CN1391742A - Relative Velocity Estimation Using TPC Commands - Google Patents

Abstract

In a system having a transmitter and a receiver that communicate over a radio channel and in which the signal transmission power of the transmitter is adjusted to compensate for fading dips in the channel, the Rayleigh fading rate of the radio channel is estimated by effectively observing the adjustment or fluctuation of the signal transmission power or amplitude of the transmitter, thereby estimating the relative velocity between the transmitter and the receiver. This is accomplished, in accordance with the exemplary embodiments of this invention, by observing TPC (transmit power control) commands that cause the transmitter to adjust its signal transmit power in order to overcome Rayleigh fading, i.e., fading dips, in the radio channel. Since the TPC commands cause the transmitter to vary its signal transmit power to compensate for the fading recess, by observing the signal transmit power fluctuations represented by the TPC commands, the fading recess and fading rate, as well as the relative velocity between the transmitter and the receiver, can be determined.

Description

Relative Velocity Estimation Using TPC Commands

Background of the invention

field of invention

The present invention relates to wireless telecommunications, and in particular to estimating the velocity of a mobile station relative to a transmitter in a mobile communication system.

technical background

In mobile communication systems, it is often necessary to accurately estimate or model radio channels in order to enhance the performance of receivers that receive signals over the radio channels.

Figure 1 illustrates an exemplary radio channel known in the art. In particular, the transmitter 102 transmits a signal over a radio channel 104 , which is affected by AWGN (Additive White Gaussian Noise), indicated by the element 106 . The signal is then received at receiver 108 .

Radio channels are often affected by multipath propagation, which is caused when the path between the transmitter and receiver includes reflections from large objects such that the rays of the transmitted signal from the transmitter meet at the receiver Traveling along different paths with different lengths. Therefore, when the rays of the signal meet, there is a phase difference between these rays because of the different path lengths. This causes Rayleigh fading, where the rays or echoes of the transmitted signal are combined constructively or destructively, depending on their phase. Thus, since the combined signal received at the receiver is composed of many smaller rays or echoes of the transmitted signal that travel along different paths on the way to the receiver, the combined signal exhibits a change randomly. Therefore, even though the transmitter outputs the transmission signal at a constant power or amplitude, the amplitude or power of the transmission signal at the receiver varies due to Rayleigh fading.

The combined signal received by the receiver is usually modeled or represented as having a Rayleigh probability distribution, which is defined as: $P\left(r\right)=\frac{{\mathrm{re}}^{-r^2/2{\mathrm{the\; s}}^2}}{S^2}$

In other words, the Rayleigh distribution describes the distribution of signal components in a Rayleigh fading channel within an envelope about the RMS value of the signal sent from the transmitter to the receiver, with the line-of-sight path between the receiver and the transmitter blocked such that A condition in which a signal transmitted by a transmitter arrives at a receiver in a multipath manner, with different rays or components of the signal reflected along different paths to the receiver. The Rayleigh distribution describes the envelope because the in-phase and quadrature components of the signal are Gaussian in nature due to the arrival of multiple multipath components or rays of different phases. Therefore, the signal envelope, which is the square root of the sum of the squares of the in-phase and quadrature components, follows a Rayleigh distribution.

Whether the rays of the transmitted signal from the transmitter are combined constructively or destructively at the receiver depends on the particular location of the receiver and transmitter and obstructive/reflective objects in the environment surrounding them. Thus, if the receiver changes position relative to the transmitter and the environment, it will move through positions where the rays combine differently. In other words, as the receiver moves, the rays constructively constructively at the receiver in an alternating fashion and then combine destructively so that the receiver experiences a series of "fading dips" in which the amplitude of the received signal falls periodically below dips, then returns to its previous level. The frequency of these fading dips, or in other words the number of fading dips per unit of time, is often referred to as the "fade rate" of the radio channel.

The fading rate corresponds to the speed of the receiver relative to the transmitter, such that as the speed of the receiver increases, the fading rate also increases. It turns out that the fading rate is equivalent to the "Doppler spread" of a Rayleigh fading radio channel, which is defined as: $f_d=\frac{2\&Center\; Dot;V\&Center\; Dot;f_c}{c}$

where F _d = Doppler spread,

V = velocity of the receiver relative to the transmitter,

F _c = the frequency of the transmitted signal from the transmitter, and

c = speed of light (3×10 ⁸ m/s).

For example, if the speed of the receiver is 50 km/h, and the transmitted signal frequency is 2 GHz, the Doppler spread will be about 185 Hz. Therefore, the fading rate will be 185Hz, which means that at the receiver the amplitude of the transmitted signal will slope downward, or cyclically fade in amplitude and then recover, 185 times per second. Conversely, if the transmitted signal frequency and fading rate are known, this equation can be used to determine the relative velocity of the receiver.

The relative speed between the receiver and the transmitter is proportional to the bandwidth of the Rayleigh distribution, which in turn is related to the properties of the radio channel, such as the second moment statistics of the radio channel. The exchange or communication between transmitter and receiver can be greatly improved if the properties of the radio channel are known. In other words, the relative speed between the receiver and the transmitter determines or reflects the properties of the radio channel which are useful for enhancing the communication between the transmitter and the receiver. Therefore, it is important to accurately determine the relative velocity between a receiver and a transmitter, for example between a mobile station (MS) and a base station (BS) in a mobile communication network. The relative velocity between the receiver and the transmitter is usually estimated eg by studying the fading properties of the radio channel seen by the receiver according to the above-mentioned Doppler spread equation.

Due to the high chip rates in mobile communication systems using CDMA (Code Division Multiple Access), receivers in such systems are usually equipped with power controllers to reduce or overcome the effects of fading dips in the radio channel Influence. The power control of the signal broadcast by the transmitter is usually based on an estimate of the receiver's SIR (Signal to Interference Ratio). SIR is typically estimated by using a pilot technique, such as a pilot signal or channel, data, or a combination of pilot technique and data. The estimate of the SIR is used by the receiver to inform or instruct the transmitter to reduce or increase the power it uses to broadcast the signal to the receiver. Essentially, the transmitter varies the signal broadcast power to compensate for Rayleigh fading. Thus, the power or amplitude of the broadcast signal at the transmitter is varied such that the power or amplitude of the broadcast signal from the transmitter will be effectively constant at the receiver, thereby maintaining a constant SIR at the receiver.

FIG. 2 shows an exemplary procedure known in the art in which, as shown in step 202, the SIR is estimated using data and/or pilot techniques. From step 202 control passes to step 204 where the SIR is compared to a reference value. Control proceeds from step 204 to step 206 where, based on the comparison in step 204, a TPC command is formed and then sent to the transmitter, informing the transmitter of how it should change the power or power of the signal it is sending to the receiver. magnitude.

Communication between a receiver and a transmitter (eg, a mobile station and a base station) regarding control of the power of a signal sent from the transmitter is typically done using TPC commands.

FIG. 3 illustrates an exemplary structure of data transmitted in downlink from a network to a mobile station in a W-CDMA (Wideband-Code Division Multiple Access) system. Specifically, superframe 302 contains 72 frames such as frame 304 . Each frame contains 15 time slots such as time slot 306 . Each slot contains symbols, including pilot symbols and TPC symbols. The number of each symbol and the total number of all symbols in a slot depends on the spreading factor used for CDMA spreading. For example, as shown in slot 306 , each slot may contain a total of 20 symbols including 4 pilot symbols 308 and a single TPC symbol 310 .

The transmit power control rate applied to the transmitter limits the maximum rate of fading that the transmitter can effectively compensate for. For example, in W-CDMA (Wideband-Code Division Multiple Access), where the power control rate is 1500 Hz, when the mobile station moves at a speed less than 30 kilometers per hour, fading dips can be effectively overcome by using TPC commands. Power control generally operates most efficiently to overcome fading when the relative speed is low relative to the power control rate. This is because the power control rate determines how quickly the transmitter can react and change its broadcast power output to overcome fading dips at the receiver, and the relative speed between the receiver and transmitter determines how much fading dips will be at the receiver Show up quickly. As the relative speed increases and thus the fading rate increases, the power control rate necessary to effectively overcome the fading must also increase, in other words, if the power control rate is increased appropriately, the higher relative speed due to the receiver The fading rate can theoretically be effectively overcome by using TPC commands.

However, the relative velocity between the receiver and the transmitter is usually estimated by studying the fading properties of the radio channel seen by the receiver. Thus, while TPC commands are effectively used to overcome Rayleigh fading seen by the receiver, this technique for estimating relative velocity is inaccurate because the compensation removes the fading seen by the receiver. In other words, the inaccuracy of speed estimation increases with the effectiveness of power control. When the speed is low relative to the power control rate such that the power control effectively overcomes fading, the speed estimate will be inaccurate.

Therefore, there is a need to accurately estimate the relative velocity between a receiver and a transmitter, such as between a mobile station and a base station, while reducing or eliminating fading.

Summary of the invention

In accordance with an exemplary embodiment of the present invention, in a system having a transmitter and a receiver communicating over a radio channel and compensating for fading at the receiver, by effectively observing fluctuations in signal power or amplitude occurring at the transmitter, Instead of observing power or quality fluctuations of the received signal at the receiver, the Rayleigh fading rate or Doppler spread of the radio channel is estimated, and thus the relative velocity between the receiver and transmitter. This is done by observing TPC commands, or signal broadcast power changes represented by TPC commands, which are sent by the receiver to the transmitter to cause the transmitter to adjust its signal transmission power so that when the signal is received by the receiver , it will have effectively constant power level or signal quality.

Since the pattern of TPC commands (or the change in power they represent) corresponds to fading dips in the channel, the TPC commands can be observed over a period of time to determine how many fading dips occurred in the channel during that time. Therefore, the TPC commands, or the signal broadcast power changes represented by the TPC commands, can be used to accurately estimate the fading rate, as well as the relative velocity between the receiver and the transmitter by using the Doppler spreading technique described above for the fading rate . Such estimation is suitable when fading sags can be overcome by using TPC commands.

Brief description of the drawings

Other objects and advantages of the present invention will be apparent to those skilled in the art when reading the following detailed description of the preferred embodiments in conjunction with the accompanying drawings. The same elements in the figures are indicated by the same reference numerals.

Figure 1 shows radio communication according to the prior art.

Fig. 2 shows an exemplary transmit power control process according to the prior art.

FIG. 3 shows an exemplary structure of data transmitted in downlink from a network to a mobile station in a W-CDMA (Wireless-Code Division Multiple Access) system.

Figure 4 shows an example of a series of TCP commands corresponding to fading notches.

Fig. 5 shows a processing procedure according to an exemplary embodiment of the present invention.

Description of preferred embodiments

In accordance with an exemplary embodiment of the present invention, in a system having a transmitter and a receiver communicating over a radio channel, and wherein signal transmission power or amplitude from the transmitter is adjusted to compensate for fading dips in the channel, by Effectively observing adjustments or fluctuations in the transmitter's signal transmission power or amplitude, estimating the Rayleigh fading rate of the radio channel and thus estimating the relative velocity between the receiver and the transmitter.

The receiver controls and adjusts the transmit power of the transmitter to compensate for Rayleigh fading, ie fading dips in the radio channel, such that constant signal power or quality is provided at the receiver. For example, the receiver may send TPC (Transmit Power Control) commands to the transmitter so that the transmitter adjusts its signal transmit power to overcome fading notches. Since transmit power fluctuations correspond to and represent fading dips in the channel, the fade rate (fading dips per unit time) or Doppler spread can be determined by observing the TPC commands. The fading rate or Doppler spread can then be used to accurately estimate the relative velocity between the receiver and transmitter by using the Doppler spreading technique described above with respect to fading rate.

Figure 4 shows an example of a series of TPC commands and corresponding signal transmit powers (representing fading notches). The x-axis is a timeline with binary TPC commands ("u"=up, "d"=down) which cause the transmitter to increase or decrease its signal transmission power. The signal transmission power is represented on the Y-axis. In this example, a slot structure such as that shown in Figure 3 is used, where each slot sent by the receiver to the transmitter includes a TPC command. The frequency of TPC commands may be eg 1500 Hz in WCDMA systems. Example signal transmit power increments and other example TPC command frequencies are well known in the art and so will not be described in detail here.

As seen in time period A, the TPC command instructs the transmitter to effectively maintain a steady state signal transmission power. During time period B, the overall effect is an increase in signal transmission power, then during time period C, the TPC command sent by the receiver to the transmitter causes the transmitter to reduce the signal transmission power to a steady state level. During time period D, the TPC command instructs the transmitter to effectively maintain a steady state signal transmission power. Thus, Figure 4 shows a single fading dip that occurs in the channel during the time period spanning time periods B and C.

A bit of noise or a change in the rate of change in signal power during a notch is also seen in the middle of period B, where one of the TPC commands causes the signal transmit power to decrease momentarily before the next TPC command causes the signal transmit power to continue increasing . Additionally, different rates of change in signal power must be represented by using a mix of "up" and "down" binary TPC commands. For example, the TPC sequence up-up-up-down-up-up-up-down represents a higher rate of change than the TPC sequence up-up-down-up-up-down. Those skilled in the art will appreciate that well-known signal processing techniques can readily be used to accurately identify fading dips that correspond to changes in signal transmit power over time, regardless of signals caused by noise or other sources other than fading dips. Fluctuations in transmit power.

FIG. 5 illustrates a method of estimating a relative velocity between a receiver and a transmitter using TPC commands according to an exemplary embodiment of the present invention. As represented in step 502, a TPC command is first observed. From step 502 control passes to step 504 where a plurality of signal transmission power level fluctuations corresponding to fading dips in the radio channel between the receiver and the transmitter are identified based on the observed TPC commands. The time frame in which the identified fluctuation occurs is also identified to determine the rate or frequency of the identified fluctuation. This rate is the estimated fade rate or Doppler spread.

From step 504 control passes to step 506 where the velocity of the receiver relative to the transmitter is estimated based on the estimated fading rate or Doppler spread. Specifically, the velocity can be estimated by substituting the estimated fading rate and the known carrier frequency of the signal sent by the transmitter into the Doppler spread equation described above.

From step 506 control passes to step 508 where the estimated velocity of the receiver relative to the transmitter is used to estimate or characterize properties of the radio channel. From step 508 control passes to step 510 where the properties of the radio channel are used to maintain or enhance the quality of communications between the transmitter and receiver over the radio channel. Control passes from step 510 to step 502, where the loop repeats.

According to an exemplary embodiment of the present invention, observing and analyzing TPC commands to determine the fading rate and the relative velocity between the mobile station and the base station may be performed within the mobile station. According to another exemplary embodiment of the present invention, observing and analyzing the TPC commands to determine the fading rate and the relative velocity between the receiver and the transmitter may be performed within the receiver. In other words, the analysis and determination may be performed at any suitable location within, or connected to, the communication system.

Those skilled in the art will recognize that a mobile station can function as both a transmitter and a receiver, and that a base station can also function as both a transmitter and a receiver, since communications between the mobile station and the base station Usually it goes both ways. Thus, the principles described here can be applied variously and reciprocally to mobile stations and base stations, as well as other transceivers.

Those skilled in the art will also recognize that the present invention can be applied to systems having a different or wider set of TPC commands available, including, for example, ) TPC commands.

Other mechanisms representing signal transmission power fluctuations at the transmitter corresponding to fading dips may also be used or observed to determine fading dips or fade rates in the radio channel, and the relative velocity between the receiver and the transmitter.

Those skilled in the art will also appreciate that the present invention may be embodied in other specific forms without departing from the spirit or essential characteristics of the invention and that the invention is not limited to the specific exemplary embodiments described herein. The presently disclosed exemplary embodiments are therefore to be considered in all respects as illustrative and not restrictive. The scope of the present invention is indicated by the appended claims rather than the above-mentioned description, and all changes which fall within the meaning and scope of the present invention and equivalents thereof are intended to be embraced in the present invention.

Claims (10)

1. A method for estimating the relative velocity between a receiver and a transmitter in a Rayleigh fading radio channel, comprising the steps of: Observe the TPC (Transmit Power Control) commands that instruct the transmitter to vary its signal transmit power to overcome Rayleigh fading in the radio channel; Estimate the Rayleigh fading rate of the radio channel from the observed TPC commands; and Based on the estimated fading rate, the velocity of the receiver relative to the transmitter is estimated. 2. The method of claim 1, wherein the steps of observing TPC commands over a period of time, and estimating the Rayleigh fading rate comprise the steps of: identifying TCP command sequences corresponding to fading dips in the channel; and The Rayleigh fading rate is estimated from the number of fading dips represented by the identified TCP command sequence occurring during the time period. 3. The method of claim 1, wherein the transmitter is a mobile station in a mobile communication network. 4. The method of claim 1, wherein the transmitter is a base station in a mobile communication network. 5. The method of claim 1, wherein the receiver is a mobile station in a mobile communication network. 6. The method of claim 1, wherein the receiver is a base station in a mobile communication network. 7. The method of claim 1, further comprising the steps of: Characterize the properties of the radio channel in terms of the estimated relative velocity; and Depending on the nature of the radio channel being characterized, the quality of the radio communication between the transmitter and the receiver is enhanced. 8. The method of claim 1, wherein the steps are performed by the receiver. 9. A method for estimating the relative velocity between a receiver and a transmitter in a Rayleigh fading radio channel, comprising the steps of: observe fluctuations in the transmission power of signals transmitted by the transmitter; determine which fluctuations correspond to fading dips in the radio channel; Estimating the Rayleigh fading rate of the radio channel based on the determined fading dips; and Based on the estimated Rayleigh fading rate, the velocity of the receiver relative to the transmitter is estimated. 10. The method of claim 1, wherein the fluctuation is observed indirectly by observing TPC (Transmit Power Control) commands instructing the transmitter to vary its signal transmit power.

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