Narrowband CDMA wireless network planning and design

Narrowband CDMA wireless network planning and design

1 Introduction

In the past 10 years, mobile communication users have increased their scale at an annual rate of 80% to 200%. According to this analysis, by 2000, China's mobile users will reach 30 million, accounting for about 10% of the world's total mobile users. The contradiction between the rapidly increasing network capacity and the limited frequency resources has become increasingly prominent.

The CDMA technology has the characteristics of large user capacity, wide coverage and good voice quality, which has won the favor of consumers and operators, and has been more and more widely used around the world. As of June 1999, there were more than 30 million mobile users on CDMA networks worldwide. In 1996, China first conducted commercial trials of CDMA networks in Beijing, Shanghai, Guangzhou and Xi'an. According to the test results and market demand, it is currently possible to build a large-scale commercial network.

2. Wireless network planning

The construction of the mobile communication network can be roughly divided into six steps: draft coverage indicators and traffic requirements to be achieved by the network; preliminary network planning; site survey of base station sites; revise the network planning and complete the engineering design; system commissioning and network optimization ; According to the optimization results or network expansion requirements, return to the first step. The design of the CDMA network also follows this step, but it differs from the GSM and TACS networks in many ways.

2.1 Wireless coverage

The wireless coverage of the CDMA network mainly depends on factors such as equipment noise figure, interference impact, fading reserve, Eb / No, etc. The specific analysis is shown in Table 1.

Table 1 Setting of wireless coverage parameters

Serial number

Project Description

Typical value

Remarks

1

Bandwidth (HZ)

1228800

CDMA single carrier frequency bandwidth

2

Blotzman constant (w / (Hz * k))

1.38E-23

Unit constant

3

Greenhouse (k)

290

4

Base station noise figure (dB)

5

Typical value

5

Receiver interference effect (dB)

4 ~ 6

6

Soft switching gain (dB)

3 ~ 4

7

Base station antenna Zeng Yi (dBi)

9 ~ 17

8

Feeder loss (dB)

1 ~ 3

9

Normal Fading Reserve (dB)

9 ~ 11

10

Building penetration loss (dB)

10 ~ 25

According to the terrain features

11

Eb / N. (dB)

6 ~ 7

12

Required C / I (dB)

-14

The bandwidth and Boltzman constant are fixed values. The noise figure of the base station depends on the device. The interference effect is determined by the network design load percentage. The fading reserve is given by the wireless signal edge coverage. Determined.

Because CDMA is a broadband system and has a higher spreading gain, it can still get a good quality of service when C / I is negative, which is much better than traditional GSM and analog systems. Under the same conditions, the propagation distance of CDMA is 1.3 to 2.1 times larger than that of GSM. The radius of CDMA base stations in high-traffic density areas of large cities can be set at a minimum of about 300 m; for suburban open areas, the characteristics of large coverage should be fully utilized, and the radius can reach more than 50 km. Table 2 shows the coverage radius of base stations in several different regions.

Table 2 Base station coverage radius of various regions

region

Urban dense area

Urban area

suburbs

rural

vehicle

Building penetration loss

18 ~ 25

15 ~ 20

10 ~ 15

10

6

CDMA base station radius (km)

0.9

1.5

4.3

21.0

33.0

2.2 Base station traffic configuration

(1) Determination of base station capacity

The main parameters that determine the capacity of a CDMA base station are: processing gain, Eb / No, voice activation factor, frequency reuse factor, and the number of base station antenna sectors.

For a single-sector single-carrier frequency base station, the maximum configuration can be 61 channels. At present, the engineering generally takes 23 omnidirectional and 20 directional, as shown in Table 3.

Table 3 Typical station capacity configuration table

GOS = 2%

GOS = 5%

Station type

Number of channels

(Piece)

Traffic

(Piece)

User number

(Piece)

Number of channels

(Piece)

Traffic

(Erl)

User number

(Piece)

O1

twenty three

15.76

631

twenty three

18.08

724

O2

46

36.53

1462

46

40.55

1622

1/1/1

20/20/20

39.54

1582

20/20/20

45.75

1830

2/2/2

40/40/40

93.00

3720

40/40/40

103.8

4152

Since the physical channel resources between CDMA base station sectors can be shared, in the actual operation of the network, the amount of traffic it can handle must be greater than the design theoretical value, which is a unique advantage that other systems do not have.

(2) Traffic configuration

Compared with the GSM network, the CDMA network base station traffic configuration has greater flexibility, therefore, it is also the focus of engineering construction. First, a detailed survey of the field is required to understand the local mobile traffic distribution; second, advanced network planning software is used to predict; and finally, base station traffic is allocated based on the prediction results. The difference between the final design value and the actual traffic volume after the network is opened should not exceed 30%.

2.3 Interference analysis and coordination

IS-95 specifies the frequency bands used by the CDMA network: uplink 824-849 MHz, downlink 869-894 MHz. The frequency of the ETACS network currently built in China is exactly 880 ~ 890 MHz, so the CDMA network to be built in the future will inevitably have a great impact on the existing network. The existence of this type of interference is a phenomenon unique to our country. In other countries and regions of the world, because there is no ETACS standard, or because there is no CDMA technology, there is no such phenomenon. Its solution is also one of the key issues affecting the construction of CDMA networks.

The effect of the CDMA originating signal on the ETACS receiving end can be expressed by the following formula:

Pr = Pt-LoA-Lb-10 lg (30/25)

Among them, Pt: CDMA single sector output maximum power; Pr: ETACS received signal strength; LoA: CDMA out-of-band loss; Lb: CDMA, ETACS antenna isolation.

In order to ensure that there is no interference, the Pr value is required to be less than the sensitivity of the ETACS receiver, that is, to adjust the antenna isolation, the theoretical calculation needs to reach more than 86 dB. Antenna isolation is divided into horizontal, vertical and inclined:

Horizontal isolation Lh = 22 + 20lg10 (d / λ)-(Gtx + Grx)

Vertical isolation Lv = 28.0 + 40lg10 (d / λ)

Slope isolation Ls = (Lv-Lh) (θ / 90) + Lh

Among them, d: horizontal spacing of antennas (meters); Gtx, Grx: antenna gain; θ: angle between the two antennas in the vertical plane.

To meet the isolation requirements, the vertical spacing between CDMA and ETACS antennas should be greater than 6 m or the horizontal spacing should be kept above 1 km. However, the actual propagation environment is not free space, and the diffraction loss caused by terrain, features, and buildings cannot be calculated by theory. To analyze these situations, the field strength of the wireless signal needs to be tested on site.

While spatially decoupling, you can also adjust the relative position of the antenna, use interference cancellers, and use antennas with narrow lobes to increase isolation. However, these cannot guarantee the fundamental solution to the interference problem. The best way is to implement frequency coordination, re-divide this frequency, and give a certain protection bandwidth.

2.4 Planning of PN-Offset

Since the frequency reuse coefficient of the CDMA system is about 1, it does not require frequency planning. However, there is a potential problem in the actual situation, that is: although all base stations use different PN-Offsets, from the perspective of the mobile station, due to propagation delay (adjacent PN-Phase interference) and PN-Offset complex Insufficient distance (interference with PN-Phase) will make some uncorrelated pilot signals look the same. Adjacent PN-Offset interference is the main factor affecting base stations in large coverage areas, and PN-Offset interference is the main factor affecting base stations in small coverage areas. Therefore, the planning of PN-Offset is a unique problem of CDMA system.

There are four types of pilot sets with the same frequency but different PN code phases: effective pilot set, adjacent pilot set, candidate pilot set and remaining pilot set. PN-Offset interference can only occur in the first two Pilot concentration.

(1) If two uncorrelated channels on both phases fall in the same effective pilot search window, both will become one of the three strongest signals, and effective pilot set PN-Offset interference will occur. The mobile station will despread and merge the uncorrelated forward traffic channel signals.

(2) If a remote traffic channel falls into the adjacent pilot set, and its Ec / Io> T-add, the adjacent pilot set PN-Offset interference will occur. The mobile station will switch to the wrong pilot and despread the wrong signal.

Their common result is strong interference and dropped calls.

The method to avoid neighbor PN-Offset interference is:

. Make the interval between adjacent PN-Offsets much different than the propagation delay.

Minimum required interval value S [chip] ≥ R × [1021 / 10a -1] + W / 2

Among them, R is the cell radius, the unit is chip (1 chip = 244 m); W is the effective pilot window size, the unit is chip; a is the path loss index.

. Large cells require large intervals, that is, increase the phase deviation of PN codes of neighboring cells.

The method to avoid interference with PN-Offset is:

. Make the difference caused by the propagation delay greater than half the size W of the pilot search window;

. The minimum value D of the PN multiplexing distance should satisfy: D> W / 2 + 2R.

2.5 Setting of soft switching area

CDMA system has three handover methods: hard handover, soft handover and softer handover. Hard handover only exists between different carrier frequencies. The so-called soft handover refers to that the mobile station does not immediately interrupt the communication with the original base station when establishing contact with the new base station during the handover process, that is, "connect before disconnecting". At present, when working on the same carrier frequency, CDMA can realize soft handover between BTS, BSC and MSC. The handover between different sectors of the same base station is called softer handover.

During the soft handover process, the mobile station establishes contact with different base stations. What remains the same is the vocoder selected for the initial call establishment. Therefore, if the vocoder is placed in the BSC, a relay direct circuit needs to be set up between different BSCs to connect the vocoder; if it is placed in the MSC, the same needs to be done. However, since the MSC has a large control range and a large number of internal vocoders, it is costly to set up a directly connected circuit. ATM is generally used to connect different MSCs to achieve soft handover between them. From this point, the integrated setting of BSC and MSC can save transmission investment.

Another important aspect is the setting of the soft handoff area. The introduction of soft handover technology has indeed reduced the call drop rate and improved the communication quality. However, in order to realize soft handover, some channel cards need to be taken out specifically as soft handover channels during base station configuration. Therefore, too much soft handover channel configuration will inevitably cause waste of resources; too little will reduce the success rate of soft handover. The proportion of soft handoff areas should be set reasonably based on the actual characteristics of various regions and the scale of CDMA network construction and development. It is generally maintained between 30% and 40% in engineering.

2.6 Application of multi-carrier

In recent years, due to the exponential growth of mobile users, in some high-traffic areas of large cities, the traffic density has grown from a few, a dozen errands per square kilometer to dozens or hundreds. It will be higher in the next two or three years. The application of CDMA multi-carrier technology is an important method to solve such high traffic density.

Because there is hard handover between multiple carriers in a CDMA system, the first factor to be considered in the design of multiple carriers is how to reduce hard handover. The following issues should be noted:

(1) Optimize hard handover to reduce the risk of dropped calls;

(2) To avoid isolation of multi-carrier base stations, multi-carrier should be implemented in a group of cells to reduce hard handover;

(3) Avoid making high-traffic cells the border cells where hard handover occurs.

When planning the network, multi-carrier base stations should be connected as much as possible. For handover, a pseudo-pilot method can be used, that is, at the edge of the multi-carrier coverage area, some base stations that only transmit pilot signals to the multi-carriers are set. When the mobile station moves here, the pilot is used to trigger soft handover, but the traffic channel of other carrier frequency is used, and then the pilot signal is switched to the carrier frequency to actually complete the hard handover. There are also methods such as loop triggering and FER discrimination to deal with the hard switching between multiple carriers, which can be used flexibly according to the specific characteristics of the project.

2.7 Application of Repeater

As an auxiliary technical means to realize wireless coverage, the mobile communication repeater is often used to solve the blind area that is difficult to cover by the base station or extend the base station signal. In the early days of network construction, it can use less investment and shorter cycles to rapidly expand wireless coverage. Its installation should fully consider the following links: mainly solve the coverage of narrow and long terrain such as the main traffic roads and railways in suburban counties; for areas with low carrier frequency utilization of base stations, the surplus communication capabilities can be transferred to Where necessary, improve equipment utilization; try to set it in a relatively isolated area to avoid wireless interference; choose a suitable base station as the signal source.

Points to note when using CDMA repeaters:

(1) Delay problem: There is a 4 ms delay between the repeater and the source base station. Therefore, when designing its coverage, the delay caused by multipath and the inherent delay must be considered at the same time so that it does not exceed one The length of chip time will not cause crosstalk between codes.

(2) Antenna setting: The introduction of repeater will cause the background noise of the base station to increase. The amount of noise increase is related to the noise factor, system gain, antenna gain and propagation loss of the repeater. While considering its coverage environment to make it have a certain propagation loss, we also need to carefully choose the antenna gain so that the introduction of repeaters will not cause the communication quality of the base station to deteriorate.

(3) Diversity technology: For areas with more multipath signals and faster mobile user speed, if repeater technology is used, the diversity antenna system must be considered to ensure the quality of the call, such as highway areas; for multipath In areas with fewer signals and slower mobile user movement, it is not necessary to use diversity systems, such as indoor distribution systems.

2.8 Other problems

In the design of mobile communication networks, high-traffic hot spots are the design difficulties and key points. The CDMA network can further reduce the radius of the macrocell base station (up to 300 m) and adopt multi-carrier technology. It can also use microcell, more sectors and smart antenna technology. Because CDMA systems share the same carrier frequency, interference control is the primary issue for microcell applications. Buildings can be used to solve interference. CDMA microcell base stations should be located indoors, underground, tunnels, or subways.

When the three sectors cannot meet the capacity requirements, if the six sector technology is used, the base station capacity can be increased by 1.8 times. The introduction of smart antennas can also expand the network capacity by about 1.3 times, while also reducing wireless signal interference.

3. Conclusion

The large-scale commercial use of CDMA technology in China is still in its infancy, and the research and solution of many problems require continuous learning and testing in practice. For engineering and technical personnel, to further understand and master its engineering characteristics is an important prerequisite for improving CDMA technology and expanding its market.

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