Tetra 2
Tetra 2
Tetra 2
1.Site Selection
Selecting a repeater site is one the most critical decisions affecting the overall performance of the system. A repeater must be located where it can receive a sufficiently powerful signal from the BTS site in order to maximize the repeaters performance. To generate an output signal of 25dBm the input would have to be at least 50 dBm after the pick-up antenna.
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Examples of repeater and accompanying antenna locations include (but are not limited to): The roof of a building adjacent to the affected area, with antennas mounted to the penthouse or building sides The top of the hill that is obstructing the donor sites coverage with the antennas mounted on top An existing utility pole with equipment mounted below any existing power lines A newly installed pole or tower
The distance from both the BTS site and the Repeater site to be covered must be taken into consideration. Ten kilometres is a typical distance from a BTS site to the repeater. The repeater will usually have a coverage radius of several kilometres. However, distances vary depending on propagation losses due to terrain, foliage and building obstructions in the area. Ideally the repeater should have line of site to both the BTS site and the area to be served in order to reduce the effects of fading. When foliage or other barriers block the line sight, repeaters even a short distance away from the donor site may not receive sufficient signal strength. Distance from the donor base station to the repeater affects the free space path loss, which in turn affects the received signal strength at the repeater. Free space loss is calculated as: L p , dB = 37.0 + 20 log 10 ( f MHz ) + 20 log 10 ( d miles ) = 32.4 + 20 log 10 ( f MHz ) + 20 log 10 ( d km )
At 400 MHz these formulas reduce to: L p , dB = 89.04 + 20 log 10 ( d miles ) L p , dB = 84.44 + 20 log 10 ( d km ) For example, at 400Mhz, a repeater 8km from the donor will have free space path loss of 102.5dB. At 12 km this loss increases to 106dB.
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Kilometres 2 4 6 8 10 12 14 16 18 20 30 Figure 1
Loss 90,46 96,48 100,00 102,50 104,44 106,02 107,36 108,52 109,54 110,46 113,98
Maximum BTS output Feeder Loss BTS Antenna Gain Path Loss (10km) Pick-up Antenna Gain Cable Loss Repeater Gain Feeder Cable Loss Server Antenna Gain Resulting ERP Figure 2.
+43dBm -2dB +6dB -104dBm +9dBi -2dB +75dB -2dB +9dBi +32dBm
F1
C O V ER A G E A NTE N A
F1
F1
DO NO R AN TE NA BASE S TA T I O N
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I= Isolation (dB) D= Distance between donor and server antennas (metres) =Wavelength (metres) Gd=Gain of donor antenna in direction of server antenna Gs=Gain of server antenna in direction of donor antenna (Valid for Gd, Gs<10dB)
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Integrating Repeaters into a TETRA Network using the Avitec CSR404 The following table can be used as a guideline for antenna separation. Antennas are assumed to be highly directional antennas pointed in opposite directions. VERTICAL ANTENNA SEPARATION
Separation(m) Isolation(dB)
3 6 9 12 15 18 21 24 27 30 Figure 4.
52 64 71 76 80 83 86 88 90 92
60.0 66.5 72.5 78.5 84.5 86.5 89.5 90.6 92.5 94.5
Theory and practice show that vertical separation yields better results than horizontal separation. When desired isolation cannot be met due to insufficient separation, external shielding has proven effective, mounting the antennas on either side of a rooftop structure or using some type of grounded metal screen or even wire mesh between antennas.
3.Call Processing
The TETRA system perceives all calls handled by the repeater as actually being handled by the BTS site; the repeater is simply an extension of the base station coverage. Therefore, the BTS handles call initiation, power control messages, hand-off request, etc., for mobiles in the repeaters coverage area. When the mobile moves from the repeaters area to a neighbouring site, the BTS site handles the hand off in the same manner as for a mobile in the base station area. When the BTS assigns a channel to the mobile, that channel is picked up by the repeater and then retransmitted at the same frequency. Since the repeater is technically part of the BTS, no hand-off takes place when a mobile unit moves from the repeaters coverage area to that of the BTS. However when deploying a channel selective repeater in the TETRA system, care must be taken to keep the overlapping coverage area between the donor base station and the repeater to a minimum. In the overlapping area the mobile receives signal from both the base station and the repeater. The effect is comparable to a mobile receiving multiple signals at various times due to multi-path propagation.
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SERVER ANTENNA
f1
f1
BTS
figure 5. Figure 5 shows a typical repeater installation. In this example, the noise contributed to the BTS can be calculated in the following manner. For the purpose of this exercise, we shall establish some reference numbers as follows: Base Station Feeder Loss Base Station receiver sensitivity Repeater noise figure Donor antenna gain Server antenna gain Repeater gain Path loss at 10km 43dBm -2dB -104 dBm 6dB 9dBi 9dBi 45-75dB -104 dB
As mentioned earlier, free space loss at 400MHz is calculated as: L p , dB = 89.04 + 20 log 10 ( d miles ) L p , dB = 84.44 + 20 log 10 ( d km )
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Gain in the repeater is set at a level to generate a forward channel output power of 25 dBm. For the purpose of this exercise we assume that the reverse channel path is set at the same gain as the forward path to provide the system signal path balance. Gain (dB) =Repeater Output Input signal =Repeater Output (Base Station ERP-Path Loss + donor antenna gain-cable loss) =25dBm (47dBm-104 dB + 9dB-2dB) =75dB
Contribution to thermal noise level (NL) at the base station is calculated as follows: (the system noise floor at a site may also be affected by other manmade noises; however, for this exercise, we shall use thermal noise as our reference point.) NLrepeater = Repeater EIN + repeater gain + donor antenna gain + server antenna gain path loss-feeder loss
EIN (equivalent input noise) is the amount of RF noise at the input of a device that would produce the observed noise at the output if the device itself were noiseless. Repeater EIN for a 25kHz bandwidth is the thermal noise (TN) present in the 25kHz bandwidth plus the noise figure (NF) of the amplifier in the repeater. The noise figure is the ratio, in dB, between the actual noise power and the amount of noise that would be produced by a similar device with perfect noise performance. Thermal noise is specified as 174dBm at ambient temperature in a 1 Hz bandwidth. The following example illustrates how to calculate thermal noise in a 25 kHz bandwidth: TN = -174 dBm/Hz + 10 log (BW) = -174 dBm/Hz + 10 log (20kHz) = -131 dBm for a 20kHz bandwidth
For a repeater with a noise figure (NF) of 6 dB EIN Repeater = TN + NF Repeater = - 131 dBm + 6 dB = -125 dBm
Therefore , NL repeater = Repeater EIN + repeater gain + donor antenna gain + server antenna gain- path loss = -125 dBm + 75dB + 9dB + 9dB 104dB = -136dBm
NL repeater
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Base Stations typically require a 11.2 dB S/N ratio (equivalent to 0.3%BER) to receive a signal. Since a base station receivers sensitivity is required to be at least 115dBm(Class B static sensitivity requirement), you can calculate the EIN and the NF of the base station. Therefore, EIN BTS = Sensitivity S/N =-115 dBm 11.2dB = -126.2 dBm NF BTS = EIN BTS TN = -126.2 dBm - (-131) dBm = 4.8 dB in the 25kHz bandwidth. When the repeater system noise contribution of 136 dBm is added to the base station EIN of 126.2dBm, the resulting composite system EIN is 125.8 dBm, or a system degradation of 0.4 dB. (Recall that signal levels must be converted from dBm to Watts in order to be added together, then converted back to dBm; dBm = 10 Log (W) + 30.) Repeater Base Station Base Station Repeater Composite Total system Noise Figure Noise Figure EIN Noise System EIN Degradation Contribution 6dB 4.8 -126.2dBm -136dBm -125.8dBm 0.4dB Figure 6. Repeater Noise Contribution
When considering the outlined specifications, only two points within the system can be adjusted in order to reduce the noise level contribution: the server antenna gain and the reverse channel gain setting in the repeater. A reduction in either will reduce the noise level contribution. Offsetting the forward and reverse path gains will affect system path balance between the repeater and the base station. Signal path balance between the repeater and the mobile is not affected. Designing a system for low reverse channel path signals will cause some system imbalance. Systems designed for levels greater than 95 dBm will experience a minimal affect. Careful overall system analysis should be made prior to any off balance increase or decrease in the reverse channel gain. Proper selection of the repeaters antennas will provide for the system requirements and minimise donor site noise contribution.
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Summary
Repeaters can be utilised to provide coverage in almost all applications. Integrating repeaters into your system can be straightforward by adhering to the following design recommendations:
Select a repeater location with line of sight to the donor site and the area to be served to allow for the best signal strength. Select repeater antennas for the system with the proper directivity and high front to back ratios in order to optimise repeater coverage and system noise level. Mount donor and server antennas so that the needed isolation is achieved. Direct repeater coverage away from the donor cell to minimise RF signal coverage overlap. Use only the required power to cover holes or areas to minimise border overlap with the donor cell. Optimise repeater gain levels to achieve system path balance and an acceptable noise level contribution.
Information contained herein is deemed to be reliable and accurate as issue of date. No responsibility is assumed for its use, nor for any infringements on the rights of others. Avitec AB reserves the right to change the design specifications of the product at any time without notice
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