WO2002037136A1

WO2002037136A1 - Method of measuring the doppler frequency-shift and the relevant device

Info

Publication number: WO2002037136A1
Application number: PCT/CN2000/000355
Authority: WO
Inventors: Cosmos Zhang; Daoben Li
Original assignee: Linkair Communications, Inc.
Priority date: 2000-10-23
Filing date: 2000-10-23
Publication date: 2002-05-10
Also published as: CN1423751A; WO2002037136A8; AU2000279004A1; HK1052971A1

Abstract

The invention discloses a method of measuring the Doppler frequency-shifts which is applied to digital mobile communication system and the device which accomplishes the method. The channel estimating information is filtered by the method, the filtered information is then calculated to get the absolute value of its first derivative. During the time, add up the above calculated results and get its average value, then combine the maximum ratio by the Doppler frequency-shifts determined by the every branch of the RAKE receiver and determine the end measuring value of the Doppler frequency-shifts. The method can be accomplished simply and can be applied widely. It also can be used for the technology used in CDMA mobile communication system such as channel estimate, power control, and switch. It can improve the performance of the receiver and capacity of the system.

Description

Method and device for measuring Doppler frequency shift

Technical collar

The present invention relates to the field of digital mobile communication systems, and in particular, to a method and a device for measuring Doppler frequency shift applied in the field. Background of the invention

The performance of wireless communication systems is mainly restricted by mobile wireless channels. The propagation path between the transmitter and the receiver is very complicated, ranging from simple line-of-sight propagation to encountering various complex features such as buildings, mountains and trees. The wireless channel is not as fixed and predictable as the wired channel, but has strong randomness and is difficult to analyze. The speed of the mobile station also affects the fading of the signal level.

Due to the relative motion of the mobile station and the base station, each multipath wave has undergone a significant frequency shift process. The receiver signal frequency shift caused by the movement is called Doppler frequency shift. It is related to the moving speed and direction of the mobile station, and the angle of incidence of the receiver's multipath wave.

Current mobile communication systems are increasingly required to be able to adapt to the communication requirements of fast-moving users. Because in the fast moving process, the channel characteristics change drastically. In view of the unique situation in this mobile environment, the receiver must be able to track the channel changes in real time in order to achieve normal communication. Therefore, the effectiveness of the channel estimation method is crucial. WCDMA (Wideband Code Division Multiple Access) uses a joint estimation of a continuous pilot channel and a dedicated pilot channel in its forward channel and an estimation of a dedicated pilot channel in its reverse channel (see 3GPP TS 25.211). This dedicated pilot channel method realizes channel estimation by time multiplexing pilot signals with service signals, estimating channel characteristics by relying on adjacent pilot channels, and obtaining channel characteristics experienced by service signals through interpolation processing. This method implicitly assumes that the channel characteristics between adjacent pilot signals change linearly. The validity of this assumption depends on the adjacent pilot signals. Time interval and mobile station's moving speed. In actual implementation, it is desirable to use as few pilot signals as possible while satisfying the requirement of as high a moving speed as possible. Therefore, a real-time processing method is often adopted, that is, the current mobile station's mobile speed is measured by the receiver, and different pilots and time division multiplexing formats between service signals are adopted according to different mobile speeds.

Furthermore, the mobile environment poses new problems for many other technologies of mobile communication systems. Such as power control. This is because the signal fading changes due to the user's mobility, so that at the same transmit power, the power received by the user's antenna constantly changes, and this is especially true for mobile communication systems, especially code division multiple access (CDMA) mobile communications. The system is extremely unfavorable. It reduces the average signal-to-interference ratio received by the user, which reduces the capacity advantage originally possessed by the CDMA system. Therefore, power control is especially important in CDMA systems. However, the current power control technology, because closed loop power control is used to track fast fading, there must be a certain processing delay and transmission delay, so power control always lags behind the current channel change. When the mobile station moves very fast, power control may not be able to track fast fading. At this time, the existence of power control will not only bring benefits to the system, but will worsen the performance of the system. Therefore, if the current moving speed of the mobile station can be measured and then the power control is determined based on this, it is undoubtedly a desirable method.

User mobility also brings similar problems to handovers. For a mobile user, determining whether it is handed over to another cell is achieved by measuring the signal strength of the base station in each cell. The strength of this signal is usually affected not only by its path loss and slow fading, but also by its fast fading. We hope that the measured signal can average out the effect of this fast fading, so as to accurately obtain the information required for handover. Therefore, it takes a certain time to average the rapid fluctuation of signal strength caused by fast fading. When the mobile station moves quickly, the average time required is shorter because the channel characteristics change rapidly; when the mobile station moves slowly, the measurement time required is correspondingly longer. On the other hand, when the mobile station moves fast, it must be measured faster Information, otherwise the mobile station may have moved away from the base station it is accessing and approached a neighboring interfering base station. Therefore, the measurement of Doppler frequency shift also helps to optimize the switching algorithm.

In addition, information on the moving speed of the mobile station will be useful in many other ways.

Current algorithms, such as obtaining Doppler frequency shifts by using phase information between multiple channels of data, have complex implementations, and are highly dependent on the modulation, spreading, and frame structure of the transmitted signal, which is not conducive to its widespread use. application. For example, in a communication system that provides data using the QPSK modulation format, the cross-product of the I component and the Q component of the QPSK signal can be used to know the relative phase of the two components, and the speed of the mobile station relative to the base station can be roughly determined (see US Patent US 07 / 981,034 "Pilot Carrier Dot Product Circuit"). This method can only roughly obtain the Doppler frequency shift size of the mobile station, and requires the QPSK modulation format, so the application range is very narrow. Summary of the Invention

An object of the present invention is to provide a Doppler frequency shift measurement method and a device thereof, which are simple and suitable for various spreading modes and modulation modes.

A further object of the present invention is to apply the Doppler frequency shift measurement technology to a mobile communication system, particularly to a channel estimation, power control, and handover technology of a CDMA mobile communication system, to improve system performance and system capacity. '

The method for measuring Doppler frequency shift applied in a digital mobile communication system provided by the present invention is implemented by obtaining an absolute value of a first derivative of a time-domain waveform of a channel that reflects the speed of a mobile station. Because the result of channel estimation is a continuous channel characteristic, it roughly reflects the change of the actual channel, and it implies the information of the mobile station's Doppler frequency shift. Therefore, the Doppler frequency shift measurement result can be obtained by finding the absolute value of the first derivative of the energy information or phase information obtained by channel estimation or a combination of the two.

Due to the influence of interference and noise, errors in channel estimation results, which affects many Spectral frequency shift measurement accuracy. Therefore, the information obtained by the channel estimation may be further filtered to smooth the channel characteristics and improve the measurement accuracy of Doppler frequency shift.

Because in a relatively short period of time, the channel change may still not completely reflect the average Doppler frequency shift at this moment. Therefore, the accumulation of the absolute value after the derivation for a period of time can be further averaged to obtain a higher Doppler frequency shift measurement accuracy.

In CDMA systems, multipath signals make it possible for Rake receivers. Rake technology can be used to obtain good diversity effects and significantly improve system performance. The present invention also uses the Rake technology to achieve similar diversity effects. That is, the size of a Doppler frequency shift is independently measured on each receiving branch of the Rake receiver according to the above method, and then the maximum ratio combining is performed according to the average received power of each branch to obtain the final Doppler frequency shift measurement. value.

According to the Doppler frequency shift measurement method provided by the present invention, the current moving speed of the mobile station can be measured, and power control can be adjusted accordingly, thereby avoiding the defect that power control cannot track fast fading when the mobile station moves faster. According to the method of the present invention, the time division multiplexing format between different pilots and service signals can be adjusted in real time according to the measured mobile station mobile speed, which solves the problems existing in the time division pilot channel estimation methods in the prior art. Inherent defect; According to the method of the present invention, the handover algorithm of the mobile user in the handover can also be optimized, that is, the measurement information calculated by the method of the present invention can reduce or eliminate the influence of fast fading, and complete the handover for the mobile station. Provide accurate information.

For the wireless communication system, especially the CDMA wireless communication system, the present invention has the simplicity of implementation incomparable with other methods, and the real-time and accuracy of measurement. The present invention can be used in channel estimation to adjust a channel estimation algorithm according to different Doppler frequency shifts, and can also be used in power control to adjust a power control algorithm according to different Doppler frequency shifts or to determine whether to turn off / on power control Loops, and other technologies and applications that require Doppler frequency shift size information and vehicle speed information.

The Doppler frequency shift measurement method provided by the present invention implements a simple order, and There is no dependency on the modulation method, spreading method, frame structure, and so on. This method can improve the performance of the receiver when applied to mobile communication systems, especially channel estimation, power control, and handover techniques in CDMA mobile communication systems, and ultimately increase the capacity of the system. Brief description of the drawings

Figure 1 is a schematic block diagram of a simple CDMA mobile communication system.

Fig. 2 is a block diagram of an implementation of a Doppler frequency shift measurement device according to the method of the present invention. Mode of Carrying Out the Invention

The present invention is described in detail below with reference to the drawings. Referring to FIG. 1, FIG. 1 shows a simple CDMA mobile communication system. The source generator 101 generates a signal, which is encoded by the encoder 102 and spread by the spreader 103. The spread spectrum signal is sent to the modulator 104 for IF and RF modulation, and the signal power is transmitted through the transmitting antenna 105. At the receiver end, the receiving antenna 106 receives the signal power, and then demodulate it into a baseband signal by the demodulator 107. The despreader 108 completes the despreading process, and the channel estimation circuit 109 completes the decoding and the source recovery circuit 110 needs the channel. It is estimated, and according to the difference between the received signal and the interference level, a power control command is generated, which is transmitted to the transmitting end via the power control loop 111 for power control.

The measurement of the Doppler frequency shift provided by the present invention is implemented by finding the absolute value of the first derivative of the time domain waveform of the channel. Although implemented in the present invention based on channel estimation, it is not limited to a specific channel estimation method. This is because no matter what kind of channel estimation method, the final result is to estimate a continuous channel characteristic, and this estimated channel characteristic at least roughly reflects the change of the actual channel. In the change of this characteristic, the information of the Doppler frequency shift of the mobile station is hidden.

First, the channel energy or phase information obtained by the channel estimation circuit 109 or both Some combination performs filtering by using a FIR (Finite Impulse Response) filter or an IIR (Infinite Impulse Response) filter to suppress errors in channel estimation caused by interference and noise, thereby affecting its accuracy. After filtering, the channel characteristics will become smooth. Through the analysis of the channel characteristics, we can know that when the mobile station moves fast, the channel characteristics change quickly, and the channel estimate value after the filter changes also fast; when the mobile station moves slowly, the reverse, after filtering The change of the channel estimation value behind the processor is also slower. The mathematical quantity that reflects the speed of this change is the absolute value of the first derivative of its time-domain waveform. Therefore, by using a differentiator widely existing in the prior art, and finding the absolute value of the output of the differentiator, a specific value reflecting the speed of the channel change can be obtained. Through theoretical analysis and computer simulation, we can know that this value has a linear relationship with the magnitude of the Doppler frequency shift, so this value reflects the relative magnitude of the Doppler frequency shift.

Although in most cases only such relative sizes are needed, such as in power control, channel estimation and handover techniques in CDMA systems. But getting its absolute size is easy. In order to further obtain the absolute magnitude of the Doppler frequency shift, computer simulation or field test can be used to measure the value of the mobile station when the mobile station's moving speed is known, to obtain a linear curve between the mobile station's moving speed and the value. Therefore, according to this curve, the absolute magnitude of the corresponding Doppler frequency shift can be obtained.

Although the obtained channel estimates can be filtered to obtain a relatively smooth and close-to-actual channel time-domain waveform, it is considered that in CDMA systems, interference and noise are common, and the fading of the signal amplitude follows an approximate Rayleigh fading Or Rice fading characteristics, in a relatively short period of time, the channel change may still not completely reflect the average Doppler frequency shift at this moment. Therefore, a cumulative average of the absolute values after derivation over a period of time is recommended for adoption. Considering that in a short period of time (such as 1-2 seconds), the mobile station's moving speed rarely changes much, and this period of time lasting about 1-2 seconds is the size of the cumulative average Doppler frequency shift It's long enough. Therefore, by e.g. 1-2 seconds accumulation It is necessary and feasible to measure the Doppler frequency shift on average to obtain higher measurement accuracy.

In CDMA systems, multipath signals make Rake receivers possible. Using Rake technology can get a very good diversity effect and significantly improve the performance of the system. In the present invention, the Rake technique is also adopted to obtain similar diversity effects. That is, each receiving branch of the Rake receiver is independently measured to obtain a Doppler frequency shift according to the above method, and then the maximum ratio combining is performed according to the average received power of each branch. The Doppler frequency shift estimated value thus obtained is The measured value of the final Doppler frequency shift provided by the present invention. Here, the average received power of each branch can be obtained in two ways: Through statistical analysis of different environments and the establishment of its channel model to obtain the power distribution of the signal at different time delays and spreads, the branches of the Rake receiver can be roughly estimated. The average power received by the channel, and the average power value is the system parameter under the environment, which is announced to each mobile station in the form of broadcast; or the received signals are measured in real time by each receiving branch of each mobile station receiver Power, and after a long time average (such as 5-10S) to remove the influence of fast fading on the received signal power, so as to obtain a more reliable average received power of each branch, and apply it to the maximum ratio combining as described above.

Through the above process, a simple and practical method for implementing Doppler frequency shift measurement in a mobile communication system, especially in a CDMA mobile communication system. In actual implementation, the absolute size of the maximum Doppler frequency shift that can be tolerated by different dedicated pilot structures can be obtained through computer simulation or field testing, so that the current Doppler frequency shift can be used to determine the current The dedicated pilot structure that should be used to obtain accurate and effective channel estimates; further, through simulation or field testing, the maximum Doppler frequency shift corresponding to the power control scheme given by the current system can be useful Therefore, according to the currently measured Doppler frequency shift, the power control method is changed or the power control is invalid when the Doppler frequency shift is large; the same method can also be applied to the switching algorithm, that is, through simulation or field measurement To obtain the best handover algorithm under different Doppler frequency shifts, and then determine the current handover algorithm that should be used according to the estimated value of the current Doppler frequency shift magnitude. These methods also do not require any substantial changes Can be applied to other related technologies.

The present invention provides a method for simple measurement of the Doppler frequency shift, and the method can be applied to mobile communication systems, in particular, it can be widely applied to multiple technologies in a CDMA mobile communication system. Although only three techniques are mentioned in the present invention: channel estimation, power control, and handover techniques, this does not exclude that the method for measuring Doppler frequency shift provided by the present invention is applied to other technical fields. . For example, the present invention only needs to be obtained through computer simulation or field test ^: The correspondence between the magnitude of the Doppler frequency shift measured by the method and the moving speed of the mobile station (this correspondence is roughly linear) can be used to obtain the measurement movement The speed of the movement of the stage, thereby generating applications related to it.

The implementation device of the present invention is described with reference to FIGS. 2 and 2. The signal amplitude and / or phase information obtained by the channel estimation circuit is filtered by a finite impulse response filter (FIR) or an infinite impulse response filter (IIR) 112 to obtain smoother channel information, which is passed through a differentiator 113 and processed The generator 114 obtains an absolute value, obtains information reflecting the magnitude of the Doppler frequency shift by measuring the absolute value of the first derivative of the channel information, and then passes the accumulator 115 and the divider 116 to obtain an average over a period of time (such as 1-2 seconds). Value, thereby increasing the accuracy of the measurement. For the Rake receiver, the measured Doppler frequency shift magnitude is then combined by the maximum ratio combiner 117 to measure the Doppler frequency shift magnitude of each -one Rake branch, so as to obtain the final multispectrum more accurately. Information on the magnitude of the frequency shift. It is obvious to those skilled in the art that according to the method disclosed in the present invention, there can be many ways to modify the disclosed invention, and in addition to the above-mentioned specific preferred modes, the present invention can also have many other embodiments. Therefore, any method or improvement that can be obtained according to the concept of the present invention shall fall within the protection scope of the claims of the present invention.

Claims

Claim

1. A method for measuring Doppler frequency shift applied in a digital mobile communication system, which is characterized by:

The measurement of the Doppler frequency shift is achieved by obtaining the absolute value of the first derivative of the channel time domain waveform obtained from the channel estimation.

2. The Doppler frequency shift measurement method according to claim 1, wherein: the channel time-domain waveform is obtained through channel estimation, and it can be energy information or phase information obtained through channel estimation or A combination of both.

3. The Doppler frequency shift measurement method according to claim 1 or 2, characterized in that: further filtering information obtained by channel estimation;

The absolute value of the first derivative of the filtered channel estimation information is used to obtain the measurement result of the Doppler frequency shift.

4. The Doppler frequency shift measurement method according to claim 3, wherein the filtering is a finite impulse response filtering (FIR) or an infinite impulse response filtering.

(IIR).

5. The Doppler frequency shift measurement method according to claim 1 or 2, further comprising: cumulating and averaging Doppler frequency shift values measured over a period of time to improve the Doppler frequency. Measurement accuracy of frequency shift.

6. The Doppler frequency shift measurement method according to claim 1 or 2, characterized in that: in the CDMA system, one Doppler frequency shift measurement value is independently measured on each receiving branch of multipath reception;

The maximum ratio combining is performed according to the average received power of each branch to obtain the final Doppler frequency shift measurement value.

7. The Doppler frequency shift measurement method according to claim 6, characterized in that The stated average received power is obtained by the following steps:

Through statistical analysis of the environment to establish its channel model to obtain the power distribution of signals with different time delays and spreads, so as to estimate the average received power of each branch;

The average received power value is broadcasted to each mobile station in the form of a system parameter.

8. The Doppler frequency shift measurement method according to claim 6, characterized in that the average received power is obtained by the following steps:

The received signal power is measured in real time by each receiving branch of each mobile station receiver; after a long time averaging, the average receiving power of each branch is obtained.

9. The Doppler frequency shift measurement method according to claim 1, wherein the method further comprises the following steps:

Filtering the energy information or phase information obtained by channel estimation or a combination of the two through a filter;

Find the absolute value of the first derivative of the filtered channel estimation information;

The accumulated absolute value over a period of time is averaged to obtain the measurement result of the Doppler frequency shift.

10. The Doppler frequency shift measurement method according to claim 9, characterized in that: in the CDMA system, one Doppler frequency shift measurement is obtained by independent measurement according to the method on each receiving branch of multipath reception. Value

11. The method for measuring Doppler frequency shift according to claim 1, characterized in that: when it is applied to channel estimation, the measured Doppler frequency shift value and a known Doppler frequency shift value can be further used. Make a comparison to determine the current dedicated pilot structure, where the known Doppler frequency shift value is the maximum Doppler frequency shift that can be tolerated by different dedicated pilot structures obtained through computer simulation or field testing. Absolute value.

12. The Doppler frequency shift measurement method according to claim 1, wherein: when it is applied to power control, the measured Doppler frequency shift value and a known Doppler frequency shift value can be further used. Make a comparison to change the current power control method or decide whether power control is required, where the known Doppler frequency shift value is a power control scheme given by the system obtained through computer simulation or field testing can have useful effects The maximum Doppler frequency shift corresponding to the time.

13. The Doppler frequency shift measurement method according to claim 1, wherein: when applied to handover, the measured Doppler frequency shift value and a known Doppler frequency shift are further used. The values are compared to determine the current switching algorithm. The known Doppler frequency shift value is the Doppler frequency shift corresponding to the best switching algorithm under different Doppler frequency shifts obtained through simulation or field measurement. value.

14. A Doppler frequency shift measurement device applied in a digital mobile communication system, which is characterized by:

The device includes a filter, a differentiator, an operator, an accumulator, a divider, and a maximum ratio combiner, wherein the channel information obtained from the channel estimation obtains smoother channel information through a filter, and the information is calculated by the differentiator. The first derivative calculates the absolute value of the operation result through an operator, calculates the average value of the operation result over a period of time through an accumulator and a divider, and obtains the final Doppler frequency shift size information through a maximum ratio combiner.

15. The Doppler frequency shift measurement device according to claim 14, wherein the filter is a finite impulse response filter (FIR) or an infinite impulse response filter (IIR).

16. The Doppler frequency shift measurement device according to claim 14, characterized in that: the maximum ratio combiner is to perform a maximum ratio d on the Doppler frequency shift measured by each branch of the RAKE receiver, and perform the maximum ratio. merge.