Point to point microwave system design

Licensed
Un-licensed

Point to multi-point microwave system design

Un-licensed

In-building Communications systems design

VHF,UHF,Cellular,PCS, 2.4GHz

• Wireless systems that meet system traffic requirements
• Wireless systems that meet reliability requirements
• Wireless systems that are constructed at a minimum cost
• Wireless systems with a proper foundation for future growth

Standard Procedures

During the initial system design, several basic decisions must be made regarding the type of equipment and the configuration which is to be employed. However, before these decisions can be made it is first necessary to establish the volume and type of traffic to be transmitted and where it is to be terminated. It is important that not only the initial traffic requirements be considered but also the needs for future growth. Inaccurate growth forecasting can result in an excessive initial investment if the growth is overstated . Alternatively, predictions, which are too small, can necessitate expensive modification or expansion of the system soon after it is operational.

Usually one of the first decisions which is made is whether the system will employ analog or digital techniques. Most new routes will favor the use of digital radio equipment, since this design offers greater economy and flexibility for the transmission of digital data traffic; a feature which will become increasingly important in the future. Early digital microwave systems were very limited in the amount of traffic, which could be accommodated within a given band, but with improved, and more sophisticated modulation techniques this factor is becoming less of a consideration. Even when voice or other analog traffic is predominant, the relatively low cost of pulse code modulation multiplexing equipment, when compared to frequency division multiplexing equipment, will often favor the use of digital radio. However, analog radio may be more desirable if there is a requirement for carrying video signals or if a very high concentration of voice circuits are needed, such as may be necessary for a long-haul telephone system.

The next step in the preliminary planning should be to decide on the degree of reliability or availability which the system is required to yield. Most non-technical people posed with such a question would probably respond with answers like: "The best possible, " "It must always be available when needed," or possibly "Communications are vital to my business and no service interruptions can be tolerated." Unfortunately, these answers are of little value or help to the microwave system design who requires a specific numeric value upon which to base the design. The appropriate selection of this value is of paramount importance, since it will affect many subsequent design decisions and the over-all cost of the system.

Table 1 provides the relationship between reliability and outage time, but it is almost impossible to predict the duration and frequency of each individual outage, which will contribute to this total value. Further, the outage time will be composed of two different reliability figures. Equipment malfunctions can be expected to be relatively rare, particularly if standby assemblies are furnished, but may be of long duration. If the microwave station is remotely located, it may take an hour or more to dispatch a technician and remedy the fault. Service interruptions due to propagation conditions will be more frequent but of short duration—typically a few seconds. The permissible outage time will affect such factors as:

• Operating frequency band
• Maximum path length
• The need for diversity
• Equipment failure protection
• Antenna size
• Transmitter output power
• Equipment selection

The major factors which influence the choice of the frequency band in which the proposed microwave system will operate are:

• The required traffic capacity
• The classification of the user
• The length of the path
• Frequency congestion in the area
• Weather conditions

The available microwave bands may be broadly classified as 2, 4, 6, 12, 18, and 23GHz. Within each of these bands, sub-classifications assign specific frequencies to various types of users and the amount of bandwidth (and hence the traffic capacity) which can be employed. The widespread acceptance of microwave radio communication by a variety of users has produced congestion of the limited number of frequencies available, particularly in and around large cities and in the lower frequency bands.

Each of the frequency bands and the regulations controlling their use has resulted in systems being developed to meet specific needs. In general, the frequencies between 4 and 7 GHz are employed for heavy density, long distance routes with repeaters spaced at intervals of about 30 to 40 miles. The 2 GHz band is typified by relatively light density, short to moderate haul systems, such as spurs feeding into a backbone route. The frequency bands between 11 and 13GHz, although capable of supporting a large number of circuits, suffer from some propagation limitations which tend to limit the distance between repeaters and thus total system length. An important role of systems operation in these bands is to relieve the congestion experienced at lower frequencies, especially as an entrance link into larger cities. Higher frequencies have not yet been exploited to the extent that the lower bands have, but they can be expected to play an important role in the future where a need for very short (2-10 miles) easily installed inter-city routes exists.

The objective of path is to design a microwave system that meets system traffic and reliability requirements at a minimum cost. The final system path design documentation contains:

• Tower types and heights
• Antenna sizes and types
• Site layout reports
• Site acquisition reports
• Radio equipment configuration plan
• System transmission design

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