Both uplink and downlink have their own transmission power loss and path fading. In cellular communication, in order to determine the effective coverage, the maximum path fading or other limiting factors must be determined. In the uplink, the limiting factor from the mobile station to the base station is the receiving sensitivity of the base station. For the downlink, the main limiting factor from the base station to the mobile station is the transmission power of the base station. By optimizing the balance between uplink and downlink, better communication quality can be achieved within the coverage radius of the cell.
Generally, by using base station resources, the link balance (uplink or downlink) of each cell in the network is improved, so that the system works in the best state. Finally, it can also promote better performance of mobile calls during handover and call establishment.
Calculation of uplink and downlink balance. For the GSM system to realize two-way communication, the balance between uplink and downlink is very important, which is the main factor to ensure the same traffic and communication quality in both directions, and also relates to the actual coverage of the cell.
Downlink refers to the link sent by the base station and received by the mobile station.
Uplink refers to the link sent by the mobile station and received by the base station.
The algorithm of uplink and downlink balance is as follows:
Downlink (in dB):
Pinms = poutbts-lduplbts-lpbts+gabts+Currie +GaMS+GdMS-islantbts-LP down.
Among them:
PinMS is the power received by the mobile station;
PoutBTS is the output power of BTS;
LduplBTS is the loss of combiner, duplexer, etc.
LpBTS is the loss of feeder cable, jumper and connector of BTS antenna;
GaBTS is the gain of the base station transmitting antenna;
Currie is the directional coefficient of the base station antenna;
GaMS is the gain of the mobile station receiving antenna;
GdMS is the diversity gain of the receiving antenna of the mobile station;
LslantBTS is the polarization loss of dual-polarized antenna;
LPdown is the downlink path loss;
Uplink (in dB):
Pinbts = poutms-lduplbts-lpbts+gabts+curry+GaMS+GdBTS -LPup +[Gta]
Among them:
PinBTS is the power received by the base station;
PoutMS is the output power of the mobile station;
LduplBTS is the loss of combiner, duplexer, etc.
LpBTS is the loss of feeder cable, jumper and connector of BTS antenna;
GaBTS is the gain of the base station receiving antenna;
Currie is the directional coefficient of the base station antenna;
GaMS is the gain of mobile station transmitting antenna;
GdBTS is the diversity gain of the base station receiving antenna;
Gta is the gain brought by using tower amplifier;
LPup is the uplink path loss.
According to the reciprocity theorem, that is, for any mobile station location, the uplink path loss is equal to the downlink path loss, that is:
LPdown = LPup
Let the system tolerance be DL, the deterioration reserve of the mobile station be DNMS, the deterioration reserve of the base station be DNBTS, the receiver sensitivity of the mobile station be MSsense, the receiver sensitivity of the base station be BTSsense, and other losses, such as building penetration loss, automobile loss, human body loss, etc. Therefore, for any point in the coverage area, the following conditions should be met:
Pinms-DL-DNMS-RalfLose & gt= MSsense
PinBTS-DL-DNMS-RalfLose & gt= BTSsense
The purpose of uplink and downlink balance is to adjust the transmission power of the base station so that the point on the boundary of the coverage area (the point farthest from the base station) satisfies:
Pinms-DL-DNMS-RalfLose = MSsense
Then, the formula for calculating the maximum transmission power of the base station is obtained:
PoutBTS & lt= ms sense-BTS sense+pout ms+GdBTS-GdMS+islandbts-Gta+DNMS-DNB ts
2 Determination of various losses
◆ Seepage loss of buildings
The penetration loss of a building refers to the attenuation of electric waves passing through the external structure of the building, which is equal to the difference between the median field strength outside and inside the building.
The penetration loss of buildings is closely related to the structure of buildings, the types and sizes of doors and windows, and the floors. The penetration loss varies with the floor height, generally -2dB/ floor. Therefore, the penetration loss of the first layer (bottom layer) is generally considered.
The following is a set of data in 900MHz frequency band, based on foreign test results:
-General reinforced concrete frame buildings in medium-sized cities, with median penetration loss 10dB and standard deviation of 7.3dB；; The median penetration loss of similar buildings in suburbs is 5.8dB, and the standard deviation is 8.7dB.
-general reinforced concrete frame buildings in big cities, with median penetration loss 18dB and standard deviation of 7.7dB；; The median penetration loss of similar buildings in suburbs is 13. 1dB, and the standard deviation is 9.5dB.
-For buildings with metal shell structure or special metal frame structure in metropolitan area, the median penetration loss is 27dB.
Because China's urban environment is very different from that of foreign countries, it is generally 8- 10 dB higher than that of similar foreign names.
For 1800MHz, although its wavelength is shorter than 900MHz, its penetration ability is greater, but its diffraction loss is greater. Therefore, in fact, buildings with 1800MHz have a penetration loss greater than those with 900MHz. According to GSM 3.30, the penetration loss of buildings in urban environment is generally 15dB, and that in rural areas is 10dB. Generally, it is 5- 10 dB larger than the penetration loss of 900MHz in similar areas.
◆ Human body loss
For the mobile phone, when it is located at the waist and shoulders of the user, the received signal field strength will be reduced by 4-7 dB and 1-2 dB, respectively, compared with when the antenna is several wavelengths away from the human body.
Generally, the human body loss is set to 3dB.
◆ In-vehicle loss
The loss in the car caused by metal structure can not be ignored. Especially in economically developed cities, people spend part of their time in cars.
General in-vehicle loss is 8 8-10dB.
◆ Feeder loss
In GSM900, 7/8 inch paper feeder is often used. Under the condition of 1000MHz, the loss per 100 meter is 4.3dB. At 2000MHz, the loss per 100 meter is 6.46dB, which is 2. 16 dB more.
3 wireless propagation characteristics
The propagation of mobile communication is shown in the curve in Figure 5-02. The overall average value decreases with the increase of distance, but the signal level will be affected by fast fading and slow fading. Slow fading is the reflection of the terrain around the receiving point to the signal, which makes the signal level change greatly within tens of meters. If the mobile station moves in an environment without any obstacles, the signal level is only related to the distance of the transmitter. So usually, the signal level at a certain point refers to the average signal level within a range of tens of meters. The change of this signal is a normal distribution. For different terrain features, the standard deviation is different, usually around 6-8 dB. The fast fading is superimposed on the slow fading signal. This fading speed is very fast, up to dozens of times per second. In addition to the terrain, it is also related to the speed of the mobile station and the wavelength of the signal. The amplitude is very large, which can be tens of dB, and the signal changes in Rayleigh distribution. Fast fading often degrades speech quality, so we should keep fast fading.
The propagation of radio waves in free space is the most basic and simple problem in the study of radio wave propagation. Free space is an ideal space satisfying the following conditions: 1. Infinite space with uniform loss, 2. Isotropy, and 3. Zero conductivity. Based on the electromagnetic field theory, the expression of transmission loss Ls under the condition of free space propagation can be derived as follows:
Ls=32.45+20lgf+20lgd
The basic transmission loss Ls in free space is only related to frequency f and distance d, and when f and d are doubled, Ls increases by 6dB, so we know that the transmission loss of GSM 1800 base station in free space is 6 dB greater than that of GSM900 base station.
The main feature of land mobile channel is multipath propagation, and the actual multipath propagation environment is very complicated. When studying communication problems, we tend to simplify and start from the simplest situation. The dual-path model considering only the direct wave from the base station to the mobile station and the ground reflected wave is the simplest propagation model. The dual-path model is shown in Figure 5-04, which can be deduced by electromagnetic field theory. The expression of transmission loss Lp is Lp=20lg(d2/(h 1*h2)).
5.4 Two Common Radio Wave Propagation Models
◆ Calculation model of radio wave propagation attenuation in Okura
GSM900MHz mainly adopts the calculation mode of Okura radio wave propagation attenuation recommended by CCIR. Based on the median field strength or path loss of large cities with quasi-flat terrain, the model corrects other factors such as propagation environment and terrain conditions in the form of correction factors. The basic transmission losses on different terrains are predicted by the following formulas respectively.
L (urban area) = 69.55+26.16 lgf-13.82 lgh1+(44.9-6.55 lgh1) lgd-a (H2)-s (a).
L (suburb) = 64.15+26.16 lgf-2 [LG (f/28)] 2-13.82 lgh1+(44.9-6.55 lgh/kloc
L (country road) = 46.38+35.33 lgf-[LG (f/28)] 2-2.39 (lgf) 2-13.82 lgh1+(44.9-6.55 lgh1).
L (open area) = 28.61+44.49lgf-4.87 (lgf) 2-13.82lgh1+(44.9-6.55lgh1) lgd-a (H2
L (forest area) = 69.55+26.16 lgf-13.82 lgh1+(44.9-6.55 lgh1) lgd-a (H2).
These include:
F-operating frequency, MHz
H 1-antenna height of base station, m
H2-mobile antenna height, m
D-distance to the base station, km.
A(H2)- height gain coefficient of mobile antenna, dB.
A (H2) = (1.1lgf-0.7) H2-1.56 lgf+0.8 (small and medium-sized cities)
= 3.2 [LG (11.75h2)] 2-4.97 (big city)
S(a)- urban building density correction coefficient, db;
s(a)= 30-25 LGA(5% & lt； a≤50%)
= 20+0. 19 LGA- 15.6(LGA)2( 1% & lt； a≤5%)
=20 (a≤ 1%)
◆ Cost-231-Walfish-Ikegami radio wave propagation attenuation calculation model.
GSM 1800 MHz mainly adopts the "Cost- 2-Walfish-Ikegami" radio wave propagation attenuation calculation model jointly recommended by European Telecommunication Science and Technology Research. The characteristics of this model are: the radio wave loss model in a small area covered by radio waves is measured in many cities.
Line of sight separation and non-line of sight:
(1) sight distance
The basic transmission loss is calculated by the following formula
L=42.6+26lgd+20lgf
(2) NLOS situation
The basic transmission loss consists of three items:
L=Lo+Lmsd+Lrts
Lo=32.4+20lgd+20lgf
A)Lo stands for loss of free space.
B)Lmsd is the diffraction loss of multiple shields.
C)Lrts is the diffraction and scattering loss from the roof to the street.
No matter which mode is used to predict wireless coverage, it is only an approximate calculation based on theory and test results statistics. Because the actual geographical environment is very different, it is difficult to describe it accurately with a mathematical model, especially the reflection, diffraction and occlusion of various dense irregular buildings on urban streets, which brings great difficulties to the prediction of mathematical models. Therefore. Although the prediction with a certain precision can be used as a preliminary setting to guide the selection and distribution of network base stations, there is always a difference between the prediction by mathematical model and the actual signal field strength. Due to the complexity and variability of the mobile environment, it is quite difficult to accurately calculate the median value of the received signal. The practice in wireless communication engineering is to find out the relationship between propagation loss (or received signal field strength) and distance, frequency and antenna height under various terrains and objects, give various charts and calculation formulas of propagation characteristics, and establish a propagation prediction model, so as to predict the median value of received signals with a relatively simple method.
5.5 Reference coverage standard
Indoor coverage level of bustling urban areas in big cities: -70dBm
General urban indoor coverage level: -80 dBm
Urban outdoor coverage level: -90 dBm