for voice, they offer low bit rates and poor interoperability. All this will change over the next three years, as operators construct 2.5 and 3G mobile networks. The aim is to provide packet-switched data to a handheld terminal with throughput measured in hundreds of Kbps.

3G has been in gestation since 1992, when the International Telecommunication Union (ITU) began work on a standard called IMT-2000 with 16 different proposals. The ITU envisaged IMT-2000 as a single global standard, but the world's regulators, vendors, and carriers were unable to reach a unanimous agreement. By 1998, the ITU was faced with 13 different 3G-radio interfaces based on CDMA. Qualcomm and the Interim Standard 95 (IS-95) CDMA industry wanted one harmonized CDMA standard based on their chosen technology, cdma2000, because it would be backward compatible. Ericsson and the GSM camp wanted their own technology, WCDMA, which is incompatible with today's IS-95 systems. In October 1999, representatives from different countries finally agreed to disagree. The result is a "federal standard," or, more accurately, fudge. IMT-2000 will have at least three optional modes of operation: W-CDMA, cdma2000 and time division duplex, an optional component of W-CDMA. So the two technologies, WCDMA and cdma2000, remains pitted against each other. WCDMA is gaining most of the industry's attention, threatening to sweep the world's footprint with its emphasis on global roaming and promises of economies of scale. Timing and cost of the technology remain as drawbacks. Below, in the table, is a summary of the family of five sets of 3G IMT-2000 wireless standards approved and published by ITU on May 2000.

The path to 3G will be gradual, and expensive to deploy, everyone wants to ensure compatibility with their existing systems.

Three IMT-2000 modes are based on Code Division Multiple Access, a system that enables many users to share the same frequency band at the same time. CDMA codes are chosen so that they cancel each other out. For exact cancellation, signals must be perfectly timed; base stations need to make very precise measurements of their time and location. They do this by using signals from Global Positioning System (GPS) satellites, which can pinpoint anywhere on Earth to within four meters and measure time more accurately than the Earth's own rotation. The only CDMA system in use so far is cdmaOne, developed by Qualcomm but now supervised by an independent organization called the CDMA Development Group (CDG). It has been standardized by the Telecommunica-tions Industry Association (TIA) as IS-95A, and is popular among cellular operators in America and Asia. Because it already uses CDMA, it is easier to upgrade to 3G compared to rival systems based on Time Division Multiple Access (TDMA).

cdmaOne spreads every signal over a 1.25MHz channel, transmitting on the entire bandwidth at once. It uses a set of 64 codes, known as Walsh sequences, so in theory up to 64 phones could use the channel at once. In practice, that number depends on the data throughput. The basic system offers voice and 14.4Kbps data rates, which facilitates between 15 and 20 users. An upgrade called IS-95b offers data rates of up to 115Kbits/sec, which would mean only two users per channel.

To reach the IMT-2000 target of 2Mbits/sec, CDMA systems need to use more codes, a different modulation scheme, and wider bandwidths. The official upgrade, developed by Qualcomm and ratified by the ITU, is known as cdma2000 3X. The 3 in 3X comes from its 3.75MHz bandwidth, the result of three cdmaOne 1.25MHz channels joined together. As an intermediary step, some cdmaOne operators are deploying a technology called cdma2000 1X, which uses the same 1.25MHz channels and doubles the number of codes to 128, thus doubling either the throughput per user or the number of users in a cell.

Qualcomm and Motorola are also pushing rival schemes that enhance 1XRT, known respectively as High Data Rate (HDR) and 1Xtreme. Both 1Xtreme and HDR work by altering the modulation scheme, or the way data is actually represented in radio waves. Most cell phones use a system called Phase Shift Keying (PSK), which interrupts a wave and moves it to a different point in its cycle. The bit rate depends on the frequency of these interruptions, known as symbols, and on the number of shapes that each symbol can take. The symbols in quadrature PSK, the system used by cdmaOne, can take four different shapes. This means that each shape can represent two bits, since two bits can take four combinations. The 8-PSK variation could represent three bits per symbol, increasing the data rate by half. HDR and 1Xtreme automatically increase the number of shapes to the highest number supported, depending on their connection quality. By late this year, most CDMA operators plan to migrate to 1X, which will give them data enhancements of up to 144 kbps with primarily a software upgrade. Shortly after, in early 2002, CDMA operators are likely to add what is known as 1X EV (evolution), which will allow them to dedicate a 1.25 Mhz channel to data services that will offer speeds up to 2.4 Mbps.

Europe, South Korea, Japan and are rolling out Wideband CDMA (WCDMA), or Universal Mobile Telecommunications System (UMTS). This requires the new spectrum assigned by the ITU, and thus won't be used in the United States without establishing new spectrum. It is technically very similar to cdma2000 3XRT but uses a slightly wider bandwidth, hence the name. The wider bands are necessary so that the system can interoperate with Global System for Mobile Communications (GSM), the most ubiquitous second-generation (2G) wireless standard. Japan's NTT DoCoMo is expected to be the world's fiirst operator to launch a WCDMA system in fall of 2001. However, DoCoMo's WCDMA technology is slightly different from the standards accepted by the ITU. The standard, dubbed Japanese WCDMA, or JW-CDMA, has certain subtleties, including proprietary protocol stacks.

In the US, TDMA operator AT&T wireless announced plans to deploy GSM services and 2.5 G general packet radio service (GPRS, and then onto WCDMA. Its alliances and affiliates in Canada: include Rogers Cantel and TelCorp PCS will follow. They are not waiting on enhanced data rates for GSM evolution (EDGE). AT& T Wireless says it still plans to migrate to EDGE, but industry dynamics are suggesting that EDGE might fail --- eroding the economies of scales that TDMA operators have sought. EDGE technology was originally envisioned as a spectrally efficient option for European operators that were not successful at winning new spectrum. All but one major European operator has obtained new spectrum licenses. Other US operators, such as Cingular, testing GPRS, are likely to follow the AT&T Wireless movement. Carriers in Europe have not announced intentions to deploy EDGE technology, but are heavily committed to GSM with GPRS. The CDMA community is just beginning to embrace the idea of subscriber identity modules that store subscriber information and are commonly used in GSM phones today.

Because the WCDMA standard is a moving target, most vendors are working on deploying Release 99 of the standard and adding in corrections, which standards bodies will complete in June of 2001. Release 2000 will be broken into two parts, Release 4 and Release 5. WCDMA Release 99 has two branches. One controls voice traffic, while the other supports data traffic. This means the core network is circuit switched with packet data running on top. Together, Release 4 and Release 5 will create an all IP network.

The sidebar icons summarize the different embedded software technologies essential to the operation of Wireless Wide Area Network (WWAN) wireless infrastructure.

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