ترجمه مقاله سیستم های رادیویی ثابت: تجهیزات نقطه به نقطه و نقطه به چند نقطه

Multipoint (MP) systems are traditionally used for rural applications where line-of-sight paths between the central station and terminal stations can be ensured by conventional path planning process. To overcome problems of non line-of-sight paths, repeaters are often used. In the urban/suburban areas, however, the situation is quite different. Radio coverage could be provided from a tower located outside a town or from a tall building in the centre of the town. In either case line-of-sight paths to all potential customer locations is more difficult to guarantee than in a rural area, and Fresnel zone clearance is not always possible. The multipath problem needs to be tackled, since the use of terminal station antennas with beamwidths in excess of 20 degrees is not uncommon.

Increasingly access technologies for MP systems are being optimized for greater spectral efficiency [1]. For example, in Direct Sequence Spread Spectrum Systems (DSSS) using Code Division Multiple Access (CDMA), synchronous transmissions are used in both directions in order to minimize inter-system interference. Additionally, by selecting orthogonal codes, inter-system interference could be reduced considerably, thereby increasing the number of channels usable simultaneously in a given bandwidth. However, multipath signals of the type experienced in urban areas could increase the noise level and hence reduce the number of users able to share the channel simultaneously. Unlike mobile systems, wireless local loop systems do not necessarily have rake receivers to combat multipath interference. The effect of multipath is to increase inter-system noise. It is the overall signal-to-(noise + interference) ratio which sets the limit on the number of users able to share a given channel simultaneously. This is particularly important where orthogonal codes are used to minimize inter-system interference.

9 Conclusions

The use of CP in urban/suburban MP systems gives an additional degree of freedom in optimizing system performance by reducing the effect of multipath in up-links. This is a particular benefit in heavily urban environments where reflections tend to be from approximately perfect conductors. If the multipath suppression allows the use of higher order modulation schemes that permit higher data rates, then the amount of data that may be transmitted per Hz per square kilometre would be greater than with linearly polarized systems. This would offset any concerns that it may no longer be possible to employ frequency re-use at a given central station site, due to the inherently poorer cross polar isolation with circularly polarized antennas. MP systems employ a number of access technologies (e.g. FDMA, FH-CDMA, DS-CDMA, TDMA) and consequently rules on how different access technology systems should co-exist are expected to be in place in due course. In order to encourage competition, national authorities may allocate spectrum for MP in blocks and/or impose geographical restriction on their use. This is one example where the use of CP could be contemplated, i.e. when block frequency allocation is made.