Radio Propagation, System Range, Reliability and Availability
The operation of a radio system within its system `range' is critical to ensuring good quality of transmission. In this chapter we discuss how to caculate the range of a radio system, and we learn about the parameters and other factors which de®ne the range and affect the performance. We discuss how the weather and the gaseous constitution of the atmosphere affect the performance and reliability of a radio system. We also learn that the `range' of a radio system is not a ®xed value Ðradio waves do not conveniently cover the intended coverage area and then stop at a boundary line we call the `range' of the system. Rather, the range is only a nominal value, de®ning the maximum distance the two endpoints of the radio link may be from one another if a given quality of link performance and reliability is to be achieved. For different link quality targets, the range will differ. We discuss the methodologies and detailed mathematical formulae for calculating the range of a radio link or system, and de®ne on the way a critical parameter for de®ning our reliability needs for the link Ðthe
`availability' (the percentage of time the link is to meet the desired quality standard).
7.1 The Reliability, `Range' and `Availability' of a Radio System
There is no standard means of de®ning the `reliability' of a radio system. Instead the standard terms are the system range and the radio link availability. The range and availability of a radio system are limited by the transmitter power output, the receiver sensitivity, and by the atmospheric and climatic conditions of the region of operation.
Before we can determine exact values, though, we must de®ne exactly what we mean by range and availability.
The reliable propagation of radio waves across a radio link is achieved when a signal of adequate strength for good quality demodulation arrives at the receiver. So that the signal arrives with suf®cient strength, there has to have been suf®cient signal power generated by the transmitter, adequate sensitivity of the receiver and only limited signal attenuation or ampli®cation (i.e. loss or gain in signal strength) caused by the atmosphere. The atmospheric or path attenuation is usually caused by interference from other signals, absorption of the signal (by atmospheric gases or by rainfall) or by path re¯ections or other effects.
Of all the factors in¯uencing the range of the system, the technical speci®cations of the radio system are most easily de®nable:
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. The transmitter power of microwave radio systems is usually quoted in Watts (W), milliWatts (mW) or dBm (decibels relative to 1 milliWatt). Typical maximum values are in the range 15 dBm (32 mW) to 25 dBm (320 mW).
. The receiver sensitivity is usually quoted in terms of the minimum threshold Received Signal Level (RSL) required to be received by the receiver for a de®ned quality (or
`accuracy') of reception. CEPT and ETSI recommendations for digital radio suggest quoting the receiver threshold as the minimum power required for the receiver to achieve a Bit Error Ratio (BER) reception of a digital signal better than 10 6: The receiver sensitivity of a digital microwave radio system is typically around
80 dBm. If the received signal level is less than than this value, then the desired threshold BER will not be achieved. In this case, the received signal will contain more errors than the target BER of 10 6, and the radio link is assumed to be out of operations limits or unavailable. But there will still be reception of a signal Ð received signal level lower than the threshold value does not necessarily mean we have no reception.
Expressed in dBm, the higher the negative value of the receiver sensitivity (i.e. the weaker the receivable signal) the more sensitive is the receiver. Sometimes, multiple values of sensitivity are quoted for a radio receiver. Thus a receiver with a sensitivity threshold (or receiver threshold) of 80 dBm for BER 10 6 will have a threshold of approximately 83 dBm for BER 10 3. In other words, the receiver can still receive even weaker signals, but at a lower level of accuracy. Said another way, the link of high quality is unavailable, but a lower quality link is available!
More dif®cult to de®ne and predict than the basic radio performance is the path attenuation or path loss of the signal. This is the loss in signal power caused by the atmosphere between the two ends of the link. This attenuation is variable, depending upon current weather and atmospheric conditions. The availability of the radio link is thus dependent upon the local climate.
Heavy rainfall, atmospheric disturbances and radio signal absorption effects tend to attenuate (i.e. weaken) the signal so reducing the RSL. During good weather the signal strength will allow good reception, while during very heavy thunderstorms the received signal may contain an unacceptably high number of errors. The principle is straightforward, but the prediction of the exact quality of performance is much harder to handle. The important question is, for what percentage of the time the Received Signal Level (RSL) exceeds the minimum value required to achieve a given BER target. This percentage of the time for which the target link transmission quality is achieved is termed the link availability.
For estimating the likely effects of the climate on the operation of a radio system, ITU-R (the Radiocommunications sector of the International Telecommunications Union) has published a large number of recommendations and reports on the propagation of radio.
These present a standardised method for calculating the range of a radio system.
The ITU-R reports and recommendations classify the regions of the world into different climate zones, each zone characterised by a typical climate pro®le drawn from extensive weather measurements over many years and named with a letter of the alphabet from A to S (Figure 7.1). The climate zones are characterised in terms of their `normal' (i.e.
statistically averaged) weather patterns, from which the `expected' radio path loss for different types of weather and at different times of year can be calculated.
The range of a radio system, when calculated according to ITU-R recommendations, is the maximum distance at which the radio terminals may be placed apart from one another, in order to ensure a minimum BER quality for a given target availability. The range of the 116 Radio Propagation, System Range, Reliability and Availability
system is usually calculated in kilometres for a given ITU-R climate zone. When quoted, the de®ned availability target and the BER target should also be quoted.
Thus, a 26 GHz shorthaul radio system may be quoted as having a range of 15 km, given:
. a target BER (Bit Error Ratio) of 10 6 (i.e. a maximum of 1 error in every million bits sent) is achieved;
. a target availability of 99.99%;
. in the worst month (i.e. maximum of 4.5 minutes unavailable in the worst monthÐor alternatively over the year as a whole Ðoutage 53 minutes per annum);
. in climate zone `E';
. while carrying 2 2 Mbit/s bitrates (the threshold RSL may vary from one bitrate to another).
The range, when quoted on its own (without also stating the other ®ve target parameters listed above), is meaningless. It is thus very dangerous to compare the system range quoted by different manufacturers of radio systems on their datasheets, without having checked that each of the ®ve parameters above have been set the same for the two system range calculations.
The Reliability, `Range' and `Availability' of a Radio System 117
Figure 7.1 ITU-R climate zones (ITU-R Recommendation PN.837, Reproduced by permission of the ITU)
Most ®xed wireless access radio systems operate in either the millimetre wave or microwave radio bands. Attenuation in these bands is particularly great during heavy rainfall. It is the high density of water between the antennas of transmitter and receiver which leads to high signal attenuation, and thus to periods of unavailability. Unavailability generally thus results from the critical rainfall rate being exceeded for the given path length (typically during a very heavy thunderstorm).
It is customary to limit the unavailability and to design radio systems for a minumum availability of 99.99% (53 outage minutes per annum, based on annual statistics of ITU-R, or about 4.5 minutes per month if worst month radio planning is conducted). A calculation set out in ITU-R recommendations allows the radio planner, based upon the climate zone, the transmitter power, the receiver sensitivity and the antenna gain to calculate the maximum system range. The calculation method is based upon an ITU-R model, named the ITU-Rtransmission loss concept, which we discuss later in the chapter. However, before we move on, we summarise the concepts of radio system range and availability to underpin our understanding of the terms.
Neither of the terms range or availability are `absolute' measures: both depend upon the targets we set for the `performance' (quality) and `reliability' of the radio link. Figure 7.2 illustrates an example of how the range of a 26 GHz radio system is affected by the target values set for climate zone, carried bitrate, BER target and availability. The values assumed are typical values for a point-to-multipoint (PMP) system operating with 908 base station sector antennas. From the diagram you will see how the range is reduced by:
. the ITU-R climate zone (the later the letter in the alphabet, generally the lower the range);
. increasing link availability target (the higher the availability needed, the lower the range);
. increasing link quality target (the lower the target bit error ratio required, the lower the range).
118 Radio Propagation, System Range, Reliability and Availability
Figure 7.2 The range of a radio system depends upon the target values for link quality (bit error ratio) and availability, as well as being dependent upon the climate zone