The Global System for Mobile (GSM)

Tuesday, 20 August 2013

GSM uses both TDMA and Frequency Division Multiple Access (FDMA) to transmit and recover information as shown in Figure 1. These systems use data packets at specific times at specific frequencies. Thus, several conversations take place simultaneously and at the same frequency using different time slots. Systems are also frequency duplex so that the transmitting and receiving frequencies are different, and both sides of the transmission (Mobile-to-Base and Base-to-Mobile) are concurrent.  The characteristics of GSM system are shown in Table 1. The spacing between the carriers in GSM system is 200 kHz. Eight time slots carry speech and data in a GSM system.  The bandwidth for the GSM system is 25 MHz, which provides 125 carriers each having a bandwidth of 200 kHz. Due to interference to other systems, the very first carrier is not used, thus reducing the number of carriers to 124.  With eight users per channel there are about 1,000 actual speech or data channels. The number of channels will double to about 2,000 as the half rate speech coder is introduced. The frequency band used for the uplink is 890 MHz to 915 MHz (from MS to base station) and for the downlink 935 MHz to 960 MHz (from base station to MS).

The modulation method in GSM is Gaussian Minimum Shift Keying (GMSK), which facilitates the use of narrow bandwidth and coherent detection capability. In GMSK the rectangular pulses are passed through a Gaussian filter prior to their passing through a modulator. This modulation scheme almost satisfies the adjacent channel power spectrum density requirement of -60 dBc specified by CCIR. The normalized pre-Gaussian bandwidth is kept at 0.3, which corresponds to a filter bandwidth of 81.25 kHz for an aggregate data rate of 270.8 Kbps. With 200 kHz of carrier spacing and this data rate, the spectral efficiency of the system is 1.35 b/s/Hz. With the bit interval of 3.7 ms, the GSM signal will encounter significant intersymbol interference in the mobile radio path due to multipath ( multipath minimal delay spread 3 ms to 6 ms in urban areas). As a consequence, an adaptive equalizer is used. There are eight time slots in a frame and 26 or 51 frames in a multiframe. With 270.8 Kbps divided among eight users in GSM, the per user data rate is 33.85 Kbps. The speech coder is a regular pulse excitation with long-term predictor (RPE-LTP) for a full rate speech that converts speech to 13 Kbps.  There are five different categories of mobile telephone units specified for the European GSM system: 0.8W, 2W, 5W, 8W, and 20W. The power level can be adjusted to vary between 3.7 mW to 20W. To optimize co-channel interference, each BS individually directs MS to use the minimum power setting that is necessary for reliable transmission. The setting is determined by BS and provided to the MS. The GSM air interface allows for frequencies to be hopped to prevent multipath problems resulting in excessive bit error rates. Both the mobile and the base station will use Discontinuous Transmission (DTx). This will allow the mobile to save the battery life and the base station to reduce co-channel interference.


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