Experimental Results - Outdoors Flight

Repeaters are bidirectional amplifiers designed to be used in locations where coverage from a cell is poor and requires enhancement. They are commonly used to extend more reliable coverage indoors from a donor cell or to fill in coverage holes that exist because of terrain. In the early rollout phase of a UMTS network they may be used to increase the general coverage rapidly and at low cost in rural areas, residential areas or along roads and railways.

4.4.2 Donor Antenna Alignment

Repeaters can be made channel selective, but Node Bs in the same HCS layer are separated only by code. This means that great care must be taken when considering the position, type and alignment of the donor antenna for a repeater. Any other Node B signals arriving at the repeater will also be amplified and reradiated in the repeater area. The result of this could be that UEs in the repeater area are in continuous soft handover with consequential loss of capacity in the system.

It is recommended that the donor antenna should be positioned such that there is at least an 8 to 10 dB margin between the donor cell signal and other cells in the repeater area. This must then be reflected in handover thresholds.

Node B

Node B

Intended donor Node B

Repeater UE in

repeater area Risk of continuous

soft handover Plan for 8 to 10 dB

margin

Figure 20

Repeaters and Antenna Alignment

SC2804/S3/v1.1

3.43 © Wray Castle Limited

4.4.3 Repeaters to Add Capacity

Where a repeater is used to add new coverage to the system in the sense that communication from the repeater area would be impossible without the repeater, it will not add capacity to the system. However, if a repeater is used to improve the reliability in an area where communication is possible to some extent without the repeater then there may be some capacity gain.

The repeater will have the effect of improving the radio link in both uplink and downlink directions. The result is that the closed loop power control process will reduce UE and Node B transmit power, thus reducing interference contribution. This in turn translates to an increase in net system capacity.

A good example of where this effect could be utilized to some advantage is illustrated in Figure 21. The building shown would be served directly by an external macro cell at network rollout. The building penetration loss means that a UE inside the building would require a disproportionately large power weighting in the downlink direction. This reduces the capacity of the cell, and also neighbour cells because ultimately more transmit power is radiated. Once the repeater is installed as shown, the reduction in overall downlink path loss means a lower proportion of downlink power is allocated to UEs inside the building. This means that more calls can be established on the cell for a given transmitter power amplifier capability. A similar argument can be made for the uplink direction.

Serving Node B

Donor Node B

Disproportionately large power weighting to compensate for building

loss reduces capacity

Reduced power weighting due to repeater gain increases capacity

Figure 21

Repeaters to Add Capacity

SC2804/S3/v1.1

3.45 © Wray Castle Limited

4.4.4 Antenna Isolation and Gain Setting

The coupling between the two antennas connected to the repeater is referred to as

‘isolation in decibels’. Normally the donor antenna will be highly directional. The antennas in the repeater area may be either omnidirectional or directional.

Any signal coupled from the output of the repeater back to the input via the antennas will be reamplified. There is a danger that this could lead to positive feedback. The resulting transmitted noise would have severe implications for capacity in the system as a whole. It is critical therefore to ensure that the repeater gain is kept below the level of isolation to prevent self-oscillation.

It is recommended that in the downlink direction repeater gain is kept at least 15 dB below the level of isolation. This margin may be reduced by up to 5 dB in the uplink direction. Typically a repeater gain can be set independently in the uplink and downlink directions up to a maximum of about 90 dB.

Isolation between the antennas should be determined once they are fixed in their final locations. Driving the repeater antenna from a suitably calibrated test transmitter and measuring the power level received at the donor antenna can achieve this. For in-building solutions, the physical structure of the building is interposed between the antennas. This should lead to a better degree of isolation between the antennas than for outdoor applications.

Repeater Gain up to c. 90 dB

Donor antenna

Isolation (dB)

Repeater antenna Recommended maximum gain 15 dB less than isolation

Figure 22

Antenna Isolation and Gain Setting

SC2804/S3/v1.1

3.47 © Wray Castle Limited

4.4.5 Node B Desensitization

Consider the repeater shown in Figure 23. Its uplink gain is set to 80 dB and it has a noise figure of 6 dB. Assuming thermal noise of –174 dBm/Hz the noise at the input to a channel selective repeater when allowing a channel bandwidth of 4.8 MHz will be –107.2 dBm. This background input level is amplified by 80 dBm and the noise figure must also be added.

–107.2 + 80 + 6 = –21.2 dBm

Assume an input sensitivity level at the input to the donor cell’s receiver of –102 dBm. If the total coupling loss between the repeater output and the donor cell’s input is less than 80.8 dBm the amplified noise being transmitted back to the donor cell will be above the threshold of –102 dBm. In these circumstances the repeater is increasing the noise rise and therefore reducing the cell’s capacity.

The coupling loss includes all antenna gains, the path loss and other forms of gain or loss between the donor cell input and the repeater output. A coupling loss in the order of 80.8 dB could occur with a spacing between repeater and donor cell of about 400 m, although exact figures will depend on the antennas used and the propagation path.

If a donor cell and repeater are closely located, then it is worth calculating the coupling loss and checking in relation to the gain of the repeater whether desensitization seems likely. A reduction of repeater gain may be necessary to correct the problem.

Repeater

Donor Node B

–107.2 dBm 80 dB

gain

6 dB NF –21.2 dBm

–102 dBm Gain 80 dB

NF = 6 dB

Coupling loss

Coupling loss = 80.8 dB

Figure 23

Node B Desensitization

SC2804/S3/v1.1

3.49 © Wray Castle Limited

4.4.6 Time Delay in Repeaters

Another consequence of using a repeater is the propagation delay through the repeater. The specific value of delay caused depends on the particular device in use;

the value will be available from the vendor as part of the device specification sheet.

Typical delays for a repeater will be in the range 5 to 8 µs. A typically delay of 6 µs translates to a distance travelled of about 1.8 km for a normally propagating radio signal.

This is not a problem for normal UMTS operation, but it could cause difficulties when trying to estimate range or position for the UE. For example this would mean that a round trip time measurement used to estimate range from a Node B would show an error of approximately +1.8 km. Also, if using the observed time difference of arrival method for position determination, the error would be at least 900 m and could be considerably higher depending on the relative positions of the Node Bs and the UE.

It may also cause a problem if the receiver can see both direct and repeated versions of the transmitted signal. The delay in the repeater could mean a delay spread greater than the search window for the rake receiver. Thus some channel paths would be treated as interference.

UE in repeater

area Repeater

Node B

e.g. 6 µs delay each way

Round trip time measurement indicates UE is 1.8 km further away than it really is.

May cause problems because of limited search window size in Node B and UE.

Figure 24

Time Delay in Repeaters

SC2804/S3/v1.1

3.51 © Wray Castle Limited