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Chapter 1
Radio Propagation
This chapter aims at establishing the link budget of a radio transmission. The objective is to link the cell coverage and the useful throughput of a wireless system with the transmission power. These calculations are performed based on the transmission power, and using an accurate model of the power scattering in the space between the transmitter and the receiver. This radio space is then called the propagation channel. Its definition may be complex, as we may, in some cases, choose to integrate some elements of the transmission and reception chains in it (especially the antenna systems). Consequently, the most important elements, which will be introduced in an incremental way in the proposed exercises, are mainly: the transmission power, the antenna gain, the noise power spectral density, the useful throughput and the communication channel’s capacity.
In an environment with obstacles, the radiocommunication channel undergoes lots of attenuations. It is modeled by propagation losses, also called path losses. Given that the same signal is transmitted, the average power of the received signal decreases as a function of the distance, according to a distribution that depends on the environment, called the path loss model. The path loss models are empirical and are obtained using radio measurements. We can distinguish between the urban environment model, the rural environment model or indoor environment models that are used inside buildings.
The radio channel also undergoes a shadowing effect. In the multipath case, it introduces fading phenomena into the signal spectrum. The multipath channel may also create inter-symbol interference.
Inter-symbol interference occurs depending on the channel selectivity. A radio channel is frequency-selective if its channel bandwidth is large with respect to its coherence bandwidth or, equivalently, if the maximum transmission delay introduced by the channel is large with respect to the modulation symbol time. In this case, equalization techniques must be used at the receiver in order to recover the transmission symbols.
The channel is time-selective if it varies faster than the modulation symbol time. This phenomenon is due to the mobile’s speed and is called the Doppler effect. It happens when the coherence time of the channel is small with respect to the symbol time. The channel is then a “fast fading” channel. If the channel varies during the training sequence, it is no longer possible to estimate the channel impulse response, and equalization can no longer be performed.
1.1. Free-space loss link budget and capacity
In this first simple introductory problem, we consider free-space propagation for an earth-to-satellite radio link. The carrier frequency is equal to 6 GHz, the transmitter power is equal to 4 W, the transmission bandwidth is equal to 200 kHz, the earth station antenna is a parabolic antenna with a diameter equal to 80 cm, the ...
