GB2166325A - A method of carrying out a radio propagation survey - Google Patents

Abstract

A method of carrying out a radio propagation survey in which a site (10) FIG 1 which may be an industrial site, such as a power station, or an urban area, is divided into a plurality of elemental areas (12) of a size greater than twenty times the wavelength of the carrier frequency of a test signal transmitted by a base station (14). An operator equipped with a receiver (18) having a communications probability indicator (22) (Figures 2 and 3 (not shown)) proceeds from one elemental area to the next, either on foot or in a vehicle, monitoring a particular characteristic of the transmitted signal and the number of occurrences of that characteristic within a predetermined sampling period in each elemental area, thereby determining the quality of reception of that characteristic. This characteristic may be either the carrier strength or SINAD (the ratio of the [signal plus noise plus distortion] to [noise plus distortion]). In order to conduct a survey for two-way radio communication the power output of the base station is adjusted to equal that of the weakest transmitter to be used in the radio system. <IMAGE>

Description

SPECIFICATION A method of carrying-out a radio propagation survey The present invention relates to a method of carrying out a radio propagation survey.

When a radio system covering a site is installed whether it be in a large building complex such as a power station or covering a large urban area it is important for an operator to know the extent of area coverage for a given service quality. Information about the extent of area coverage is of interest not only when the system has been newly installed but also after it has been operating for several years and climatic conditions may have caused deterioration in the performance of parts of the system. In conducting a radio propagation survey different parts of a site have to be visited either on foot or by mobiles and assessments made. There are two commonly used methods of making assessments.In one of these two methods a person at a base station communicates with another person equipped with a transceiver who moves about the site and makes a subjective assessment of the strength and quality of the signal received from the base station. In the other of these two methods absolute field measurements are made from different points on the site and probability curves are drawn-up and a medium value of signal intensity determined. For each sector of the area to be covered the former case is labour intensive in requiring at least two operators and provides a subjective result whilst the latter case requires complex test equipment and further extensive analysis.

It is an object of the present invention to provide a simple but effective way of carrying out a radio propagation survey.

According to the present invention there is provided a method of carrying-out a radio propagation survey, comprising dividing a site to be surveyed into a plurality of elemental areas, transmitting a signal from a base station, measuring the duration of presence of the transmitted signal by sampling a plurality of times the transmitted signal as received in each of the elemental areas, and producing an indication of the effectiveness of the reception of the transmitted signal in that elemental area.

The method in accordance with the present invention only requires one operator, the transmitter simply transmitting a carrier wave. It does not only rely on a subjective assessment of the quality and strength of the received signal.

By measuring the duration of presence of the transmitted signal above a set threshold then this can be checked by an apparatus which is sufficiently compact that it can be incorporated into a standard portable transceiver thus avoiding the need for expensive test equipment.

An alternative to measuring the duration of presence is to measure distortion by for example detecting the ratio of the [signal plus noise plus distortion] to [noise plus distortion] (otherwise known as SINAD). A benefit of detecting SINAD over measuring the carrier strength is that account is taken of the true quality of the signal as received.

The communications probability indicating circuit may comprise means for detecting a characteristic of the transmitted signal, and means for storing the number of occurrences of the characteristic within a sampling period formed by a sequence of clock periods and providing an indication of said number.

In an embodiment of the present invention the recording and indication producing means comprise a counter and digital display.

The present invention will now be described, by way of example, with reference to the accompanying drawings, wherein: Figure 1 illustrates diagrammatically the method of conducting a radio propagation survey in accordance with the present invention, and Figures 2 and 3 are two alternative embodiments of a receiver and communications probability indicator, In the drawings the same reference numerais have been used to indicate the same features.

Figure 1 illustrates the measuring method in accordance with the present invention. A site 10 is divided into a plurality of elemental areas 12 of a suitable shape and size. A base station transceiver 14 is coupled to an antenna system 16, which may be a distributed system including leaky feeder cables extending for example through lift shafts. The transceiver 14 transmits continuously through the surveying period(s). An operator with a transceiver 18 of a type which is in current use in the site radio system and to which a communications probability indicator 22 (Figures 2 and 3) is connected moves around each elemental area 12 and obtains an indication of the duration of presence of the transmitted signal.In the present example the characteristic used to detect the duration of presence is carrier strength and this is detected by sampling the condition of a transceiver's 18 squelch circuit during a specified sampling period.

At the end of this period a reading is given by the communications probability indicator 22 which tells the operator the percentage of time that the squelch remained open. Thus a reading of 100 indicates that the squelch was open during the whole of the sample period and effectively that the signal level was above threshold for that same period. For the purpose of illustration, in Figure 1 a number of values have been entered into a few of the elemental areas 12. If these values are arbitrarily banded so that 99% to 95% may be rated as Good 94% to 90% may be rated as Fair and less than 90% may be rated as Poor then contour lines can be drawn in different colours for each rating so that a person can assess quickly the probability of holding a two way communication in different parts of the site 10.

It is desirable that the signal propagation measurements are related to the transmitter output power of the least powerful transceiver so that the probability of establishing two way communication can be determined. The most convenient way of doing this is to adjust the transmitter output power of the base station transceiver 14 to equal that of the mobile or portable transceiver 18.

The site of the elemental areas 12 is dependent on a number of factors including the frequency of operation of the radio system and the size of the site 10 which may be say 1 square kilometre to several hundred square kilometres. It is desirable for the length of the elemental area 12 to be many, say greater than 20, wavelengths long so that the signal level will follow a Rayleigh distribution. Consequently exactly where in the elemental area a surveyor moves is inimportant. Thus in the former case the elemental areas are typically 10 metres by 10 metres and the transceiver 18 is a portable one and the survey is done on foot. However in the latter case the elemental areas are say 1km by 1 km and the transceiver 18 is a mobile one and the survey is carried out in a vehicle.

Once the site 10 has been split into elemental areas then a surveyor takes a reading by moving within an elemental area during an overall sampling period within which a plurality, for example 100, of samples are taken in succession in order to determine the duration that a characteristic, for example the signal strength or SINAD, the ratio of [signal + noise + distortion] to [noise + distortion], of the receiver signal is above a threshold value.

An embodiment of a system for carrying-out a radio propagation survey in accordance with the present invention will now be described with reference to Figure 2. In this embodiment communications probability indications will be measured by determining the duration that the squelch is above its threshold. The transceiver 18 shown in Figure 2 is a portable transceiver and may comprise a standard body worn FM transceiver, such as a "Pocketfone" selected from the P5000 or PFX Series Personal Radiotelephones manufactured by Pye Telecommunications Limited of Cambridge, England, having a communications probability indicator 22 built into a separate microphone and loudspeaker which are coupled by a flexible lead and facilitates connector to the transceiver proper.

The communications probability indicator 22 comprises a buffer constituted by an NPN bipolar transistor 30 which is connected as an emitter follower in that an input for a squelch signal derived from the transceiver 18 is applied to its base electrode and its output is taken from its emitter electrode. This output is coupled to the J and K inputs of a J-K flip-flop 32. The 0 output of the flip-flop 32 is connected to a two digit display formed by respective counter, display and driver integrated circuits having a seven segment L.E.D. display 34. A clock generator comprising a Schmitt NAND circuit 36 formed by a quarter of 4093 integrated circuit is coupled to an input 40 of another Schmitt NAND circuit 38 which functions as a gate and determines by virtue of the logical state of its other input 42 the overall length of the sampling period.The clock generator frequency can be selected as desired but a typical frequency would be 10Hz.

A series R.C. circuit is coupled between "the output of the circuit 36 and ground and a junction of the R and C is connected to one input of the circuit 36. Another input of the circuit 36 has a logical "high" signal H is applied to it. The output from the gate 38 is applied to clock inputs of the J-K flip-flop 32 and of a ripple counter 44 such as a type 4024 counter. The counter 44 has three outputs 46, 48 and 50, which are coupled to a NAND gate 52, and a SET input 54. The output 46 is also connected to a clock input of another J8K flip-flop 56, whose J and K inputs have a logical "high" signal H applied to them. The flip-flop 56 functions to extend the counter's range.A Q output of the flip-flop 56 is applied as a forth input to the NAND gate 52 whose output serves as a "display-inhibit" signal to the display 34 so that a result of the sampling is displayed at the end of the overall sampling period. The output from the gate 52 also comprises a "STOP" signal which is applied to the input 42 of the circuit 38 to block the clock signal and thereby terminate the sampling period.

A reset circuit 58 including a switch 60 is coupled to the display 34 which is reset on a logical "low". An inverter 62 is coupled to the reset circuit 58 and is coupled to a SET input 64 of the flip-flop 32 and to SET input 54 of the counter 44, both of which require a logical "high" to reset them.

The basic operation of the communication probability indicator 22 is as follows: the clock circuit produces a square wave having a 50/50 duty cycle.

This signal is applied to the counter 44 via the NAND circuit 38, the input 42 being high and the digital display 34 being inhibited. Ignoring the flipflop 56 for the moment, the counter 44 counts to say a maximum of 100 and produces logical "high" signals on the outputs 46, 48 and 50 which, ignoring the Q output from the flip-flop 56, causes the output of the NAND gate 52 to go low thereby inhibiting the gate 42 and removing the inhibit from the digital display 34 which then displays the condition of the squelch signal during the preceding 100 cycles of the clock signal, the condition being stored by the counter (not shown) incorporated in the display 34.

In order to avoid jitter on the squelch causing a false count in the display, the flip-flop 32 acts as a safety device and ensures that only the clock pulse is passed to its output.

The other flip-flop 56 extends the range of the counter and ensures that a position "stop" signal is produced when the count in the counter 44 reaches a count of 1-98. After the count stored in the digital display 34 has been recorded then by actuating the reset switch 60 the counter in the display 34 is reset and the flip-flop 32 is set and the counter 44 is reset. The indicator 22 is then able to take another set of samples.

Figure 3 is an embodiment of a communication probability indicator circuit which is more suited to mobile radio usage in which the propagation survey is carried-out over larger sites.

The communication probability indicator 22 comprises a squelch buffer 70 which controls the oper ation of a switch 72 so that for example when the squelch signal is above its threshold the switch 72 is closed connecting a clock generating circuit 74 to a counter 76, and when the squelch signal is below its threshold the switch 72 is opened interrupting the supply of clock pulses to the counter 76.

In this embodiment an overall sampling period corresponds to the period of 999 clock pulses and a detector 78 for 999 clock pulses is coupled between the output and a "finish" input of the clock generating circuit 74. The output of the detector 78 is also connected to the two most significant stages 34A and 34B of a counter display and driver integrated circuit, and one input of a two input NAND gate 80 which is coupled to the least significant stage 34C of the counter display and driver integrated circuit. A second input of the NAND gate is an enable line from a detector 82 which produces an output signal if the count in the counter 76 is greater than 900.

The operation of the circuit shown in Figure 3 is much the same as described with reference to Figure 2 and in the interests of brevity it will not be repeated. As mentioned above the overall sample period is equivalent to 999 clock pulses and when this count has been detected by the detector 78 then the clock 74 is inhibited and the result is displayed. If the count is less than 900 then only the first two displays are enabled. Alternatively for a count greater than 900, then the detector 82 enables the third stage 34C of the display and a decimal point appears after the second of the two most significant stages 34B so that the top end of the count is expanded to give greater accuracy.

Although not shown another variant of the present invention would be to store the count together with an address of the associated elemental area and after all the measurements have been completed then the stored counts and associated addresses are transferred to a computer controlled map drawing apparatus.

Claims (11)

1. A method of carrying out a radio propagation survey, comprising dividing a site to be surveyed into a plurality of elemental areas, transmitting a signal from a base station, measuring the duration of presence of the transmitted signal by sampling a plurality of times the transmitted signal as received in each of the elemental areas, and producing an indication of the effectiveness of the reception of the transmitted signal in that elemental area.

2. A method as claimed in Claim 1, in which the duration of presence of the transmitted signal is measured by detecting the percentage duration of the received signal strength above the squelch threshold of the receiver.

3. A method as claimed in Claim 2, in which the duration of presence of the carrier above a threshold is measured by determining the operation or otherwise of a squelch circuit during a predetermined number of samples made within an overall sampling period.

4. A method as claimed in Claim 1, in which the duration of presence of the transmitted signal is measured by detecting the duration of presence of audio distortion and spurious noise, with reference to a set threshold.

5. A method as claimed in Claim 4, in which the level of distortion of noise is measured by detecting the ratio of the[signal plus noise plus distortion to the [noise plus distortion] (otherwise known as SINAD).

6. A method as claimed in Claim 1, in which the receiver comprises a transceiver and in which the output power of the signal transmitted signal by the base station is adjusted to be substantially equal to the output power of the transceiver.

7. A method of carrying out a radio propagation survey, substantially as hereinbefore described with reference to the accompanying drawings.

8. An apparatus for use with the method as claimed in Claim 1, comprising a receiver having a signal output coupled to a communications probability indicating circuit comprising means for detecting a characteristic of the transmitted signal, and means for storing the number of occurrences of the characteristic within a sampling period formed by a sequence of clock periods and providing an indication of said number.

9. An apparatus as claimed in Claim 8, in which the receiver includes a squelch circuit and the detecting means comprises means for detecting the operation or otherwise of the squelch circuit.

10. An apparatus as claimed in Claim 8 or 9, wherein the recording and indication producing means comprise a counter and digital display.

11. An apparatus for use with the method as claimed in Claim 1, constructed and arranged to operate substantially as hereinbefore described with reference to Figures 2 and 3 of the accompanying drawings.