Design of Outdoor Repeater in GSM Network
In GSM wireless, it is impossible to achieve complete seamless coverage for various reasons. In some remote factories and mines in blind areas, outdoor repeaters are used for coverage. The design of the outdoor repeater system mainly includes stability design and coverage design.
1. Repeater stability
The repeater is actually a special amplifier. On the downlink, its input is the upstream antenna interface of the amplifier, and its output is the downstream antenna interface. The opposite is true on the uplink. At the same time, it is also a feedback amplifier with signal coupling between the uplink and downlink antennas. According to the stability of the amplifier, the stability of the amplifier must meet the amplitude balance condition: AF <1 where A is the open-loop gain of the amplifier and F is the amplifier The feedback coefficient must also satisfy that the feedback signal is in phase with the input signal, which is called the phase balance condition.
dB, as long as the downlink amplifier is stable, the stability of the entire repeater can be guaranteed. The stability analysis of the repeater is actually the stability analysis of the downlink. Due to the multipath propagation of wireless signals, the retransmitted signal in the repeater system always has some signal components in phase with the input signal through the feedback path. To stabilize the system, the amplitude balance condition must be considered. Express AF = 1 in dB form:
Gï¼L = 0 or G ï¼ L (1)
G is the open-loop gain of the amplifier, that is, the gain between the two antenna interfaces of the main device of the repeater, and L is the loss of the feedback path, and the loss of the signal from the downlink antenna interface to the uplink antenna interface.
Under normal circumstances, the maximum gain of the repeater is fixed, which is equal to the sum of the maximum gain of each amplification link in the equipment. In practice, the device does not work in the state of maximum gain, but operates at a gain that meets the coverage requirements, which is called the working gain (Gw).
Gw = Po-Pi (2)
Among them, Po is the output power of the repeater (two carrier frequency devices are generally around 33dB), and Pi is the input power of the device.
Po = Poa-Gtx (3)
Pi = Pia + Grx (4)
Gtx and Grx are the gains of the uplink and downlink antennas respectively, and Poa and Pia are the output and input signal power of the entire repeater system (including the antenna).
According to the above relationship, the amplitude balance condition Gwï¼L <0 can be expressed as
Po-Pi In the above formula, L is the isolation between the input port and output port of the repeater equipment, which is composed of the gain of the transceiver antenna and the spatial isolation between the antennas, L = L space-(Gtx + Grx). It can be seen that the stability of the repeater system is directly related to the three factors of the repeater's input signal strength Pi, output signal strength Po, and transceiver antenna isolation L, where Po is known and L The propagation environment is related to the gain of the transceiver antenna, and the propagation environment of Pi from the base station to the repeater station is related. Slave Po-Pi Of course, for the repeater system to operate stably, Po-Pi 2. Estimation of the isolation of transceiver antenna It can be seen from the above that the isolation between the uplink and downlink antennas is very important for the stability of the entire system, and the isolation needs to be correctly estimated during the site selection of the repeater. The isolation between antennas is the result of many factors, including: space isolation and building isolation. (The isolation below is the signal loss from the downlink antenna interface to the uplink antenna interface) 2.1 Space isolation Spatial isolation refers to the space loss caused by a certain distance between transmitting and receiving antennas. It can be calculated with the following semi-empirical formula: (1) Horizontal isolation Lhu = 22 + 20lg (d / λ) + Dt (θ) + Dr (θ)-(Gtx + Grx) (6) In the formula, Dt and Dr are the loss caused by the horizontal directional function of the two antennas (horizontal circular diagram). The specific value can be found in the antenna pattern, as shown in the figure at right, at 55 There is an additional loss of 3dB at the corner. When the uplink and downlink antenna angle is 180. , The directivity loss is the front-to-back ratio of the antenna. (2) Vertical isolation Lv = 28 + 40lg (d / λ) + Dt (θ) + Dr (θ)-(Gtx + Grx) (7) In this formula, θ is the pitch angle of the antenna. d is the antenna spacing, and Dt and Dr are the vertical directivity functions of the two antennas, similar to the horizontal directivity function. (3) Tilt isolation Ls = (Lv-Lh) (α / 90) + Lh (8) Where α is the angle between the two antennas in the vertical plane. 2.2 Building isolation Building isolation is the isolation caused by the fading of the signal caused by the blocking of the building. This isolation does not work well, and generally adopts the method of directly substituting the empirical value. For example, the isolation of a wall is 10 ~ 20dBm. 3. Measurement of isolation Because the propagation of wireless signals is affected by many factors, the isolation can only be roughly determined by calculation. In actual engineering design, if a more accurate isolation value is required, it can be obtained by field measurement. Add a signal with known strength p to the downlink antenna, and use the test mobile phone to measure the received signal strength r at the uplink antenna. The isolation is: L = p + GrD (9) Among them, G is the downlink antenna gain, and D is the front-to-back ratio. 4. The nominal power and actual output power of the repeater The specifications of the repeater often indicate the single carrier frequency power of the device (usually 36dB), that is, the output power of the device when only one frequency point is amplified. In actual operation, the output power of the device is reduced by 3dB every time the number of frequency points doubles. 5. Coverage forecast The ultimate goal of the repeater opening is to meet the needs of coverage. It is necessary to predict the coverage of equipment during the design process. 5.1 Okumura / Hata formula The Okumura / Hata model is a more extensive coverage prediction model. It is based on the urban area with quasi-smooth terrain, and the impact of the remaining areas appears in the form of correction factors. The basic transmission loss mode of the Okumura / Hata model urban area is: Lb = 69.55 + 26.16lgf-13.82lghb-α (hm) + (44.9-6.55lghb) lgd (10) Lb: Median propagation loss of quasi-smooth terrain in urban areas (dB) Where a is the building density. Generally, the effective height of the mobile phone antenna is 1.5 meters. In the GSM900 system, α (hm) is about 0. The above formula can be expressed as: Lb = 146.833-13.82lghb + (44.9-6.55lghb) lgd-s (a) Lbs = Lb (urban area)-2 [lg (f / 28)] 2-5.4 (12) Apply the following amendments to the village Lbs = Lb (urban area)-[lg (f / 28)] 2-2.39 (lgf) 2 + 9.17lgf-23.17 (13) Use the following correction method for open areas Lbq = Lb (urban area)-4.78 (lg f) 2 + 18.33lg f-40.94 (14) 5.2 Application of Okumura / Hata formula in repeater coverage estimation Before applying the Okumura / Hata formula, the building characteristics of the coverage area should be taken seriously, not blindly applied, and the appropriate correction method should be selected according to the specific situation. If the village model is used, the prediction result will be quite different from the actual result. The distribution of villages in our country at this stage is scattered and uneven. Most of the villages are open land (farmland), the village area is small, the buildings in the village are often small in height, and the building shielding is small, but they are very dense and their density is no less than Big city buildings. Most of the buildings are residential houses, and public buildings account for a small proportion. The narrow streets are not conducive to signal propagation. In general, the signal strength in the village is 10-30dB lower than in the village. In hills and mountains, villages are often in low-lying depressions or valleys, and between villages are highlands or mountains. Seriously affect the signal propagation. In view of the characteristics of rural buildings in China at this stage, the urban correction method can be used to predict rural coverage. The building density can be calculated by the following formula: a = number of households × 150 / village area (15) 6. Design steps of outdoor repeater system The design of the outdoor repeater mainly includes the stability design and coverage design of the system. Through the above discussion, the design steps of the repeater can be summarized as follows: (1) According to the received signal strength, coverage and geographical factors, choose the location of the station. 7. Other From the perspective of the current network, in addition to following the above principles in the design and construction of repeaters, the following points should also be noted: (1) The uplink antenna should be aligned with the base station. If the uplink antenna deviates from the base station, the uplink signal of the repeater may cause interference to other nearby base stations, increasing the number of dropped calls. (2) The upstream gain should be properly set. If the upstream gain is too large, the upstream signal will be too strong, and the base station will be saturated with the amplifier to receive the signal. If the upstream signal is too weak, it is susceptible to interference from other co-channel or adjacent channels, which increases call drop. (3) The downlink gain should not be too large. Some repeaters set too large a downlink gain to meet the coverage, so that the power amplifier exceeds the rated power to work, resulting in distortion of the output signal, and the increase of harmonic components brings unnecessary interference. Call drop increased. Specification:
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Lh = 31.5 + 20lgd + Dt (θ) + Dr (θ)-(Gtx + Grx) (GSM900 case)
Lv = 47.1 + 40lgd + Dt (θ) + Dr (θ)-(Gtx + Grx) (GSM900 case)
f: working frequency (MHz)
hb: effective height of base station antenna (m)
hm: effective height of mobile station antenna (m)
d: distance between mobile station and base station (km)
α (hm): mobile station antenna height factor
s (a): building density factor (11)
For the suburbs, the following correction method is adopted:
(2) Determine the required isolation according to the received signal strength, the output power of the device, and the parameters of the uplink and downlink antennas, and reserve a certain margin.
(3) Comprehensive use of vertical isolation, horizontal isolation, building isolation to ensure the required isolation. Determine the height of the antenna and the distance between the uplink and downlink antennas. If necessary, measure isolation on site.
(4) Coverage prediction model such as: Okumura / Hata model for coverage prediction. If the coverage cannot be guaranteed, the downlink antenna of the repeater should be adjusted to be too high to meet the coverage requirements.
(5) Implement the project according to the design plan and verify the system stability and coverage.
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