Second generation mobile systems are digital systems. The GSM system has been described in Section 18.5. This section describes the GSM1800 system, the personal communications system (PCS1900, sometimes known as personal communications network or PCN) and third generation systems.
The GSM1800 system operates in the 1840–1880 MHz band, with a carrier spacing of 200 kHz. Each carrier gives multiple access through time division multiple access (TDMA). It is a lower power version of GSM900, which operates in the 900 MHz band. A GSM1800 handset has a maximum power output level of 1 watt, rather than the 2 watts of a GSM900 handset.
Voice coding can be either: half-rate using VSELP at 5.6 kbps, full-rate using RPE-LTP at 13 kbps, or enhanced full rate (EFR) using ACELP at 12.2 kbps. Convolutional coding is used at a rate of1 to2 reduce the effects of noise and errors on the radio channel. The radio carrier modulation is Gaussian minimum shift keying (GMSK).
The personal communications system (PCS) is based on GSM and used time division multiplexing to provide separate channels over each radio frequency. Convolutional coding is used at a rate of
1 2to reduce the effects of noise and errors on the radio channel. Each carrier is spaced 30 kHz apart and offset quadrature phase-shift keying (OQPSK) is used to modulate the carrier.
The voice-coder (vocoder) used by PCS1900 is either full-rate using VSELP at 7.95 kbps or enhanced full rate (EFR) that uses ACELP at 7.4 kbps. The EFR scheme was developed for use in PCS1900 and has since been adopted by both GSM900 and GSM1800 networks because it improves the audio quality. Previously, GSM used either a half-rate (HR) or full-rate (FR) vocoder.
The so-called 3G systems have been described using a number of terms: UMTS, IMT-2000, cdma-2000, w-cdma, etc. There are variations between the proposed standards, but in general they all use code division multiple access (CDMA), convolutional coding, quadrature phase shift keying and time or frequency division duplexing.
Third generation systems will operate in the 1885–2025 MHz band and in the 2110–2200 MHz band. Initially the lower band between 1980 and 2010 MHz will not be allocated, nor will the upper band between 2170 and 2200 MHz. However, the full bands will be available when existing services in these bands can be moved (such as PCS1900, PHS and DECT).
The code division multiple access (CDMA) modulation scheme allows several channels to occupy the same frequency band. CDMA is a form of spread spectrum encoding. The digitized speech signals are coded using a unique code, known as a spreading code, so that one bit of speech code produces many encoded bits. When demodulated and mixed with the same spreading code, the original data is recovered. Receivers that use a different spreading code are unable to recover this data, but they are able to recover data intended for them.