Upgraded Digital Cellular System (Generation 2.5)

The types of upgraded 2nd generation digital cellular systems (generation 2.5) include GPRS, EDGE, and CDMA2000^TM , 1xRTT.

General Packet Radio Service (GPRS)

General Packet Radio Service (GPRS) is a portion of the GSM specification that allows packet radio service on the GSM system. The GPRS system adds (defines) new packet channels and switching nodes within the GSM system. The GPRS system provides for theoretical data transmission rates up to 172 kbps.

Enhanced Data Rates for Global Evolution (EDGE)

Enhanced Data Rates for global Evolution (EDGE) is an evolved version of the global system for mobile (GSM) radio channel that uses new phase modulation and packet transmission to provide for advanced high-speed data services. The EDGE system uses 8 levels Phase Shift Keying (8PSK) to allow one symbol change to represent 3 bits of information. This is 3 times the amount of information that is transferred by a standard 2 level Gaussian Minimum Shift Keying (GMSK) signal used by the first generation of GSM system. This results in a radio channel data transmission rate of 604.8 kbps and a net maximum delivered theoretical data transmission rate of 384 kbps. The advanced packet transmission control system allows for constantly varying data transmission rates in either direction between mobile radios.

CDMA2000™, 1xRTT

CDMA2000^TM is a 3G standard that allows operators to evolve from their existing IS-95 networks to offer 3G services. The original CDMA2000^TM proposal contained two distinct evolutionary phases, the first known as 1xRTT used the same 1.25 MHz channels as IS-95 but delivered increased capacity and data rates compared to IS-95. The second phase was known as 3xRTT that uses three times the spectrum of IS-95, that is 3.75 MHz. The 3xRTT concept would deliver data rates up to 2 Mbps, a requirement for any 3G technologies. However recent evolutions of 1xRTT are offering data rates in excess of this and therefore it is unlikely that 3xRTT is required.

By the middle of 2003 there were a total of 60 commercial 1xRTT networks offering service.

Evolution Data Only (1xEVDO)

The evolution of existing systems for data only (1xEVDO) is an evolved version of the CDMA2000^TM 1xRTT system. The 1xEVDO system uses the same 1.25 MHz radio channel bandwidth as the existing IS-95 system that provides for multiple voice channels and medium rate data services. The 1xEVDO version changes the modulation technology to allow for data transmission rates up to 2.5 Mbps. The 1xEVDO system has an upgraded packet data transmission control system that is allows for bursty data transmission rather than for more continuous voice data transmission.

Evolution Data and Voice (1xEVDV)

The evolution of existing systems for data and voice (1xEVDV) is an evolved version of the CDMA2000^TM 1xRTT system that can be used for data and voice service. The 1xEVDV system provides for both voice and high-speed data transmission services in the same 1.25 MHz radio channel bandwidth as the existing IS-95 system. The 1xEVDV Vision allows for a maximum data transmission rate of approximately 2.7 Mbps.

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Global System for Mobile Communication (GSM)

The Global System for Mobile Communications (GSM) system is a global digital radio system that uses Time Division Multiple Access (TDMA) technology. GSM is a digital cellular technology that was initially created to provide a single-standard pan-European cellular system. GSM began development in 1982, and the first commercial GSM digital cellular system was activated in 1991. GSM technology has evolved to be used in a variety of systems and frequencies (900 MHz, 1800 MHz and 1900 MHz) including Personal Communications Services (PCS) in North America and Personal Communications Network (PCN) systems throughout the world. By the middle of 2003, 510 networks in 200 countries offered GSM service (Fig: 1).

Fig 1: GSM Network

The GSM system is a digital-only system and was not designed to be backward-compatible with the established analog systems. The GSM radio band is shared temporarily with analog cellular systems in some European nations.

When communicating in a GSM system, users can operate on the same radio channel simultaneously by sharing time slots. The GSM cellular system allows 8 mobile telephones to share a single 200 kHz bandwidth radio carrier waveform for voice or data communications. To allow duplex operation, GSM voice communication is conducted on two 200 kHz wide carrier frequency waveforms.

The GSM system has several types of control channels that carry system and paging information, and coordinates access like the control channels on analog systems. The GSM digital control channels have many more capabilities than analog control channels such as broadcast message paging, extended sleep mode, and others. Because the GSM control channels use only a portion (one or more slots), they typically co-exist on a single radio channel with other time slots that are used for voice communication.

A GSM carrier transmits at a bit rate of 270 kbps, but a single GSM digital radio channel or time slot is capable of transferring only 1/8th of that, about 33 kbps of information (actually less than that, due to the use of some bit time for non-information purposes such as synchronization bits).

Time intervals on full rate GSM channels are divided into frames with 8 time slots on two different radio frequencies. One frequency is for transmitting from the mobile telephone; the other is for receiving to the mobile telephone. During a voice conversation at the mobile set, one time slot period is dedicated for transmitting, one for receiving, and six remain idle. The mobile telephone uses some of the idle time slots to measure the signal strength of surrounding cell carrier frequencies in preparation for handover.

On the 900 MHz band, GSM digital radio channels transmit on one frequency and receive on another frequency 45 MHz higher, but not at the same time. On the 1.9 GHz band, the difference between transmit and receive frequencies is 80 MHz. The mobile telephone receives a burst of data on one frequency, then transmits a burst on another frequency, and then measures the signal strength of at least one adjacent cell, before repeating the process.

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