CDMA and 3G Cellular 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 third-generation (3G; see "Mobile Generations" below) mobile networks. These aim to provide packet-switched data to a handheld terminal with throughputs measured in hundreds of Kbits/sec.

Mobile Generations

1G: Analog voice

2G: Digital voice and messaging

3G: Broadband data and Voice over IP (VoIP)

3G has been in gestation since 1992, when the International Telecommunications Union (ITU) began work on a standard called IMT-2000. IMT stands for International Mobile Telecommunications; the number 2000 initially had three meanings: the year that services should become available, the frequency range in MHz that would be used, and the data rate in Kbits/sec. The name has stuck, though all these criteria were eventually abandoned . The year slipped back to 2002, North America is already using the recommended frequencies for other services, and high speeds will only be available through special indoor "pico- cells ."

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. The path to 3G will be gradual, and everyone wants to ensure compatibility with their existing systems. The FCC's failure to license any new spectrum further complicated things: American operators must upgrade their existing networks rather than build new ones.

In October 1999, representatives from different countries finally agreed to disagree . The result is a "federal standard," or, more accurately, a fudge. IMT-2000 will have at least three modes of operation, with no guarantee that phones for one mode will work with the others.

Access Codes

Two of the three IMT-2000 modes are based on Code Division Multiple Access (CDMA), a system that enables many users to share the same frequency band at the same time (see Table 1). Each signal is encoded differently so that it can be understood by a receiver with the same code. CDMA actually predates both computers and mobile phones, though it was considered too complicated for use in cellular phone networks until the late '90s. Many of the innovations in cell networks were due to the company Qualcomm (www. qualcomm .com), whose growth in the cellular technology market has echoed that of Microsoft and Cisco Systems.

CDMA is often compared to an airport transit lounge, where many people are speaking in different languages. Each listener only understands one language, and therefore concentrates on his or her own conversation, ignoring the rest. The analogy isn't exact, because a roomful of people all talking at once soon becomes very loud. Everyone ends up trying to shout above the background noise, which just makes the problem worse .

To prevent such a vicious circle, 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.

CDMA Flavors

Table 1: Prick Up Your Ears . Code Division Multiple Access (CDMA) systems enable many users to share the same frequency band at the same time. Currently, cdmaOne is the only such system in use.

Code Division Multiple Access (CDMA) system	Channel bandwidth	Chip rate	Maximum capacity	Real capacity
cdmaOne IS-95b	1.25MHz	1.2288MHz	115Kbits/sec	64Kbits/sec
cdma2000 1XMC	1.25MHz	1.2288MHz	384Kbits/sec	144Kbits/sec
cdma2000 1Xtreme	1.25MHz	1.2288MHz	5.2Mbits/sec	1.2Mbits/sec
cdma2000 HDR	1.25MHz	1.2288MHz	2.4Mbits/sec	621Kbits/sec
cdma2000 3XMC	3.75MHz	3.6864MHz	4Mbits/sec	1.117Mbits/sec
Wideband CDMA (W-CDMA)	5MHz	4.096MHz	4Mbits/sec	1.126Mbits/sec
Source: Motorola

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 Telecommunications Industry Association (TIA) as IS-95a, and is very 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 14.4Kbit/sec 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.

Open Wide

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 3XMC. The 3 comes from its 3.75MHz bandwidth, the result of three cdmaOne 1.25MHz channels joined together.

As an intermediary step, most cdmaOne operators are deploying a technology called cdma2000 1XMC, which uses the same 1.25MHz channels. It 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 1XMC, 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: Poor connections cannot cope with a system using many shapes per symbol, as they become hard to distinguish.

To make things more confusing, many parts of the world are rolling out an entirely new system called Wideband CDMA (W-CDMA), or Universal Mobile Telecommunications System (UMTS). This requires the new spectrum assigned by the ITU, and thus can't be used in the United States. It is technically very similar to cdma2000 3XMC 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 popular second-generation (2G) standard.

Though W-CDMA is not a direct upgrade from GSM, it was designed for use in areas that are already well-covered by GSM networks. The theory is that operators can build W-CDMA "hot spots" in major cities, and rely on GSM in the beginning for the rest of their coverage. All W-CDMA phones sold in Europe will work with GSM; they'll even be able to hand over calls so that users can move between cells based on the different systems without any interruption.

Even in countries where GSM is not prevalent , W-CDMA is likely to prove popular because it offers instant coverage nearly worldwide. GSM-free Japan is rolling out the world's first W-CDMA networks because its existing Personal Digital Cellular (PDC) system has run out of capacity. Mobile operators NTT DoCoMo and J-Phone both began trials in early 2000 and hope to have commercial services in operation during 2001.

Time Again

Not all 3G systems use CDMA. A third variant of IMT-2000, called EDGE, applies the same modulation tricks as HDR to the TDMA-based General Packet Radio Service (GPRS, described in Network Magazine's April 2000 Tutorial). Operators of GSM and Digital Advanced Mobile Phone System (D-AMPS) who don't have a license for a new spectrum need GPRS; this includes all mobile providers in the United States, as the FCC hasn't issued any licenses.

EDGE requires a channel bandwidth of 200KHz, the same as GSM. This makes it more difficult for D-AMPS operators, who are used to 30KHz channels, but still more attractive than the alternative: A CDMA system requires a minimum of 1.25GHz, which a busy network may not be able to spare from its existing service. Dual-mode handsets are also easier to make if they support two TDMA systems-rather than both CDMA and TDMA-though this problem must be overcome by European manufacturers if UMTS is to be compatible with GSM.

From layer 2 upward, EDGE inherits virtually every property of GSM and GPRS. At the radio level, it works by replacing the Gaussian Minimum Shift Keying (GMSK) modulation of GSM with 8-PSK. GMSK is a binary system, permitting only one bit per symbol compared to 8-PSK's three. The raw data rate is thus tripled from GPRS's 171Kbits/sec to 513Kbits/sec. Some of this must be allocated toward error correction, with nine different throughput speeds defined depending on the quality of the connection.

There are also 29 different terminal classes; each class grants users access to the channel at different times while transmitting, receiving, or both. The result: Users have access to a wide range of speeds, from 8.8Kbits/sec to 474Kbits/sec. This doesn't quite meet the original IMT-2000 wish list, but the ITU has ratified it anyway, on the understanding that the 2Mbit/sec requirement can be satisfied by combining it with a wireless LAN technology.

Resources

The official International Mobile Telecommunications (IMT)-2000 site at www.itu.int/imt2000/ has plenty of background information on the standardization process.

The Third Generation Partnership Project is an alliance of vendors and regulators that tries to stay neutral in the standards war. Its Web site is located at www.3gpp.org.

The Code Division Multiple Access (CDMA) Development Group's Web site (www.cdg.org) promotes all forms of CDMA and provides detailed mathematical information about how they work.

Detailed information on the European Wideband CDMA (W-CDMA) system can be found at www.umts-forum.org.

The Universal Wireless Communications Consortium explains how EDGE works and promotes Time Division Multiple Access (TDMA) as a third-generation (3G) technology at www.uwcc.org.

This tutorial, number 146, by Andy Dornan, was originally published in the September 2000 issue of Network Magazine.