In GSM wireless networks, due to various reasons, it is impossible to achieve complete sequetless coverage. In some remote rural, factory mines in blind areas are covered with outdoor direct discharge stations, which is a fast and economical way. The design of the outdoor direct discharge system mainly includes two aspects of stability design and overlay design.
1. Stability analysis of straight discharge station
The straight discharge station is actually a special amplifier, on the downlink, its input is the uplink antenna interface of the amplifier, and the output is the downlink antenna interface. Conversely on the uplink. At the same time, it is another feedback amplifier with signal coupled between the upper and downstream antennas. According to the stability theory of the amplifier, it is necessary to satisfy the amplitude balance condition: AF <1 type A is the open loop gain of the amplifier, f is The amplifier feedback coefficient, but also satisfies the feedback signal as in phase, which is called phase balance conditions.
The stability of the straight discharge station is analyzed. The direct discharge station is a two-way amplifier that amplifies the upper and downlink signals. Generally, the uplink gain is a few DBs, as long as the downlink amplifier is stable, it can guarantee the stability of the entire discharge station, and the stability analysis of the direct discharge station is actually the downlink. Stability analysis of the road. Due to the multipath propagation of the wireless signal, the retransmission signal in the direct discharge system has a feedback path that has some signal components with the input signal, and the system stability must be considered from amplitude balance. Represents AF = 1 as DB form:
G-l = 0 or g = L (1)
G is the gain between the open loop gain of the amplifier, the gain between the two-day interface of the main device, L is the loss of the feedback path, the signal from the downlink antenna interface to the upstream antenna interface.
Under normal circumstances, the maximum gain of the straight discharge station is fixed, it is equal to the sum of the largest gains in each enlargement in the equipment. In practical applications, the device is not working in the maximum gain state, but is operated under the gain of coverage requirements, called work gain (GW).
GW = PO-Pi (2)
The PO is the output power of the straight discharge station (the two carrier frequency devices are typically about 33 dB), and PI is the input power of the device.
PO = POA-GTX (3)
Pi = PIA GRX (4)
GTX, GRX is the gain of uplink and downlink antennas, and PoA, PIA is the output and input signal power of the entire discharge system (including antenna), respectively.
According to the above relationship, the amplitude balance condition GW-L <0 can be expressed as
PO-PI In the above formula, the isolation between the discharge station apparatus input port and the output port, that is, the two parts of the gain and the space between the transmission and reception antenna, L = L space - (GTX GRX) It can be seen that the stability of the direct discharge system is directly related to the input signal strength Pi of the straight discharge station, the output signal strength Po, the transmission and reception signal antenna isolation L is directly related, where PO is known, L and Transit Anti-Internet The propagation environment and the gain of the transceiver antenna are related, and Pi is related to the propagation environment between the base station to the straight station. As seen from the formula PO-Pi Of course, it is necessary to make the direct discharge system stable operation except that PO-PI 2. Estimation of transmitting and receiving letter antenna isolation It can be seen from above that the isolation between the upper and downstream antennas is critical to the stability of the entire system, and the isolation is required to be properly estimated during the direct discharge survey site. Isolation between antennas is the result of multifaceted factors, including: spatial isolation and building isolation. (The separation calculated below is the loss of the signal from the downstream antenna interface to the upstream antenna interface) 2.1 Space isolation Spatial isolation refers to a spatial loss formed by a distance between a distance of the receiving letter antenna. The following semi-experience formula is calculated: (1) Horizontal isolation Lhu = 22 20LG (D / λ) DT (θ) DR (θ) - (GTX GRX) (6) LH = 31.5 20LGD DT (θ) DR (θ) - (GTX GRX) (GSM900) In the formula DT, DR is the loss caused by the horizontal direction of the two-day line to the function (horizontal direction circle), and the specific value can be found in the antenna direction, such as the direction shown in the right figure, at 55. There are 3dB additional losses during the angle. When the upper and downstream antenna angle is 180. When the directional loss is the front and rear ratio of the antenna. (2) Vertical isolation LV = 28 40LG (D / λ) DT (θ) DR (θ) - (GTX GRX) (7) LV = 47.1 40LGD DT (θ) DR (θ) - (GTX GRX) (GSM900) In this formula, θ is the pitch angle of the antenna. D is antenna spacing, DT, DR is the vertical direction function of the two antenna, similar to the horizontal directional function. (3) tilt isolation LS = (LV-LH) (α / 90) LH (8) The α in the formula is an angle of the two antenna in the vertical plane. 2.2 Building isolation The isolation of buildings is isolated due to the barrier of the building causes signal fading. This isolation calculation does not have a more effective way, generally using a method of direct substitution of experience. As a wall is separated from 10 to 20 dBm. 3. Measurement of Isolation Since the propagation of wireless signals is affected by many factors, it is only possible to determine the size of the isolation through calculation, and if the more accurate isolation value is required, it is necessary to obtain a measuring method in the actual engineering design. Add signals that are known to be p on the downstream antenna, and test the phone to measure the received signal strength r (as shown) at the uplink antenna. The separation is: L = P G-R-D (9) Where g is the downlink antenna gain, D is the front and rear ratio. 4, the nominal power and actual output power of the direct discharge station In the instructions of the straight station, the single-load frequency power (generally 36dB) of the device is often indicated, that is, the output power when the device is only enlarged, and the number of frequency points in the actual operation is reduced by a double device output power. 3dB. 5, cover forecast The final goal of the direct discharge station is to meet the cover needs, and it is necessary to predict the coverage of the device during the design process. 5.1 Okumura / HATA formula The Okumura / Hata model is a wide range of coverage prediction models, which is based on urban areas that are quasi-smooth terrain, and the impact of the remaining regions appear 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: Urban Accurate Smoothing Tile Electric Wave Proposal Medium Value (DB) f: Working Frequency (MHz) HB: Base station antenna effective height (m) HM: Mobile station antenna effective height (m) D: Distance between mobile stations and base stations (km) α (hm): mobile station antenna height factor S (a): Building Density Factor (11) Where A is a building density. Generally, the effective height of the mobile phone antenna is 1.5 meters, and 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) For suburban, the following amendments are used: LBS = LB (urban) -2 [lg (f / 28)] 2-5.4 (12) For rural industries, the following revision LBS = LB (urban) - [LG (F / 28)] 2-2.39 (LGF) 2 9.17LGF-23.17 (13) For openly adopt the following revision LBQ = LB (urban) -4.78 (LG F) 2 18.33LG F-40.94 (14) 5.2 OKUMURA / HATA Formula Application in Power Discount Overlay Estimation Before applying the Okumura / Hata formula, you should carefully analyze the characteristics of the coverage area, and cannot be blindly used, and the appropriate correction method is selected according to the specific situation. For rural areas, if the rural model prediction results will be different from the actual results. my country's currently distribution is uneven, there are many open-wide land (farmland) between the village and villages, the village is small, the building is often high, the building is smaller, but it is very intensive, its intensive degree is not Big city buildings. Most of the buildings are houses, and public buildings account for a small proportion. Street stenosis is not conducive to signal transmission. In general, the internal signal strength is 10-30 dB in the village. In the hills and mountains, the village is often in the lower deposition or valley, and the village is high or mountains. Severely affecting the transmission of signals. In view of my country's current stage, the rural correction methods can be used to predict rural coverage. Building density can be obtained by the following formula: A = number × 150 / village area (15) 6, design steps for outdoor direct discharge system The design of the outdoor straight station mainly includes two aspects of the system's stability design and overlay design, and through the above discussion, the design steps of the straight discharge station can be summarized as follows: (1) Select the top station location based on the received signal strength, override and geographic factors. (2) Determine the required isolation based on the received signal strength, the device output power, and the parameters of the upper and downstream antennas, and retain a certain amount of margin. (3) Comprehensive utilization of vertical isolation, horizontal isolation, building isolation guarantees the required isolation. Determine the antenna hint, up and down the antenna distance. The isolation is measured immediately. (4) Application coverage prediction models such as OKumura / Hata models for overlay prediction. If it is not guaranteed, adjust the downstream antenna of the direct station to meet the coverage needs. (5) Implement the project according to the design of the design, verify system stability and coverage. 7, other questions Outside the impact of the current network, in addition to the above principles in the design construction process of the direct discharge station, pay attention to the following points: (1) The uplink antenna should be an quasi-base station. If the uplink antenna deviates from the base station, the uplink signal of the straight discharge station may cause interference to other base stations nearby, and increase the drop. (2) The uplink gain should be properly set, and if the uplink gain is too strong, the base station accepts the amplifier saturation so that the signal cannot be received. If the uplink signal is weak, it is easily interfered with other matches or adjacent frequencies. (3) The downlink gain should not be too large, some direct discharge stations set too large downlink gain, so that the amplifier exceeds the rated power work, causing the output signal distortion, the harmonic component increases unnecessary interference, is The drop will increase.
