Expert: Cezar L. Palconet Date: 4/9/2007 Subject: GSM Bandwidth reduction
Question hello
I am intersted to understand the benefits of bandwidth reduction in GSM . Please give some information or site addresses to start....
Finanly,I apologize about my bad english.
Thanks alot.
Answer Hi, Morteza
Bandwidth reduction in GSM, simply imply that instead of using multiple frequency carriers for the A-Bis link between many BTS's, one could aggregate them into a single carrier, this is like using an LMDS network to connect many BTS’s, and aggregate them into a single back haul carrier.
In the ADC applications it has a different function, the process of sub-sampling or folding can be thought of as mixing the ADC input signal with the sampling frequency and its harmonics. This means that many frequencies can be mixed down to DC and their original frequency can no longer be determined. Sub-sampling cannot be used if the original input frequency must be determined at the ADC output. This is because the Nyquist criteria is violated. Sub-sampling still proves useful
if there is no need to determine the carrier frequency at the ADC output. This is true for many communication systems such as cellular base-station receivers since the receiver only needs to recover the information on the carrier and not the carrier itself.
Many Frequencies are mixed down towards DC when Sub-Sampling, consider a GSM/EDGE base-station with RF carrier frequencies of 900MHz Europe and 1800MHz USA. In the receive chain of a mobile base-station, the high frequency RF carrier signal is first down-converted in the mixer and local oscillator stage to an IF frequency in the range of 150 -190MHz for analogue to digital conversion. Shannon theory shows that the required sampling frequency is a function of the signal bandwidth or 200kHz in a GSM/EDGE system. The dynamic range specifications of a GSM system require a minimum ADC resolution of 10 bits although 12 bits are used in practically all cases. Given the vast choice of high speed ADCs available in the market a digital receiver system designer has to make a choice based upon system dynamic range requirements and also component cost. For these reasons, ADCs with sampling rates of 50 -70MSPS are the most popular choice for GSM receivers. Although the 150 – 190 MHz signal is under sampled at for example 66MSPS, Nyquist is not violated for the required information bandwidth of 200kHz. This provides more than enough headroom for the 200kHz bandwidth information signal as well as providing over 20dB's of processing gain.
Note that for many reasons, it is not practical to continually increase processing gain by
increasing sampling frequency. Higher sampling rate 12-bit ADCs such as the 12-bit 80MSPS ADCs exist for specialized applications but the cost differential between sub-100MSPS and >100MSPS sampling rate ADCs is quite significant.
