1.2.1 Attenuation
Attenuation is the drop in the signal power when transmitting from one point to another. It can be caused by the transmission path length, obstructions in the signal path, and multipath effects. Figure 2 shows some of the radio propagation effects that cause attenuation. Any objects that obstruct the line of sight signal from the transmitter to the receiver can cause attenuation.
Figure 2 Radio Propagation Effects
Shadowing of the signal can occur whenever there is an obstruction between the transmitter and receiver. It is generally caused by buildings and hills, and is the most important environmental attenuation factor.
Shadowing is most severe in heavily built up areas, due to the shadowing from buildings. However, hills can cause a large problem due to the large shadow they produce. Radio signals diffract off the boundaries of obstructions, thus preventing total shadowing of the signals behind hills and buildings. However, the amount of diffraction is dependent on the radio frequency used, with low frequencies diffracting more then high frequency signals. Thus high frequency signals, especially, Ultra High Frequencies (UHF), and microwave signals require line of sight for adequate signal strength. To over come the problem of shadowing, transmitters are usually elevated as high as possible to minimise the number of obstructions. Typical amounts of variation in attenuation due to shadowing are shown in Table 6.
| Description | Typical Attenuation due to Shadowing |
| Heavily built-up urban centre | 20dB variation from street to street |
| Sub-urban area (fewer large buildings) | 10dB greater signal power then built-up urban center |
| Open rural area | 20dB greater signal power then sub-urban areas |
| Terrain irregularities and tree foliage | 3-12dB signal power variation |
Table 6 Typical shadowing in a radio channel (Values from [11])
Shadowed areas tend to be large, resulting in the rate of change of the signal power being slow. For this reason, it is termed slow-fading, or log-normal shadowing.
1.2.2 Multipath Effects
1.2.2.1 Rayleigh fading
In a radio link, the RF signal from the transmitter may be reflected from objects such as hills, buildings, or vehicles. This gives rise to multiple transmission paths at the receiver. Figure 3 show some of the possible ways in which multipath signals can occur.
Figure 3 Multipath Signals
The relative phase of multiple reflected signals can cause constructive or destructive interference at the receiver. This is experienced over very short distances (typically at half wavelength distances), thus is given the term fast fading. These variations can vary from 10-30dB over a short distance. Figure 4 shows the level of attenuation that can occur due to the fading.
Figure 4 Typical Rayleigh fading while the Mobile Unit is moving (for at 900 MHz)[15]
The Rayleigh distribution is commonly used to describe the statistical time varying nature of the received signal power. It describes the probability of the signal level being received due to fading. Table 7 shows the probability of the signal level for the Rayleigh distribution.
| Signal Level (dB about median) | % Probability of Signal Level being less then the value given |
| 10 | 99 |
| 0 | 50 |
| -10 | 5 |
| -20 | 0.5 |
| -30 | 0.05 |
Table 7 Cummulative distribution for Rayleigh distribution (Value from [15])
1.2.2.2 Frequency Selective Fading
In any radio transmission, the channel spectral response is not flat. It has dips or fades in the response due to reflections causing cancellation of certain frequencies at the receiver. Reflections off near-by objects (e.g. ground, buildings, trees, etc) can lead to multipath signals of similar signal power as the direct signal. This can result in deep nulls in the received signal power due to destructive interference.
For narrow bandwidth transmissions if the null in the frequency response occurs at the transmission frequency then the entire signal can be lost. This can be partly overcome in two ways.
By transmitting a wide bandwidth signal or spread spectrum as CDMA, any dips in the spectrum only result in a small loss of signal power, rather than a complete loss. Another method is to split the transmission up into many small bandwidth carriers, as is done in a COFDM/OFDM transmission. The original signal is spread over a wide bandwidth and so nulls in the spectrum are likely to only affect a small number of carriers rather than the entire signal. The information in the lost carriers can be recovered by using forward error correction techniques.
1.2.2.3 Delay Spread
The received radio signal from a transmitter consists of typically a direct signal, plus reflections off objects such as buildings, mountings, and other structures. The reflected signals arrive at a later time then the direct signal because of the extra path length, giving rise to a slightly different arrival times, spreading the received energy in time. Delay spread is the time spread between the arrival of the first and last significant multipath signal seen by the receiver.
In a digital system, the delay spread can lead to inter-symbol interference. This is due to the delayed multipath signal overlapping with the following symbols. This can cause significant errors in high bit rate systems, especially when using time division multiplexing (TDMA). Figure 5 shows the effect of inter-symbol interference due to delay spread on the received signal. As the transmitted bit rate is increased the amount of inter-symbol interference also increases. The effect starts to become very significant when the delay spread is greater then ~50% of the bit time.
Figure 5 Multipath Delay Spread
Table 8 shows the typical delay spread for various environments. The maximum delay spread in an outdoor environment is approximately 20 us, thus significant inter-symbol interference can occur at bit rates as low as 25 kbps.
| Environment or cause | Delay Spread | Maximum Path Length Difference |
| Indoor (room) | 40 nsec - 200 nsec | 12 m - 60 m |
| Outdoor | 1 m sec - 20 m sec | 300 m - 6 km |
Table 8 Typical Delay Spread
Inter-symbol interference can be minimized in several ways. One method is to reduce the symbol rate by reducing the data rate for each channel (i.e. split the bandwidth into more channels using frequency division multiplexing, or OFDM). Another is to use a coding scheme that is tolerant to inter-symbol interference such as CDMA.
No comments:
Post a Comment