Direct Path
Reflect
ion from Building
TX
RX
Figure 7
Multipath Propagation
3.4 Fast Fading
Fading is an unwanted change in signal level at the receiver. This change may be so slow as to be unnoticeable, or it may be fast. When fast fading occurs, moving the receiver antenna just 8 cm (λ/2 at 1900 MHz) would be enough to move from a peak to a trough in signal strength. Thus, if the transmitter and/or receiver are mobile, the received signal strength will fluctuate rapidly, causing an effect variously known as flutter, short sector fading or, more commonly, fast fading.
In practice there will still be a very large number of paths between transmitter and receiver and the signals arriving from different paths will be likely to differ not just in phase, but also in amplitude. This is because of the differing path lengths, and also because of the effects of reflection, where waves will be scattered and attenuated further. In addition, the waves encounter both fixed and moving reflectors, which further complicate the analysis.
The typical overall effect on signal strength for a mobile receiver is summarized in Figure 8, which shows the effect of fast fading superimposed on the local mean signal level, which will vary slowly.
Log Distance
Local Mean Signal Level
Received Signal Strength
Fast Fading
Figure 8 Fast Fading
3.5 Rayleigh Fading
In a mobile radio environment there is often no direct path or Line of Sight (LoS), and the total signal received is the phasor sum of many indirect signals. This leads to a fast fading envelope characterized by deep fades of 20–30 dB in some cases. The fading envelope has a Rayleigh distribution, and the mobile receiver is said to be suffering Rayleigh fading, or working in a Rayleigh environment. This is typified by an urban or in-building environment. See Figure 9.
Obstructions e.g. buildings, hills
Figure 9 Rayleigh Fading
3.6 Rician Fading
If there is a relatively strong direct component of signal together with relatively few indirect signals, fast fading will still occur but fades will be less deep. The fading envelope has a Rician distribution. This type of fading is most likely to occur in rural environments. See Figure 10.
TX
RX
Figure 10 Rician Environment
3.7 Time Dispersion and Inter Symbol Interference (ISI)
The multipath environment causes another potential problem for digital systems like GSM: time dispersion.
Many versions of the same signal arrive at the receiver, all having travelled different distances. The recovered data from each path will therefore be dispersed in time as illustrated in Figure 11, which shows a simple three-path case.
The delay spread is the difference in time between the earliest and latest arriving signals.
3.7.1 Delay Spread for GSM
If the delay spread is significant in relation to one bit period (about 3.7 µs for GSM), then significant Inter-Symbol Interference (ISI) will occur and the receiving system will find unambiguous decoding of each symbol difficult because of interference from delayed data. For example, in Figure 11 correct decoding of the second bit (0) will prove difficult if seriously delayed versions of the first bit (1) are still arriving.
Serious ISI will lead to an increased number of decoding errors and unacceptable Bit Error Rate (BER).
The Viterbi channel equalizer used in the GSM receiver is designed to minimize the effects of time dispersion, and can compensate for delay spreads up to approximately 16 µs.
3.7.2 Assessment of Delay Spread
In general, the delay spread will depend on how open the environment is. To achieve a delay spread of 1 µs between two radio paths requires a path length difference of 300 m. A 30 m path difference will produce a 0.1 µs (100 ns) spread, and so on.
Clearly, in an indoor environment, path length differences of hundreds of metres are likely only in very large open buildings such as factories and shopping areas.
Indirect Path 2
3.8 Slow (Shadow) Fading
This is caused by the effect of obstruction or shadowing caused by clutter such as buildings, vehicles, terrain, and trees. As shown in Figure 12, the effect manifests itself as relatively slow variation in received signal strength around a local mean. It can be shown that the signal strength values have a log-normal distribution about the local mean and this allows relatively simple statistical analysis of appropriate fade margins. For GSM a typical standard deviation of σs = 7 dB is quoted, and this can be used to compute an approximate fade margin based on a required probability of coverage.
Dynamic power control helps to offset slow fading. As the mobile moves behind an obstruction, it will be commanded to power up, and vice versa. Similarly, receiver Automatic Gain Control (AGC) will help to compensate for slow fading by increasing sensitivity.
Slow fading can also be termed macroscopic fading.
ReceivedSignalStrength
Path of MS
Log Distance (Metres)
• Note scale
Mean
Figure 12 Slow Fading
3.9 Co-channel Interference
Co-channel interference results from radio transmissions on the same frequency as the serving cell. This type of interference is inevitable in a cellular system, where frequency reuse is essential to achieve required traffic capacity for the network.
Since it is most likely to be caused by other cells in the same network, the level of co-channel interference can be controlled by careful frequency planning. The Carrier to Co-channel Interference ratio (C/I) for GSM should in theory be 9 dB or better; in practice, values of 12 dB or more should be the planning goal.
3.10 Adjacent Channel Interference
Adjacent channel interference is interference caused by radio transmissions using frequencies adjacent to that being used by the serving cell. In the case of GSM networks, this means frequencies at ƒc + 200 kHz and ƒc – 200 kHz, where ƒc is the frequency of the serving cell. Once again, this type of interference is almost certain to emanate from other cells in the same network and can therefore be controlled by careful frequency planning. Theoretically for GSM, the Carrier to Adjacent Channel Interference ratio (C/A) should be –9 dB or better; in practice, values of over 0 dB and possibly 3 dB should be the planning goal.