金曜日, 1月 27, 2006

Reflected power is a good indicator of antenna and transmit path performance



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Part 3. The concept of fading also needs to be understood. Fading generally refers to the degradation of the signal of the receiver. There are four main fading effects related to physical conditions:

- Attenuation due to distance;
- Attenuation due to environmental features;
-Raleigh fading;
-Multipath fading.

Naturally, as a receiver moves away from the transmitter or places large objects between itself and the transmitter, the signal level will decrease and lower the call quality. When the subscriber moves a signifigant distance away from the transmitter, the fading that occurs is called large scale path loss, as the attenuation is due to the distance. Shadowing generally refers to a type of fading that occurs when the subscriber moves behind a large object or perhaps in a tunnel or the like. These two fades are naturally an obstacle to all wireless systems, although some systems are able to overcome the problem easier than others.

Multipath fading is another type of fading caused by interference between two or more waves leaving the transmitter at the same time, but taking different paths to the receiver and thus arriving at the receiver at different times. This can cause the signals to have different phases as they enter the receiver, and this will cause a destructive process, lowering the actual received level. It should be understood that the multipaths can arrive in phase, and thus would add constructively--a sought after condition.

If the multipaths can somehow be demodulated seperately, and then combined coherently, this would substantially improve the system. This is what CDMA one does with its Rake receiver.
Problems in digital systems related to signals taking different paths to the receiver, and thus arriving at different delays, are also often called intersymbol interference.

Fading related to the motion of the terminals (i.e., driving in a car) is often called Rayleigh fading. Because of the movement of the headset, the received signal rays undergo what is known as a Doppler shift. This causes a shift in the wavelength of the signal, and like the multipath problem described earlier, causes multipath signals at different phases to enter the receiver and degrade (or sometimes improve) reception. The nature of this kind of fading causes the changes to occur very quickly, and sometimes this type of fading is called fast fading.

Fading can either be frequency selective, acting much like a notch at a specific frequency, perhaps as wide as 300kHz, or the fades can be flat, causing a problem across a very large bandwidth. One of the best defenses against deep fading is to widen the bandwidth of the signal. Considering that fading can often be frequency selective, One can easily understand how asystem that uses a wider frequency band will be able to withstand a fading environment better than a narrowband system. This is one of the main reasons that spread spectrum systems are becoming popular. Also the fact that fades can be frequency selective explains why the mobiles transmission to the base station may be terrible, but yet the base station connection to the mobile will be fine, as these two channels will operate on different frequency bands.

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