CN102647226A - Method for carrier frequency offset compensation of receiving signals under high-speed rail environment - Google Patents

Abstract

A method for the carrier frequency offset compensation of receiving signals under high-speed rail environment comprises the steps of obtaining the position information of a current train by a high-speed rail train, calculating the horizontal distance between the train and a base station; calculating a carrier frequency offset value fd needing to be compensated currently; and conducting direct carrier frequency offset compensation on the signals received by a receiver of the high-speed rail train. According to the method, the carrier frequency offset value can be calculated without channel estimation, and the simplicity and the high efficiency are achieved.

Description

Carrier frequency offset compensation method for received signal in high-speed railway environment

The technical field is as follows:

the invention relates to the technical field of wireless communication, in particular to a method for compensating carrier frequency offset of a received signal by a high-speed railway train when a base station performs downlink communication to the high-speed railway train in a high-speed railway environment.

Background art:

after the railway of China is greatly accelerated for a plurality of times, the running speed of the train is faster and faster. The speed of the high-speed railway train which is running at present reaches and exceeds 350km/h, the requirements on the type and quality of communication service in a high-speed environment are higher and higher, and the requirements on railway wireless communication are undoubtedly more severe.

In high-speed mobile communication, doppler shift is one of important factors affecting system performance. When the electromagnetic wave emission source and the receiver move relatively, the received propagation frequency changes. When the speed of motion reaches a certain threshold, a significant change in the transmission frequency is caused, which is referred to as doppler shift. In a multipath environment, different doppler shifts occur in the spectral structure of the received signal, resulting in doppler spread. When the doppler spread is larger than the signal bandwidth, time-selective fading occurs in the channel, which greatly affects the error performance of the system.

At present, there are two main methods for solving the doppler shift problem: compensation methods and feedback methods. In the former, a receiver calculates a frequency offset value by adopting channel estimation, and then compensation of frequency offset is carried out by using an equalization or synchronization method without the help of a control channel; in the latter, the receiver feeds back the estimated frequency deviation to the transmitting end, and the transmitting end automatically adjusts the frequency of the transmitted signal. However, these methods require accurate channel estimation, and it is very difficult to achieve fast channel estimation in the case that the receiver has a large moving rate and the channel is in fast fading.

The invention content is as follows:

aiming at the problems in the prior art, the invention aims to provide a method for carrying out carrier frequency offset compensation on a received signal under a high-speed railway, which can quickly estimate a frequency offset value and solve Doppler frequency shift caused by high-speed movement of a train in a direct compensation mode of a receiver.

The main idea of the invention is: because the high-speed rail train can acquire the position information of the current train through the technologies of GPS positioning, wheel track positioning and the like, the carrier frequency offset value needing to be compensated currently can be calculated by using the position information instead of a channel estimation mode, and the real-time performance of carrier frequency offset compensation is ensured.

According to the above idea, the present invention adopts the following technical solutions:

a method for carrying out carrier frequency offset compensation on a received signal under a high-speed railway is characterized by comprising the following specific steps:

(1) in a high-speed railway communication system, a high-speed train can acquire the position information of the current train through technologies such as GPS positioning or wheel track positioning. When the high-speed train receives the downlink communication sent by the base stationIth symbol y of data_iThen, according to the position information of the current train, the horizontal distance d between the train and the base station at the moment is calculated_i；

(2) Calculating the carrier frequency offset f to be compensated_dThe calculation formula is:

<math> <mrow> <msub> <mi>f</mi> <mi>d</mi> </msub> <mo>=</mo> <mi>f</mi> <mo>&CenterDot;</mo> <mfrac> <mi>v</mi> <mi>c</mi> </mfrac> <mo>&CenterDot;</mo> <mfrac> <msub> <mi>d</mi> <mi>i</mi> </msub> <msqrt> <msubsup> <mi>d</mi> <mi>i</mi> <mn>2</mn> </msubsup> <mo>+</mo> <msubsup> <mi>h</mi> <mi>i</mi> <mn>2</mn> </msubsup> </msqrt> </mfrac> <mo>-</mo> <mo>-</mo> <mo>-</mo> <mrow> <mo>(</mo> <mn>1</mn> <mo>)</mo> </mrow> </mrow> </math>

wherein f represents the central frequency point of the received signal, v represents the running speed of the current train, and d_iRepresents the horizontal distance, h, of the train from the base station_iIndicating the altitude of the current base station and c the speed of light. When the train running direction faces the base station, v takes a positive value, the carrier frequency shift is positive at the moment, the receiving frequency is increased, when the train running direction faces away from the base station, v takes a negative value, the carrier frequency shift is negative at the moment, and the receiving frequency is decreased;

(3) according to the obtained carrier frequency offset value f_dAnd the receiver of the high-speed rail train performs direct carrier frequency offset compensation on the received signals:

<math> <mrow> <msub> <mover> <mi>s</mi> <mo>^</mo> </mover> <mi>i</mi> </msub> <mo>=</mo> <msub> <mi>s</mi> <mi>i</mi> </msub> <mo>&CenterDot;</mo> <msup> <mi>e</mi> <mrow> <mo>-</mo> <mi>j</mi> <mn>2</mn> <mi>&pi;</mi> <msub> <mi>f</mi> <mi>d</mi> </msub> <msub> <mi>T</mi> <mi>s</mi> </msub> </mrow> </msup> <mo>-</mo> <mo>-</mo> <mo>-</mo> <mrow> <mo>(</mo> <mn>2</mn> <mo>)</mo> </mrow> </mrow> </math>

wherein,representing the received sequence, s, after direct compensation_iRepresenting the received sequence before direct compensation, f_dIndicating the current carrier frequency offset value, T_sRepresenting the sampling period of the received signal.

Compared with the traditional carrier frequency offset compensation method, the method can calculate the carrier frequency offset value without channel estimation, has simple and efficient calculation method, and can obtain parameters required by calculation, such as train position information and the current running speed of the train, in real time at the train end, so that the carrier frequency offset compensation method can obtain good performance even in a high-speed moving environment.

Description of the drawings:

fig. 1 is a schematic diagram of carrier frequency offset calculation from a base station to a high-speed train.

The specific implementation mode is as follows:

the invention is further described with reference to the following figures and specific embodiments. The present embodiment is implemented on the premise of the technical solution of the present invention, and a detailed implementation manner and a specific operation process are given, but the protection scope of the present invention is not limited to the following embodiments.

As shown in fig. 1, the method for compensating carrier frequency offset in a high-speed railway environment of the present invention specifically includes the following steps:

(1) in a high-speed railway communication system, a high-speed train acquires the position information of the current train through technologies such as GPS positioning or wheel track positioning, and when the high-speed train receives the ith symbol y of downlink communication data sent by a base station_iThen, according to the position information of the current train, calculating the horizontal distance d between the train and the base station at the moment_i。

Horizontal distance d between train and base station_iThe calculation method of (2) is different due to different positioning methods of the high-speed train, and in this embodiment, only two cases of obtaining the position information of the current train by using the GPS positioning method and the wheel track positioning method are listed, and other various positioning methods are not described in detail here.

If d is calculated by GPS positioning technology_iFirstly, the train acquires the current three-dimensional coordinate (x) through the GPS equipment_i，y_i，z_i) The coordinates are the current train position information, and then are combined with the three-dimensional coordinates (x ') of the base station stored in the train receiver in advance'_i，y′_i，z′_i) Calculating the horizontal distance between the train and the base station at the moment according to the position information of the high-speed train

<math> <mrow> <msub> <mi>d</mi> <mi>i</mi> </msub> <mo>=</mo> <msqrt> <msup> <mrow> <mo>(</mo> <msubsup> <mi>x</mi> <mi>i</mi> <mo>&prime;</mo> </msubsup> <mo>-</mo> <msub> <mi>x</mi> <mi>i</mi> </msub> <mo>)</mo> </mrow> <mn>2</mn> </msup> <mo>+</mo> <msup> <mrow> <mo>(</mo> <msubsup> <mi>y</mi> <mi>i</mi> <mo>&prime;</mo> </msubsup> <mo>-</mo> <msub> <mi>y</mi> <mi>i</mi> </msub> <mo>)</mo> </mrow> <mn>2</mn> </msup> <mo>+</mo> <msup> <mrow> <mo>(</mo> <msubsup> <mi>z</mi> <mi>i</mi> <mo>&prime;</mo> </msubsup> <mo>-</mo> <msub> <mi>z</mi> <mi>i</mi> </msub> <mo>)</mo> </mrow> <mn>2</mn> </msup> </msqrt> <mo>;</mo> </mrow> </math>

If d is calculated by the wheel-track positioning technique_iFirstly, the train obtains the current train running distance l through the wheel track positioning equipment_iThe distance is the position information of the current train, and then the lookup table (d) stored on the train is searched in sequence_i，l_i) Obtaining the horizontal distance d between the train and the base station_i；

(2) Calculating the carrier frequency offset f to be compensated_dThe calculation formula is:

<math> <mrow> <msub> <mi>f</mi> <mi>d</mi> </msub> <mo>=</mo> <mi>f</mi> <mo>&CenterDot;</mo> <mfrac> <mi>v</mi> <mi>c</mi> </mfrac> <mo>&CenterDot;</mo> <mfrac> <msub> <mi>d</mi> <mi>i</mi> </msub> <msqrt> <msubsup> <mi>d</mi> <mi>i</mi> <mn>2</mn> </msubsup> <mo>+</mo> <msubsup> <mi>h</mi> <mi>i</mi> <mn>2</mn> </msubsup> </msqrt> </mfrac> <mo>-</mo> <mo>-</mo> <mo>-</mo> <mrow> <mo>(</mo> <mn>1</mn> <mo>)</mo> </mrow> </mrow> </math>

wherein f denotes the received signalCenter frequency point, v represents the current running speed of the train, d_iRepresents the horizontal distance, h, of the train from the base station_iIndicating the altitude of the current base station. When the train running direction faces the base station, v takes a positive value, the carrier frequency shift is positive at the moment, the receiving frequency is increased, when the train running direction faces away from the base station, v takes a negative value, the carrier frequency shift is negative at the moment, and the receiving frequency is decreased;

(3) according to the obtained carrier frequency offset value f_dAnd the train receiver performs direct carrier frequency offset compensation on the received signals:

<math> <mrow> <msub> <mover> <mi>s</mi> <mo>^</mo> </mover> <mi>i</mi> </msub> <mo>=</mo> <msub> <mi>s</mi> <mi>i</mi> </msub> <mo>&CenterDot;</mo> <msup> <mi>e</mi> <mrow> <mo>-</mo> <mi>j</mi> <mn>2</mn> <mi>&pi;</mi> <msub> <mi>f</mi> <mi>d</mi> </msub> <msub> <mi>T</mi> <mi>s</mi> </msub> </mrow> </msup> <mo>-</mo> <mo>-</mo> <mo>-</mo> <mrow> <mo>(</mo> <mn>2</mn> <mo>)</mo> </mrow> </mrow> </math>

wherein,

representing the received sequence, s, after direct compensation_iRepresenting the received sequence before direct compensation, f_dIndicating the current carrier frequency offset value, T_sRepresenting the sampling period of the received signal.

Claims (1)

1. A method for carrying out carrier frequency offset compensation on a received signal under a high-speed railway is characterized by comprising the following specific steps:

(1) in the high-speed railway communication system, the high-speed train obtains the position information of the current train, and when the high-speed train receives the ith symbol y of the downlink communication data sent by the current base station_iThen, according to the position information of the current train, calculating the horizontal distance d between the train and the base station at the moment_i；

(2) Calculating the carrier frequency offset f to be compensated_dThe calculation formula is：

<math> <mrow> <msub> <mi>f</mi> <mi>d</mi> </msub> <mo>=</mo> <mi>f</mi> <mo>&CenterDot;</mo> <mfrac> <mi>v</mi> <mi>c</mi> </mfrac> <mo>&CenterDot;</mo> <mfrac> <msub> <mi>d</mi> <mi>i</mi> </msub> <msqrt> <msubsup> <mi>d</mi> <mi>i</mi> <mn>2</mn> </msubsup> <mo>+</mo> <msubsup> <mi>h</mi> <mi>i</mi> <mn>2</mn> </msubsup> </msqrt> </mfrac> <mo>-</mo> <mo>-</mo> <mo>-</mo> <mrow> <mo>(</mo> <mn>1</mn> <mo>)</mo> </mrow> </mrow> </math>

Wherein f represents the central frequency point of the received signal, v represents the running speed of the current train, and d_iRepresents the horizontal distance, h, of the train from the base station_iIndicating the altitude of the current base station and c the speed of light. When the train running direction faces the base station, v takes a positive value, the carrier frequency shift is positive at the moment, the receiving frequency is increased, when the train running direction faces away from the base station, v takes a negative value, the carrier frequency shift is negative at the moment, and the receiving frequency is decreased;

(3) according to the obtained carrier frequency offset value f_dAnd the receiver of the high-speed rail train performs direct carrier frequency offset compensation on the received signals:

<math> <mrow> <msub> <mover> <mi>s</mi> <mo>^</mo> </mover> <mi>i</mi> </msub> <mo>=</mo> <msub> <mi>s</mi> <mi>i</mi> </msub> <mo>&CenterDot;</mo> <msup> <mi>e</mi> <mrow> <mo>-</mo> <mi>j</mi> <mn>2</mn> <mi>&pi;</mi> <msub> <mi>f</mi> <mi>d</mi> </msub> <msub> <mi>T</mi> <mi>s</mi> </msub> </mrow> </msup> <mo>-</mo> <mo>-</mo> <mo>-</mo> <mrow> <mo>(</mo> <mn>2</mn> <mo>)</mo> </mrow> </mrow> </math>

wherein,representing the received sequence, s, after direct compensation_iRepresenting the received sequence before direct compensation, f_dIndicating the current carrier frequency offset value, T_sRepresenting the sampling period of the received signal.

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