WO2009065255A1

WO2009065255A1 - Cdma base station and signal processing method used therein

Info

Publication number: WO2009065255A1
Application number: PCT/CN2007/003295
Authority: WO
Inventors: Hairong Xie; Steven Shio
Original assignee: Lucent Technologies Inc
Current assignee: Nokia of America Corp
Priority date: 2007-11-21
Filing date: 2007-11-21
Publication date: 2009-05-28
Anticipated expiration: 2010-05-21
Also published as: CN101884243B; CN101884243A

Abstract

The present invention provides a new solution to solve the fiber delay issue in optical repeater. According to the present invention, a CDMA base station is proposed which comprises: a timing advance unit for generating a signal ahead of the time when it should be transmitted from an antenna of a local cell by a predetermined timing advance value; a splitter for splitting the signal generated by the timing advance unit into two branches; a delay unit for delaying a first branch of split signals by the predetermined timing advance value; and a radio processing unit for receiving both the first delayed branch of split signals and a second non-delayed branch of split signals, performing the same Digital-Analog conversion and RF processing to the first and the second branches of split signals respectively, and then outputting the processed first delayed branch of split signals for the usage of the local cell, and outputting the processed second non-delayed branch of split signals to a local unit of an optical repeater to be transmitted for the usage of a remote cell. According to the present invention, a corresponding signal processing method for use in a CDMA base station is also proposed.

Description

CDMA BASE STATION AND SIGNAL PROCESSING METHOD USED THEREIN

BACKGROUND OF THE INVENTION FIELD OF INVENTION

The present invention relates to the field of radio communication, in particular the invention relates to a novel CDMA base station and a signal processing method used to improve the performance of the CDMA optical repeaters.

DESCRIPTION OF PRIOR ART

Fiber optic repeaters are widely used in the Code Division Multiple Access (CDMA) wireless networks. They are cost effective; however, they also introduce some issues to the network. One of them is the handoff issue between the repeater and the neighbor cells. If the delay relationship between the repeater coverage and the neighbor cells' coverage is not properly configured, the mobile station could not successfully search the pilot signals of the neighbor cells, thus the handoff between them would be problematic.

According to current solutions, the active/neighbor search window size must be enlarged to compensate for the fiber delay to solve the above handoff issues. However, it requires very careful configuration effort and Radio Frequency (RF) optimization effort. Basically speaking, a current repeater is transparent to the network, and it does not change any characteristic of the signals except amplifying them. So, the pilot PN (Pseudo Noise) offset is the same as that transmitted from the donor Base Transceiver Station (BTS). In the downlink, it amplifies the signals from the donor BTS and broadcasts them to all the mobile stations. And in the uplink, the repeater receives the signals from the mobile stations and feeds them to the donor BTS. Fig. 1 illustrates the block diagram for the connection of a conventional repeater. A repeater antenna 1501 is for the remote coverage, while a local antenna 140 is for the local coverage. The remote unit 1801 of the repeater can be connected with the local master unit 120 of the repeater through the optical fiber 1951. The local master unit 120 is connected with the base station by using the three-port RF component: coupler 160. Normally this coupler 160 has a high coupler loss, for example, 4OdB, thus the loss for the local coverage signal is very small and can be negligible. The base station 110 is used to generate the forward link CDMA signal and process the reverse link signal transmitted by mobiles.

Fig. 2 illustrates the transition area between the repeater and the donor cell. The donor BTS provides a local coverage 210, while a repeater provides a remote coverage 220. There is a fiber 230 connected between the local unit and the remote unit of the repeater. As indicated in Fig. 2, the repeater and the donor sector have the same PN offset configuration. A mobile station (MS) 240 is initially located at the repeater coverage 220.

First, we need to determine the delay relative to system time of the signal that reaches the MS 240 when it is in the repeater coverage area 220. The delay includes three parts: the fiber delay (D _F), repeater internal delay (D_Λ ) and the propagation delay (D _P__repealer ) from the repeater remote antenna to the MS 240.

Assume the mobile station 240 is moving from the repeater area 220 to the donor sector area 210, then the MS 240 will try to find the forward link signal broadcast directly from the donor BTS antenna. Of course, this signal also has the delay from donor BTS antenna to MS 240, and it can be noted as D_P__bls . The mobile station

240 will use the timing information demodulated from the repeater as the reference timing, and it will place the center of its pilot search window at that reference time.

Only if the signal from donor BTS arrives within this search window, the mobile station 240 can find the forward link from the donor BTS. So the delay difference of the signals from the repeater and the BTS must be less than the half of the active set search window:

W

"epeait

where W , is the search window size of active set.

Only if the delay fulfills the inequation above, the call can move smoothly from repeater coverage 220 to the donor BTS coverage 210, otherwise the call will be dropped.

Similar analysis can be done for the handoff between the repeater and the other sector/cell. The similar inequality must be satisfied for the neighbor set search window. Hence, there is a fiber delay issue for the fiber optic repeater. The active set search window size and the neighbor set search window size of the related cell must be updated to compensate for the fiber delay. In a commercial CDMA network, a lot of call issues for the repeater are related to the fiber delay issue. For example, if it is assumed that the repeater has an internal delay of 5 μs and 20 km fiber with a delay for each km fiber of 4.9 μs, then a total delay of 103 μs (= 5 + 20*4.9) needs to be compensated. As such, the search window size needs to be increased by about 253 chips (=2*(103*1.2288)) in this case. According to these analyses, a careful RF optimization is definitely required when deploying the fiber optic repeater. If the search window sizes of the donor cell and each neighbor cell are not properly set, the handoff between them will be problematic.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a new solution to solve the delay issue for fiber optic repeater, thereby avoiding or reducing problems related to handoff. The combination of the timing advance and timing delay technology is used to synchronize the donor cell signal and the remote repeater signal. According to a first aspect of the present invention, a CDMA base station is proposed which comprises: a timing advance unit for generating a signal ahead of the time when it should be transmitted from an antenna of a local cell by a predetermined timing advance value; a splitter for splitting the signal generated by the timing advance unit into two branches; a delay unit for delaying a first branch of split signals by the predetermined timing advance value; and a radio processing unit for receiving both the first delayed branch of split signals and a second non-delayed branch of split signals, performing the same Digital-Analog conversion and RF processing to the first and the second branches of split signals respectively, and then outputting the processed first delayed branch of split signals for the usage of the local cell, and outputting the processed second non-delayed branch of split signals to a local unit of an optical repeater to be transmitted for the usage of a remote cell.

According to a second aspect of the present invention, a signal processing method used in a CDMA base station is proposed which comprises the steps of: generating a signal ahead of the time when it should be transmitted from an antenna of a local cell by a predetermined timing advance value; splitting the generated signal into two branches; delaying a first branch of split signals by the predetermined timing advance value; performing the same Digital-Analog conversion and RF processing to the first and the second branches of split signals respectively; and outputting the processed first delayed branch of split signals for the usage of the local cell, and outputting the processed second non-delayed branch of split signals to a local unit of an optical repeater to be transmitted for the usage of a remote cell.

The present invention will effectively improve the fiber optic performance. Thereby, network designers do not need to update active/neighbor set search window sizes since the repeater signal has been synchronized with the system timing

BRIEF DESCRIPTION OF THE DRAWINGS The above and other objects, features and advantages of the present invention will be clearer from the following detailed description about the non-limited embodiments of the present invention taken in conjunction with the accompanied drawings, in which: Fig. 1 , already described, is a block diagram showing a conventional connection structure of an optical repeater;

Fig. 2, already described, is a schematic diagram illustrating the transition area between the repeater and the donor cell; and

Fig. 3 is a block diagram showing the system construction according to the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Hereunder, the present invention will be described in accordance with the drawings. In the following description, some particular embodiments are used for the purpose of description only, which shall not be understood as any limitation to the present invention but as examples thereof. For the sake of simplicity, conventional structures or constructions which are not relevant for the understanding of the invention will be omitted.

Fig. 3 shows a block diagram of the system construction according to the present invention. Since the solution provided by the invention is mainly applicable to the forward link of a base station, only the forward link configuration is described in this figure.

In Fig. 3, element T.A. 310 represents a timing advance module. The timing advance function is provided by the CDMA cell site modem chip, so that the new traffic frame can be generated ahead of the time when it should be transmitted from the antenna of the cell. The typical timing advance budget (that is, the upper limit of the timing advance value) for 3G1x is about 400 chips. The splitter 320 splits the base band data into two branches. The first branch goes through a radio processing module 340 and is fed into the local unit of the optic repeater 350, while the other branch is delayed by several chips (same as the T.A. chips) in a delay unit 330 and then goes through the radio processing module 340. The other branch of signal is the original donor cell's coverage signal, and thus it passes through a high power amplifier 360 and then is fed into a local coverage antenna 370.

The radio processing module 340 performs a number of functionalities which are described as follows. It will firstly convert the base band digital signal into an analog signal using a Digital-Analog converter. Then, a conventional mixer is used to change the signal into a high-frequency signal. Some filters and gain blocks would be used for the RF processing. The local unit of optical repeater 350 basically receives the RF signal from the module 340 and changes the RF signal into an optical signal and feeds it into the remote unit 380 through the fiber connection which is denoted as 395. The remote unit of the fiber optical repeater 380 will convert the optical signal from local unit 350 into an RF signal. Then the RF signal will be amplified and fed to the antenna 390 for the remote coverage.

Additionally, only one radio processing module 340 is shown in this embodiment, but the present invention is not limited to only one such module. For example, as it may be known by those skilled in the art, the radio processing module can be separated into two sub-modules, one for the first branch, and the other for the second branch.

Assuming the physical fiber length is L km, and β (chips/km) is a fiber delay per unit fiber length. Then the delay for the fiber is D _f {chips) = βL . The parameter D_f needs to be configured by MSC/BSC (Mobile Switching Center/Base Station Controller), which will notify to the cell the delay information. Then the cell will notify the parameter D_f chips to both the T.A. 310 and the delay unit 330.

For the local coverage antenna 370, the traffic data is first "timing advanced" (by element TA. 310) and then delayed by the same time interval (by the delay unit

330), so the traffic frames are still aligned with the system timing. For the remote repeater coverage (the remote coverage antenna 390), the traffic data is first "timing advanced" (by element T.A. 310) and then delayed by the optical fiber 395 by the same interval D_f , so the traffic frames are also aligned with the system timing. Thus, the signals from both the local antenna and the remote antenna are synchronized with the system timing, so no delay issue will occur.

Furthermore, the invention also proposes a signal processing method for use in a CDMA base station. According to the method of the invention, firstly, a signal is generated ahead of the time when it should be transmitted from an antenna 370 of a local cell by a timing advance value. Then, the generated signal is split into two branches. For these two branches of signals, a first branch of split signals is delayed by the timing advance value, whereas a second branch of split signals is remained non-delayed. Thereafter, the same Digital-Analog conversion and RF processing are performed to the first and the second branches of split signals respectively. Finally, the processed first delayed branch of split signals is outputted for the usage of the local cell, and the processed second non-delayed branch of split signals is outputted to a local unit of an optical repeater 350 to be transmitted for the usage of a remote cell.

In the proposed inventive method, the timing advance value is also determined as aforementioned so that the signals from both the local antenna and the remote antenna are synchronized with the system timing, and thus no delay issue will occur.

The above embodiment is provided for the purpose of example only, and is not intended to limit the present invention. It is to be understood by those skilled in the art that there may be various modifications or replacements to the embodiments without departing from the scope and the spirit of the present invention, and they shall fall into the scope defined by the appended claims.

Claims

What is claimed is:

1. A CDMA base station, comprising: a timing advance unit for generating a signal ahead of the time when it should be transmitted from an antenna of a local cell by a predetermined timing advance value; a splitter for splitting the signal generated by the timing advance unit into two branches; a delay unit for delaying a first branch of split signals by the predetermined timing advance value; and a radio processing unit for receiving both the first delayed branch of split signals and a second non-delayed branch of split signals, performing the same Digital-Analog conversion and RF processing to the first and the second branches of split signals respectively, and then outputting the processed first delayed branch of split signals for the usage of the local cell, and outputting the processed second non-delayed branch of split signals to a local unit of an optical repeater to be transmitted for the usage of a remote cell.

2. The CDMA base station according to Claim 1 , wherein the predetermined timing advance value is achieved from a CDMA cell site modem chip.

3. The CDMA base station according to Claim 2, wherein the predetermined timing advance value is calculated as a fiber delay of a fiber between the local unit and a remote unit of the optical repeater.

4. The CDMA base station according to Claim 3, wherein the fiber delay Df is calculated as below:

D_f = βL in which L denotes a physical fiber length, and β is a fiber delay per unit fiber length.

5. The CDMA base station according to any one of Claims 2 - 4, wherein an upper limit of the timing advance value is about 400 chips for 3G1 x CDMA scheme.

6. The CDMA base station according to Claim 1 , further comprising an amplifier connected between the radio processing unit and the antenna of the local cell for amplifying the processed first delayed branch of split signals outputted from the radio processing unit.

7. The CDMA base station according to Claim 1 , wherein the radio processing unit comprises: a first radio processing unit for receiving and performing the Digital-Analog conversion and RF processing to the first delayed branch of split signals, and then outputting the processed first delayed branch of split signals for the usage of the local cell; and a second radio processing unit for receiving and performing the Digital-Analog conversion and RF processing to the second non-delayed branch of split signals, and then outputting the second processed non-delayed branch of split signals to the local unit of the optical repeater to be transmitted for the usage of the remote cell.

8. The CDMA base station according to Claim 7, further comprising an amplifier connected between the first radio processing unit and the antenna of the local cell for amplifying the processed first delayed branch of split signals outputted from the first radio processing unit.

9. A signal processing method for use in a CDMA base station, comprising the steps of: generating a signal ahead of the time when it should be transmitted from an antenna of a local cell by a predetermined timing advance value; splitting the generated signal into two branches of split signals; delaying a first branch of said split signals by the predetermined timing advance value; performing the same Digital-Analog conversion and RF processing to the first and the second branches of split signals respectively; outputting the processed first delayed branch of split signals for the usage of the local cell, and outputting the processed second non-delayed branch of split signals to a local unit of an optical repeater to be transmitted for the usage of a remote cell.

10. The signal processing method according to Claim 9, wherein the predetermined timing advance value is achieved from a CDMA cell site modem chip.

11. The signal processing method according to Claim 10, wherein the predetermined timing advance value is calculated as a fiber delay of a fiber between the local unit and a remote unit of the optical repeater.

12. The signal processing method according to Claim 11 , wherein the fiber delay D_f is calculated as below:

D_f = βL

in which L denotes a physical fiber length, and β is a fiber delay per unit fiber length.

13. The signal processing method according to any one of Claims 10 - 12, wherein an upper limit of the timing advance value is about 400 chips for 3G1x CDMA scheme.

14. The signal processing method according to Claim 1 , further comprising a step of amplifying the processed first delayed branch of split signals before said signals are fed into the antenna of the local cell.