2.4 Performance requirements

The GSM standard is based on a mixture of time-division multiple access (TDMA) and frequency-division multiple access (FDMA). Each user is allocated a particular frequency channel of width 200 kHz and a time slot of duration 576.9 μs. Over the frequency range 925–960 MHz there are 175 available channels and over a period of 4.16 ms there are eight available time slots. The information rate is typically 9.6 kb/s which, after channel coding and training bits are added, translates into a physical bit rate of 270.833 kb/s. The type of modulation used is Gaussian minimum shift keying (GMSK) with a BT factor of 0.3. Responsibility for the GSM standard now lies with the 3rd-Generation Partnership Project (3GPP) [6].

Full details of the performance requirements relevant to the design of the receiver can be found in Reference 7. These relate to a ‘small’ mobile terminal receiving fullrate speech. There are several different fading conditions under which the terminal must operate, the most relevant of which are the static, TU50 and EQ50 profiles. With a static profile (i.e. no fading) the requirement on bit error rate (BER) is generally 2 per cent unless specified otherwise. For the TU50 profile, which is intended to represent a typical urban situation where the terminal is limited to a velocity of 50 km/h, the BER requirement is 8 per cent. A 3 per cent BER usually applies to the EQ50 profile. The most important of the receiver characteristics for GSM are listed in Table 2.1.

Table 2.1: Receiver characteristics
Maximum input	−15 dBm for 0.1 per cent BER, static channel
Reference sensitivity	−102 dBm for 8 per cent BER, TU50 channel (c.f. state of the art −108 dBm)
Cochannel interferer	−9 dB with respect to wanted signal at −82 dBm, 8 per cent BER, TU50 channel
Adjacent-channel interferer	9 dB with respect to wanted signal at −82 dBm, 8 per cent BER, TU50 channel
Alternate-channel interferer	41 dB with respect to wanted signal at −82 dBm, 8 per cent BER, TU50 channel
Largest blocking signal (CW)	−23 dBm, 3 MHz offset, wanted signal at −99 dBm, 2 per cent BER, static channel
AM interferer	−31 dBm, 6 MHz offset, wanted signal at −99 dBm, 2 per cent BER, static channel

It should be noted that although the GSM specification only calls for a reference sensitivity of −102 dBm, most receivers in current use actually achieve a sensitivity of −108 dBm and therefore this is now the more relevant performance objective. The AM interferer test is intended to verify that a mobile terminal will continue to function in close proximity to a GSM basestation belonging to another operator. The bursting of the large signal from the basestation can cause second-harmonic distortion in the receiver frontend which is particularly damaging if the rising edge of the burst coincides with the training sequence in the middle of the burst of the wanted signal. Zero-IF receivers are particularly prone to this kind of interference since the intermodulation products are mostly located around DC. Low-IF receivers, on the other hand, are more resilient because of the offset of the wanted signal, as previously explained.