This article introduces the basic concepts of the cellular handset through a brief historical background, details about the evolution of the GSM, and the physical-layer specifications. We will also detail the principles of data sampling, voice compression, digital modulation, and demodulation.
The Evolution of GSM
Although the first wireless telephone was invented by Bell Labs as early as 1940, the first analog cellular network—called AMPS (Advanced Mobile Phone System)—was not commercially available until 1978. In 1979, frequency allocation started the movement towards the GSM (Global System for Mobiles) standard. The technology took time to develop—the 900MHz GSM commercial kick-off did not occur until 1993. In 1996, the DCS (Digital Communication System) began using a second frequency band near 1800MHz. By 1997, the mobile phone had evolved into an everyday accessory for millions. This trend shows no signs of slowing in the future (Figure 1).
Figure 1: Number of cellular phone subscribers worldwide (actual and estimated)
In the near future, UMTS (Universal Mobile telephone system) will be launched. The new communication standard will feature real-time video, handled by an high-speed protocol that will not be compatible with the GSM standard and existing infrastructure (Figure 2).
Figure 2: Cellular phone development from the GSM standard to the 3G UMTS standard
GSM Level 1 Specifications
The specification level 1 refers to a definition proposed by the International Standard Organization (ISO) for an open system interconnection. The level 1 for the handset is a physical layer, in charge of the sound sampling, coding, modulation, and transmission. In the embedded software of the handset, we also find layer 2 specifications, dealing with the data exchange protocol.
Figure 3: Frequency assignment in GSM and DCS bands
The handset in the GSM standard is called the mobile station (MS). The base station is referred to as BS. The maximum GSM emission power is 2W in 900MHz and 1W in 1800MHz. The base station may reach 300W, but many efforts are undergoing to avoid the use of such power in urban environments, as base stations and mobile phones may harm health.
Figure 3 describes the frequency allocation for up and down links. A significant separation of the Tx and Rx sub-bands eases the coupling avoidance between transmitter and receiver parts.
Figure 4: Network channel capacity and separation between operators
The theoretical network capacity is the number of available channels in the sub-band. As each frequency channel is 200KHz in size, up to 125 channels are available for GSM and 375 channels for DCS (Figure 4). This number seams quite large, however, operators buy licenses to exclusively use a portion of the channels. For example, Operator 1 in Figure 4 exploits the frequency band from 1710 to 1735MHz, 125 channels in DCS.
Figure 5: Sharing the frequency resource between adjacent cells
A further reduction of channels is due to the cellular structure of the network (Figure 5). To avoid frequency conflicts, the channel resource is split into seven portions, each one allocated to a cell. Consequently, the number of available channels in our example is 125/7, which equals 17 available channels.
Time-Domain Multiplexing
GSM introduced the concept of time domain multiple access (TDMA), a multiplexing approach that enables eight phones to share a single frequency channel. This approach adds considerable complexity to the protocol, compared to a single user for each channel. Over a frame of 4.61 ms, each phone takes the lead on the channel and transmits its information in a burst of 577µs (Figure 6).
Figure 6: An illustration of time domain multiplexing
Figure 7: Block diagram of a mobile station
The block diagram in Figure 7 provides a simplified description of the mobile station. A microphone captures the sound, which is sampled in a numerical format, compressed, coded, and modulated. A high-frequency oscillator translates the modulated signal to a valid transmission frequency. The received signal (less than 1mV) is amplified before down-conversion to a low-frequency, demodulation, decoding, and sound reconstruction.
Speech Sampling and Compression
The sound captured by the microphone is filtered to remove harmonics lower than 300Hz and higher than 3000Hz (Figure 8). A gain-control stage then keeps the signal envelope more or less constant, before a sampling at the rate of 8000Hz, in a 13-bit format. This leads to a rate of 104 Kbit/s, meaning that a compression algorithm is mandatory to obtain an acceptable rate of 12Kb/s.
Figure 9: The GSM speech coder is based on adaptive filtering techniques
The voice coder implemented in the GSM handset splits the sound into portions of 20ms, with a time-domain aspect reported in the top portion of Figure 9. The coder removes the redundant periodic structure of the sound (80% of the speech). You use linear prediction
and root mean square minimization as the mathematical basis for computing the best coefficients to approximate each portion of sound by an adaptive filter. Consequently, the filter coefficients are transmitted, not the sampled sound. This features a significant reduction of data. Low-order coefficients are coded in a 6-bit format while high-order coefficients are in a 3-bit format.
Modulation
The modulation used in GSM is derived from quadrature phase shift keying. Each byte of data is split into four pairs of bits (Figure 10). To each pair of bits corresponds a particular phase for I(t) and Q(t). The modulated signal is the sum of the two sinusoidal waves, with a phase shift depending on the logic symbol. Smooth transitions in the Gaussian Modulated shift prevent the spread of harmonics in the emission spectrum.
Figure 10: The GSM modulation, based on phase change
Demodulation
The principles for numerical demodulation of phase modulated-signals are based on the multiplication of the received signal by a sine and cosine with the same frequency. The result shown inFigure 11 is a composition of two effects: a sinusoidal wave with twice the initial frequency (removed by filtering) and steps of voltage that correspond to the initial bits sent by the transmitter. Half the bits are obtained from multiplication by the cosine, the other half from multiplication by the sine. At the price of bit manipulation, the bytes are easily reconstructed (Figure 12).
Figure 11: The GSM demodulation obtained using direct down-conversion
Figure 12: Byte reconstruction using cosine and sinus multiplication
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