Analysis and countermeasure of introducing noise into CDMA repeater station

With the rapid popularization of mobile communication service, mobile communication operators and equipment suppliers take various measures to improve the system capacity and business type to meet the growing market demand. China Unicom in the construction and operation of CDMA network in the early stage, due to the constraints of various conditions, it is impossible to build a lot of base stations, the number of base stations will cause inadequate signal coverage, affecting the quality of network services, will ultimately affect the return on investment. To this end, Unicom's new space-time CDMA network in the initial operation of the low capacity, large coverage of wireless network layout of the leading ideas, repeater station with its unique advantages to become an integral part of the network, in the network construction plays an important role.

How to build a good CDMA network is a complex problem. This paper only analyzes the influence of the noise of CDMA repeater on the system, and puts forward the corresponding countermeasures.

Noise analysis of CDMA repeater

Repeater as an active two-way amplification equipment, in the amplification of useful signals at the same time, will inevitably introduce a certain noise, even in the absence of any input signal, will also emit stray signals, equivalent to an in-band interference source. Moreover, the CDMA system is a self jamming system, system capacity and quality of service are closely related to multi-user interference, so the system always hope that the interference of repeater to the smaller the better, and the actual equipment noise coefficient is not 0, in the equipment specification of the noise coefficient requirements are less than 5dB, therefore, in the following analysis, The noise coefficient takes the maximum limit value 5dB to participate in the calculation. (Computer science)

1. Noise analysis of Forward link

Whether it's an optical transmission repeater or a wireless repeater station, the signal-to-noise ratio of the forward link signal is very high, the noise level is much lower than the Gaussian ambient noise, the useful signal level is much higher than the Gaussian ambient noise, after the repeater amplification, the signal-to-noise ratio of the signal is still good, basically will not affect the system.

Take the wireless repeater as an example: according to the equipment working conditions, to reach the repeater signal intensity is greater than -80dbm, the Gaussian ambient noise level is -113dbm, when the signal-to-noise ratio is (s/n) in=33db, the repeater's noise coefficient is 5dB, then amplified signal signal-to-noise ratio (s/n) output= 33-5=28DB, it satisfies the requirement of ec/no>-14, so the forward link can not consider the influence of repeater in practical application.

2. Noise Analysis of Reverse link

(1) Effect of reverse link noise on thermal noise of base station

Because of the thermal noise in the repeater, the inverse link signal is superimposed on the thermal noise in the amplification process, which will increase the noise level of the donor base station and reduce the sensitivity of the donor base station receiver.

Assuming that the noise coefficient of the repeater reverse link is frep-rev, the gain of the Repeater reverse link is grep-rev, the ambient temperature t=290k, and the thermal noise power spectral density generated by the repeater is Nrep, then: NREP=10LG (KT) +frep-rev

When the thermal noise is amplified by the space link into the base station receiver, the noise power spectral density generated at the base station receiver is defined as NI, then:

NI=10LG (KT) +frep-rev+grep-rev-lrep+grep-ant-lpath+gbts-ant-lbts

Which: Lrep is the loss of the feed feeder of the repeater, the grep-ant is the gain of the donor antenna of the repeater station, the Lpath is the air loss between the donor base station and the repeater station, gbts-ant the gain of the forward antenna of the base station, the loss of the feeder between the base station transmitter and the forward antenna.

For the specific project, NI is only related to Grep-rev, the rest of the parameters are fixed constants, it can be seen that the repeater gain is directly related to the size of the donor base station noise level. (Because the bandwidth of a carrier frequency of CDMA system is usually 1.23MHz, the noise level of the base station is fixed with the noise power spectral density.) ）

In order to measure the change of the noise level of the base station receiver after introducing the repeater in the system, define the noise injection margin nim (Noise injection Margin):

NIM=10LG (Pbts/prep) wherein: PBTS is the base station receiver's own noise level, expressed as an absolute value (DBW).

Prep is the equivalent noise level generated by the repeater at the donor base station receiver, expressed as an absolute value (DBW).

To facilitate comparison, define the increased amount of thermal noise at the input end of the donor base station receiver (Rise over thermal): Rot=10lg ((pbts+prep)/pbts) =10LG (1+10)

From the above formula and the relationship curve, we can see that the increase of thermal noise at the input end of the base station receiver determines the effect of the reverse link of the repeater on the reverse link of the donor base station, that is, the greater the NIM value, the smaller the noise increase caused by the repeater at the receiving end of the donor base station, the greater the Vice. Nim is directly dependent on the Repeater reverse link gain grep-rev, if the Grep-rev value, the smaller the NIM, resulting in the increase of the rot, so that the donor base station to receive sensitivity reduction, and the base station coverage of the edge of the user will be a single pass, voice quality drop and drop words and so on. If the Grep-rev value is smaller, the NIM is bigger, at this time the rot increases little, to the donor base station receives the sensitivity not to have the obvious influence, but the repeater station's coverage scope will be very small. Considering the three factors such as reception sensitivity of base station receiver, coverage range of base station and coverage of repeater, it is possible to optimize the reverse link gain of repeater in practical application.

In practical applications, it is necessary to compromise the amount of the noise injected into the margin nim and the increase of the noise of the base station. Through a lot of reference and computational analysis, generally in highways, suburbs, rural areas and other applications, the need for repeater coverage distance, take nim=0db, then rot=3db, and indoor coverage and other occasions, can be appropriate to take large NIM, in the premise of the application to minimize the impact of the rot on the system.

(2) Effect of reverse link noise on coverage area of repeater

The repeater station is connected with the donor base station wirelessly, and can be equivalent to a series amplifier. The influence of the users in the coverage area of the repeater can be analyzed by the series equivalent noise coefficient, and the noise coefficients of the base station receivers used to compute the reverse link power budget of the base station are replaced by the series equivalent noise coefficients. If the series equivalent noise coefficient increases, the reverse power budget of the repeater coverage area decreases, and the coverage range is reduced.

(3) The relationship between the reverse link noise and the system capacity of Repeater station

From the system of CDMA system can know that decision cell capacity is multi-user interference mai, when a certain number of users successfully connected to the system, at this time if there are users to try to access, in the role of CDMA power control, may appear no matter how to increase the power of transmission can not overcome the interference of the situation, Causes the user to be unable to connect to the system, at this time the system capacity reaches the limit. From figure (b), it can be seen that the direct station itself thermal noise caused by the injection noise is static noise, similar to environmental interference, it only raised the base station receiver noise level, in the role of CDMA power control, may need to increase the launch power of Mobile station, resulting in the upstream coverage radius is reduced, That is, the thermal noise of the repeater has no effect on the system capacity.

Through the above analysis, we know that the Repeater reverse link gain directly determines the repeater to the system impact, then how to set the repeater gain? The key is the tradeoff between the amount of noise injected into the margin nim and the increase in the noise of the base station.

Methods to reduce the influence of the introduction of noise in repeater stations on the system

In engineering application, we can rationally set up the reverse link gain by measuring and calculating to achieve the goal of optimizing the network.

1. Firstly, the pure path loss of the donor antenna and the base station receiver is tested, and then the inverse link gain grep-rev is calculated based on the above noise analysis. The maximum allowable noise tolerance of the facility main base station is M, then: the noise coefficient of the grep-rev=pl+fbts-frep-m Fbts is the base station receiver.

2. With the strong cooperation of the staff of the Bureau, can also use the base station backstage statistics receiver noise level to reasonably set the repeater station reverse link gain Grep-rev method: Before installing the repeater station, through the long-term statistics, obtains the early morning user very small base station noise level P1, after installs the repeater station, Constantly adjust the Repeater reverse link gain grep-rev, continue to statistics, the early morning users very young base station noise level P2, set up the project planning and design of the base station noise increase in the amount of rot, then the P2=p1+rot gain is the best value.

This paper analyzes the effect of the noise on the system and puts forward the corresponding countermeasures, because the CDMA system is quite complex, the factors and parameters need to be considered, the other aspects of the analysis can refer to the relevant articles, this article is not to repeat.