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From GSM to LTE: An Introduction to Mobile Networks and Mobile Broadband by Martin Sauter

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4.3 FDD Air Interface and Radio Network

The major evolution in LTE compared to previous 3GPP wireless systems is the completely revised air interface. To understand why a new approach was taken, a quick look back at how data was transmitted in previous generation systems is necessary:

GSM is based on narrow 200-kHz carriers that are split into eight repeating timeslots for voice calls. One timeslot carries the data of one voice call, thus limiting the number of simultaneous voice calls on one carrier to a maximum of eight. Base stations use several carriers to increase the number of simultaneous calls. Later on, the system was enhanced with GPRS for packet-switched data transmission. The decision to use 200-kHz carriers, however, remained the limiting factor.

With UMTS, this restriction was lifted by the introduction of carriers with a bandwidth of 5 MHz. Instead of using dedicated timeslots, CDMA, where data streams are continuous and separated with different codes, was used. At the receiving end, the transmission codes are known and the different streams can hence be separated again. With HSPA, the CDMA approach was continued but a timeslot structure was introduced again to improve user data scheduling. The timeslots, however, were not optimized for voice calls but for quickly transporting packet-switched data traffic.

With today's hardware and processing capabilities, higher datarates can be achieved by using an increased carrier bandwidth. UMTS, however, is very inflexible ...

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