Use fiber optic repeater to optimize the CDMA mobile communication system network

“With the large-scale growth of mobile communication users, the scale of wireless networks is getting larger and larger, and the requirements for network quality are getting higher and higher. However, due to the uneven distribution of traffic density and the suboptimal network configuration, the existing network service quality is not satisfactory. Therefore, as the provision of mobile services moves towards marketization, network optimization has become a key issue.

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Author: Lou Li

With the large-scale growth of mobile communication users, the scale of wireless networks is getting larger and larger, and the requirements for network quality are getting higher and higher. However, due to the uneven distribution of traffic density and the suboptimal network configuration, the existing network service quality is not satisfactory. Therefore, as the provision of mobile services moves towards marketization, network optimization has become a key issue.

1 Wireless network optimization

The so-called wireless network optimization is the process of adjusting the network after it is completed and opened. It uses testing and statistical methods to understand network operation conditions, find network problems, analyze and adjust parameters and adopt certain technical methods to gradually improve system performance. Improve to achieve the best service quality, best coverage, satisfactory signal strength, best call sound quality, and lowest call drop rate under the existing system configuration.

The normal procedure of the mobile communication network project can be roughly represented as shown in Figure 1.

2 Repeater

As far as remote towns are concerned, there are many optimization methods. Among them, using repeaters to cover blind areas and extending base station signals has great advantages.

A repeater (repeater) is a co-frequency amplification device, which refers to a radio transmission relay device that enhances the signal in the process of wireless communication transmission. His basic function is a radio frequency signal power booster. In the downlink, the repeater receives the signal from the coverage area of ​​the donor antenna, and isolates the signal outside the band pass through a band-pass filter, and the filtered signal is amplified by the power amplifier and re-transmitted to the area to be covered . In the uplink path, the mobile phone signals of the mobile stations in the coverage area are processed by the uplink amplification link and transmitted to the corresponding base station in the same working manner, so as to achieve the signal transmission between the base station and the mobile phone.

There are the following categories of mobile communication repeaters:

(1) There are GSM repeater and CDMA repeater from the transmission signal;

(2) There are outdoor type and indoor type from the installation site;

(3) From the transmission bandwidth, there are broadband repeaters and frequency selective (channel selection) repeaters;

(4) From the transmission mode, it is divided into a repeater, an optical fiber transmission repeater and a frequency shift transmission repeater.

The use of CDMA repeaters can expand the coverage of CDMA system base stations and greatly save the investment in CDMA network construction (the investment of a CDMA repeater is about 1/10 of that of a CDMA base station). Especially in special environments such as underground (such as subway) and blind areas, CDMA repeaters can give full play to their advantages.

The salient feature of CDMA mobile communication repeater is the use of super linear power amplifier to ensure no clutter in multi-channel operation, two-way relay wireless signal to extend the wireless coverage area, to achieve coverage of special terrain, eliminate coverage blind areas, deploy cell services, and balance the performance of each cell Traffic volume, etc., in order to achieve the purpose of expanding wireless network coverage and optimizing the network at low cost.

2.1 Fiber optic repeater

Due to the different requirements of the environment and conditions in various places, the types of CDMA repeaters required are also different. For remote towns with a small number of users, the cost of setting up base stations is too high, and the infrastructure is also more complicated, and the cost-effective equipment with small base station functions-CDMA fiber optic repeaters can well solve such problems. The use of fiber optic repeaters as one of the necessary means to achieve the goal of “small capacity, large coverage” is mainly because the network coverage is guaranteed without increasing the number of base stations, and its cost is much lower than that of a microcellular system with the same effect . Therefore, the fiber optic repeater is the most economical and effective means to solve the mobile communication in remote urban areas, and it can meet the requirements of users for call services to the greatest extent.

The optical fiber relay mobile communication repeater station is composed of two parts, the near-end machine on the side of the base station and the remote machine on the side of the coverage area. Set up a site when there is an optical cable between the base station and the coverage area. The fiber optic repeater has both broadband, band selection, frequency selection and other functions. The transmission distance can be up to 20km. Due to the good spatial isolation and no co-frequency interference, the retransmission direction can be covered by an omnidirectional antenna to improve the coverage effect.

The fiber optic repeater transmits through the fiber optic cable, and uses the optical signal receiver and converter to connect to the remote area. It has stable work, good coverage, more flexible design and construction, avoids co-frequency interference, can cover omni-directionally, has less interference, single-stage transmission distance is up to 50km or more, can increase gain without self-excitation, and has It is beneficial to increase the downlink signal transmission power and other characteristics. Therefore, its signal transmission is not restricted by geographical conditions and expands the coverage area. It is especially suitable for remote towns or mountainous areas with complex terrain.

2.2 The working principle of optical fiber repeater

The principle diagram of the optical fiber repeater is shown in Figure 2, which is mainly composed of optical near-end machine, optical fiber, and optical remote machine (covering unit).

Both the optical near-end machine and the optical remote machine include a radio frequency unit (RF unit) and an optical unit. After the wireless signal is coupled from the base station, it enters the optical near-end machine, through electrical-optical conversion, the electrical signal is converted into an optical signal, from the optical near-end machine to the optical fiber, and transmitted to the optical remote machine through the optical fiber. The signal is converted into an electrical signal and enters the RF unit for amplification. After the signal is amplified, it is sent to the transmitting antenna to cover the target area. The working principle of the uplink is the same. The signal transmitted by the mobile phone passes through the receiving antenna to the optical remote machine, then to the near-end machine, and returns to the base station.

2.3 Engineering design principle of optical fiber repeater

When setting up the coverage area of ​​an optical fiber repeater, one basic principle and three mutually restrictive elements must be mastered.

Principle: Try to make the system margins of the uplink and downlink channels equal, so as to ensure that the communication distance, voice quality, and communication probability of the uplink and downlink channels are roughly the same. Elements: coverage radius, voice quality, communication probability (reliability).

In addition to the above three elements, factors such as the propagation environment, topographic features, frequency bands used, and available system parameters should also be considered. Therefore, the following design equations are given.

SM=SG-SL (1)

SG=Pt+Gt+Gr－Pmin (2)

SL=La+Lt+Lr (3)

Pmin=Pr+d (4)

Among them: SM is the system margin (dB);

SG is the system gain (dB);

SL is the system attenuation (dB);

Pt is the transmitter output power (dBW);

Gt is the transmit antenna gain (dB);

Gr is the gain of the receiving antenna (dB);

d is the amount of deterioration;

Pmin is the minimum protection power electronics (dBm) at the input of the receiver;

La is the actual path loss median (dB);

Lt is the additional attenuation at the transmitting end (dB);

Lr is the additional attenuation at the receiving end (dB);

Pr is the noise threshold level of the receiver.

For the radio wave propagation of mobile communication, its fading characteristics are characterized by the following known formula and Figure 3.

Propagation attenuation in free space:

Lbs=32.45+201gd(km)+201gf(MHz)(5)

Median propagation loss of quasi-smooth terrain urban path:

Ltt = Lbs + Am (f, d)-Hb (hb, d)-Hm (hm, f) (6)

Am (f, d), Hb (hb, d), Hm (hm, f) are the corresponding correction factors.

Among them: Am (f, d) is the median value of basic attenuation;

Hb (hb, d) is the base station antenna height gain factor;

Hm (hm, f) is the height gain factor of the mobile antenna.

3 Application examples of optical fiber repeater

There is a small town 40km away from the base station (with a coverage radius of 5km). Its terrain is quasi-smooth terrain. Because the coverage area of ​​the town is far from the base station and the signal attenuation is large, the wireless coverage signal of the base station cannot be obtained in this place. Therefore, the communication quality cannot be guaranteed.

After designing, it was determined to build a fiber optic repeater station here. Using the topographical features, the antenna of the repeater station was placed on the top of a 20m iron tower on a 15m high roof. The feeder selected SDY-50-22’s low loss (0 .05dB/m) The cable is 40m. Here, it is necessary to ensure that the probability of edge communication above 5km is 50%.

The built repeater can determine the channel output power is 1W (0dBW), the transmit omnidirectional antenna gain is 8dB, and the mobile station’s receiving sensitivity (A) is 110dBm (0.7μV), ensuring a 50% communication probability.

Then, the coverage radius d=9.3km can be obtained from the design equation. Therefore, the coverage area of ​​the radius 9.3km can satisfy the blind area with a 50% edge communication probability.

In addition to the coverage radius and edge communication probability, it is also necessary to determine the distance between the repeater and the donor base station; the introduction of repeater equipment adds thermal noise to the signal between the mobile phone and the base station, and the direct consequence of increasing thermal noise is to reduce the base station The receiving sensitivity. Therefore, attention should be paid to the influence of the gain of the repeater and the path loss from the base station transmitter to the repeater, as well as the correct calculation of the downstream gain of the fiber optic repeater and the issue of the balance between the upstream and downstream of the repeater and the mobile phone.

4 Conclusion

Examples have shown that using fiber optic repeaters to optimize CDMA networks in remote towns has achieved good results, and the quality of wireless network services in this area has been improved to a certain extent.

CDMA mobile communication system network optimization is a long-term, arduous, and complex system project, which runs through the entire process of planning, design, project construction, and maintenance management, and adjustments in all aspects are interrelated and affect each other. There are many ways to optimize the network, and we still need to further explore and improve it in order to comprehensively improve the quality of network services and strive for greater economic and social benefits.