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

3.3 Code Division Multiple Access (CDMA)

To show the differences between the UMTS radio access network and its predecessors, the next paragraph gives another short overview of the basic principles of the GSM/GPRS network and its limitations at the time Release 99 UMTS networks were rolled out. As discussed in Chapter 2, some of those limitations have been reduced or overcome in the meantime and are now not as severe as the description below.

In GSM, data for different users is simultaneously transferred by multiplexing them on different frequencies and timeslots (Frequency and Time Division Multiple Access, FTDMA). A user is assigned one of eight timeslots on a specific frequency. To increase the number of users that can simultaneously communicate with a base station the number of simultaneously used frequencies can be increased. However, it must be ensured that two neighboring base stations do not use the same frequencies as they would otherwise interfere with each other. As the achievable speed with only a single timeslot is limited, GPRS introduced the concept of timeslot bundling on the same carrier frequency. While this concept enabled the network to transfer data to a user much faster than before, there were still a number of shortcomings, which were resolved by UMTS.

With GPRS, it was only possible to bundle timeslots on a single carrier frequency. Therefore, it was theoretically possible to bundle up to eight timeslots. In an operational network, however, it was rare that ...