W O R K S H O P

Generation Wireless

June 11, 2001
By Blesson Mathews

Whenever people talk about wireless networks and services these days, they use the phrases third generation and next generation. Every wireless service provider, equipment manufacturer and wireless application developer is concentrating on delivering these services. At the Cellular Telecommunications & Internet Association (CTIA) Wireless 2001 event in March, many of the major U.S. cellular telephone carriers promised to start providing higher-speed wireless services, based on 2.5G technologies, this year.

The first generation, or 1G, comprised analog, voice-centric mobile systems that emerged in the early 1980s. These systems were typically limited in capacity. The second generation (2G) appeared about 10 years later, with the first digital mobile, circuit-switched networks. These systems provide better voice quality, higher capacity, lower power requirements and global roaming capabilities. 2G systems also offer support for simple nonvoice services like the SMS (short message service), which sends messages of up to 160 characters to and from mobile phones.

Because they're digital, 2G cellular systems can also transmit data over an airlink. Circuit-switched cellular data is now the most widely used wireless data service. However, the effective data rates of 2G circuit-switched wireless systems are still too slow to facilitate comfortable Internet access. Other services, like CDPD (Cellular Digital Packet Data), and noncellular proprietary services, like Cingular Interactive's Mobitex, Motient Corp.'s Ardis and WorldCom's SkyTel, provide packet-switched solutions with nationwide coverage.

CDPD is a two-way digital packet-switching technology that allows for data to be transmitted at speeds of about 19.2 Kbps (though the actual throughput averages 9.6 Kbps), across voice-based analog 1G AMPS (Advanced Mobile Phone Service) networks. CDPD makes use of the existing cellular infrastructure by sending data during idle times when voice packets are not being sent over the channel. The Ardis, Mobitex and SkyTel networks are also non-IP based. That is why there is an emphasis on the development of an IP-based cellular wireless network that builds on the existing infrastructure.

2G Systems

Cellular-phone systems use the 800-MHz (806 to 902 MHz) and 1.9-GHz (1,850 to 1,990 MHz) frequency bands. The 1.9-GHz frequencies are allocated to PCS (personal communication services), but many cellular systems use these frequencies as they bundle PCS capabilities on their voice-centric service. 1G analog systems operate in the 800-MHz frequencies, while the digital 2G systems operate in both the 800-MHz and the PCS bands.

Cellular systems use three different techniques for sharing an RF (radio frequency) spectrum: FDMA (Frequency Division Multiple Access), TDMA (Time Division Multiple Access), and CDMA (Code Division Multiple Access). TDMA and CDMA are the dominant techniques (see "Comparing FDMA, TDMA and CDMA").

TDMA Systems. TDMA-based systems divide an RF channel into time slots, each of which lasts for a fraction of a second, and then allocates those slots to multiple calls. TDMA divides a 30-KHz channel into six time slots that are allocated in pairs, resulting in three usable TDMA channels. Any given conversation can use one or more of every third time slot on an ongoing basis during a call.

TDMA was defined by the IS-54 standard Digital AMPS, or D-AMPS, in North America. GSM (Global System for Mobile Communications) and the Japanese PDC (Personal Digital Cellular) are variants of TDMA. TDMA is now specified in IS-136, which also incorporates SMS, and has a data capacity of about 9.6-Kbps. Ericsson, Lucent Technologies and Nortel Networks are the largest vendors of TDMA-based equipment, and Cingular is the largest TDMA carrier in the United States.

GSM Systems. To standardize the cellular phone system throughout Europe, Groupe Spécial Mobile (now called Global System for Mobile Communications) was formed. The greatest strength of this nonproprietary system is its international roaming capability. Users have seamless, same-number roaming in more than 170 countries.

Most GSM systems operate in the 900-MHz and 1.8-GHz frequency bands, except in North America where they operate in the 1.9-GHz band. GSM uses a combination of FDMA and TDMA multiple access schemes on its 25-MHz-wide frequency spectrum. FDMA divides the 25-MHz bandwidth into 124 carrier frequencies of 200 KHz each. Each 200-KHz channel is then divided into eight time slots using TDMA. GSM systems support data speeds of 9.6 Kbps and the SMS feature. The prominent vendors of GSM equipment are Alcatel, Ericsson, Lucent, Nokia and Nortel. VoiceStream Wireless is the largest provider of GSM in this country.

CDMA Systems. Developed by Qualcomm, CDMA is a 2G technology that uses digital encoding and spread-spectrum RF techniques to let multiple users share the same RF channel. CDMA divides the radio spectrum into channels that are 1.25-MHz wide, larger than that in typical AMPS or TDMA systems.

CDMA differentiates each user's signal by encoding it uniquely. At the transmitting end, this is used to encode the signal, which is then spread across the frequency spectrum. At the receiving end, this code is detected and used to extract the user's information.

Existing IS-95-based CDMA systems, also called cdmaOne, support circuit-switched data services at speeds of 9.6 Kbps to 14.4 Kbps. The IS-95A protocol supports data speeds of up to 14.4 Kbps. IS-95B is a next-generation transitional step to the IS-95A CDMA standard and offers data rates of about 64 Kbps while maintaining compatibility with existing IS-95A systems. In the United States, Lucent, Motorola and Nortel are the biggest CDMA equipment vendors, and Sprint PCS and Verizon Wireless are the largest CDMA carriers.

CDMA increases system capacity by about 10 to 15 times compared with AMPS and by more than three times compared with TDMA. Other strengths include improved voice quality, greater capacity and enhanced privacy.

The industry recognizes CDMA as a superior air interface technology compared with that used in GSM/TDMA, but what makes GSM popular is its international roaming feature. Many advocates of CDMA say that they do not see this as a sales driver. Down the road, however, CDMA systems will also include roaming capabilities.