tor CODEC Audio Earpiece Ringer front end Processor Memory Synth + mod Keypad Display Figure 34.12 Block diagram of a typical CT2 handset cells increase, and the usable cell size, which is determined by the carrier to interference ratio, automatically reduces. 2 msec Another possible method for sharing spectrum among a multi- plicity of users is by the use of code division (spread spectrum) techniques. In CDMA systems the available bandwidth is not divided into separate channels. Each user has a unique code known to both handset and base station, which is used to spread the voice signal over the whole available bandwidth at the transmitter and to reassemble the transmitted signal at the receiving end. Although such cordless systems are complex compared with analogue sys- items, the low powers used, and the extensive use of digital tech- niques lead to highly integrated products. A block diagram of a typical CT2 handset is shown in Figure 34.12. Control 34.8.3 Standards Figure 34.13 CT2 frame structure Commercial pressures towards open standards lead to the adoption of common air interface standards to encourage competition and facilitate roaming and public access services. The first two common 10 msec air interface standards for cordless systems are CT2 and DECT and are both European in origin. CT2 (second generation cordless telephone) and DECT (Digital European Cordless Telephony) both provide cordless speech bearer- S by using 32kbit/s ADPCM (adaptive differential pulse code modu- lation) to the CCITT G.721 standard developed for transmission on the trunk telephone network. The ADPCM coding takes the 64kbit/s PCM bit stream comprising 8-bit samples at the 8kHz rate, and reduces the 8-bit samples to 4 bits. CT2 uses a 4.0MHz block of radio spectrum between 864MHz and 868MHz to provide 40 duplex channels, and DECT uses the 20MHz from 1880MHz to 1900MHz to provide 120 duplex chan- nels. Average power levels are around 10 milliwatts per channel for both systems, and both systems employ dynamic channel allocation. 