From GSM to LTE: An Introduction to Mobile Networks and Mobile Broadband

1.7 The Base Station Subsystem (BSS) and Voice Processing

While most functionality required in the NSS for GSM could be added via additional software, the BSS had to be developed from scratch. This was mainly necessary as earlier generation systems were based on analog transmission over the air interface and thus had not much in common with the GSM BSS.

1.7.1 Frequency Bands

In Europe, GSM was initially specified only for operation in the 900-MHz band between 890 and 915 MHz in the uplink direction and between 935 and 960 MHz in the downlink direction as shown in Figure 1.18. ‘Uplink’ refers to the transmission from the mobile device to the network and ‘downlink’ to the transmission from the network to the mobile device. The bandwidth of 25 MHz is split into 125 channels with a bandwidth of 200 kHz each.

Figure 1.18 GSM uplink and downlink in the 900-MHz frequency band.

It soon became apparent that the number of available channels was not sufficient to cope with the growing demand in many European countries. Therefore, the regulating bodies assigned an additional frequency range for GSM, which uses the frequency band from 1710 to 1785 MHz for the uplink and from 1805 to 1880 for the downlink. Instead of a total bandwidth of 25 MHz as in the 900-MHz range, the 1800-MHz band offers 75 MHz of bandwidth, which corresponds to 375 additional channels. The functionality of GSM is identical ...