10.6 RADIO SYSTEM DESIGN ISSUES

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Radio system design poses special problems because of the special nature of the radio signal propagation. The important design issues are:

Frequency of operation: Radio systems cannot be operated in any frequency of our choice. Frequency allocation needs to be obtained from the centralized authority of the government. Only certain bands such as ham radio band and Industrial, Scientific, and Medical (ISM) band are unlicensed, and anyone can use these bands without getting a license from the government authorities.

Radio survey: Radio frequency propagation characteristics depend on many factors, such as natural terrain (presence of hills and valleys, lakes) and artificial terrain (presence of high-rise buildings). A radio survey must be carried out to decide where to keep the antennas to achieve the maximum possible coverage. Multipath fading causes signal degradation. Measures have to be taken to reduce the effect of multipath fading.

Note

The propagation characteristics differ for different frequency bands. A number of mathematical models are available to analyze radio propagation. The natural terrain (presence of hills, lakes, greenery) and artificial terrain (presence of tall buildings) also affect radio propagation.

Line of sight communication: Some radio systems are line of sight systems, that is, there should not be any obstructions such as tall buildings/hills between the transmitting station and the receiving station. In the case of broadcasting applications, the transmitting antennas have to be located at the right places to obtain the maximum coverage. Systems such as AM broadcast systems do not have this limitation because the radio waves are reflected by the ionosphere, and hence the range is very high.

Path loss calculations: When a signal is transmitted with a particular signal strength, the signal traverses a large distance and becomes attenuated. The loss of signal strength due to the propagation in the atmosphere and attenuation in the communication subsystems (such as filters and the cable connecting the radio equipment to the antenna) is called path loss. The path loss calculations have to be done to ensure that the minimum required signal strength is available to the receiver to decode the information content. The receiver should be sensitive enough to decode the signals. The required BER, SNR, gain of the antenna, modulation technique used, rain attenuation, and gain of the amplifiers used are some of the parameters considered during the path loss calculations.

Note

For all radio systems, path loss calculations are very important. Based on the path loss calculations, the receiver sensitivity, antenna gain, amplifier gains, etc. are calculated when designing radio systems.

Rain attenuation: The attenuation of the radio signals due to rain varies, depending on the frequency band. For instance, in satellite communication, at 17/12GHz the rain attenuation is very high as compared to 6/4GHz. This aspect has to be taken into consideration in the path loss calculations.

Radio bandwidth: Radio spectrum being a limited natural resource, the bandwidth of a radio channel has to be fully utilized. To achieve this, efficient source coding techniques have to be used. For example, to transmit voice over a radio channel, it is not advisable to use 64kbps PCM (though many systems still use it). A better approach would be to use low bit rate coding techniques (such as ADPCM, LPC, or its variations) so that in a given radio bandwidth, more voice channels can be pumped in.

Note

As radio spectrum is a limited natural resource, the radio channel has to be fully utilized. Using low bit rate coding of voice/video signals and choosing an efficient modulation technique are very important in radio system design.

Radio channels: A radio channel consists of a pair of frequencies—one frequency from base station to the end station and one frequency from end station to the base station. A minimum separation is required between the uplink and downlink frequencies.

Multiple access: Radio systems use multiple access techniques to make efficient use of bandwidth. FDMA, TDMA, and CDMA systems, as discussed earlier, have different spectrum requirements and different complexities.

All these issues need to be kept in mind when designing radio systems.

Design of radio systems involves special issues to be addressed. These include frequency of operation, radio propagation characteristics, path loss calculations, rain attenuation, efficient usage of radio spectrum through low-bit rate coding of voice and video signals, and usage of multiple access techniques.

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