CN1190976C - Dynamic time interval structure regulating method in channel estimation - Google Patents

Abstract

The present invention provides a method for dynamically adjusting the time slot structure of channel estimation in mobile communication, which adaptively adjusts the pilot symbols in the time slots, and adjusts the pilot symbols in each time slot according to the measured moving speed of the mobile station quantity. When the measured moving speed of the mobile station increases, the number of pilot symbols in each time slot is increased; and when the measured moving speed of the mobile station decreases, the number of pilot symbols in each time slot is decreased. The present invention can maximize channel utilization and simultaneously maximize the maximum allowable moving speed.

Description

A Dynamic Adjustment Method of Slot Structure in Channel Estimation

The invention relates to a channel estimation method in mobile communication, in particular to a dynamic adjustment method for a time slot structure in channel estimation, which adaptively adjusts the pilot symbols in the time slot.

The mobile communication transmission signal is sent out after encoding, interleaving, spreading, and modulation. Due to the complexity of the land receiving environment on the receiving side, the multipath characteristics of signal propagation are caused. The receiving station performs a search for each path signal After demodulation and despreading, each transmitted symbol is estimated, combined by Rake, and provided to the deinterleaving and decoding module. The signal will be "randomly tuned" by the channel during channel propagation:

$\mathrm{<span\; class="notranslate">\; r</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; =</span>\underset{\mathrm{<span\; class="notranslate">\; l</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 1</span>}}{\overset{\mathrm{<span\; class="notranslate">\; L</span>}}{\mathrm{<span\; class="notranslate">\; \&Sigma;</span>}}}{\mathrm{<span\; class="notranslate">\; a</span>}}_{\mathrm{<span\; class="notranslate">\; l</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; )</span>{\mathrm{<span\; class="notranslate">\; e</span>}}^{<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; <figure-callout\; id="2"\; label="j"\; filenames="C0010070000111.png"\; state="\{\{state\}\}">j</figure-callout></span>}\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; \&pi;</span>}{\mathrm{<span\; class="notranslate">\; f</span>}}_{\mathrm{<span\; class="notranslate">\; t</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; \&tau;</span>}}_{\mathrm{<span\; class="notranslate">\; l</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; )</span>}\mathrm{<span\; class="notranslate">\; the\; s</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; \&tau;</span>}}_{\mathrm{<span\; class="notranslate">\; l</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; )</span>$

Among them, τ ^l t and ^an t represent the delay and fading factor of the nth path respectively, both of which are random.

Therefore, estimating the channel parameters τ ^l t and ^an t at each data symbol is a necessary prerequisite for the receiver to decipher the transmitted data. Under the time-division channel structure, it is a key technology in mobile communication to estimate the channel parameters of the data part through the pilot value at the pilot symbol.

Generally speaking, the land mobile channel is a kind of multipath channel. Due to the different arrival delays of the signals of each path at the receiver, the vector summation of the received signals causes different degrees of fading of the received signal strength. When the mobile station moves relative to the base station at a certain speed, the received signal has a certain fading rate; on the other hand, it also causes the so-called Doppler effect. The Doppler effect causes Doppler frequency shift to the transmitted signal, causing the received signal In the communication system based on phase modulation, the data of the receiver and the transmitter are inconsistent. Therefore, a certain channel estimation algorithm is used to compensate for these phase differences, so that the transmitted data can be restored correctly. However, under the current time-division pilot structure, as shown in FIG. 1 , although the positions of the pilot symbols in the time slot structure are different: either they are placed at the beginning of a time slot, or they are placed somewhere in the middle. But the number of pilot symbols is fixed: either 2 or 4. Under this fixed time slot structure, when the speed of the mobile station reaches a certain speed, Doppler frequency shift and Rayleigh fading will occur, which makes channel estimation face insurmountable difficulties. The following takes QPSK modulation as an example for specific analysis.

QPSK is four-phase keying modulation, which is to modulate the transmitted signal to π/4, 3π/4, 5π/4, 7π/4, and the receiver can recover the transmitted data by recovering the phase of the transmitted signal through the channel estimation method . Under the time-division pilot structure, the received data is demodulated by determining the phase difference of the data symbols in each time slot relative to the pilot symbols of the time slot. In this case, it is generally only possible to ensure accurate recovery of sending information at low speeds. Assume that the time interval between pilot symbols (ie, the duration of data symbols in a slot) is T. Below we give the analysis from two angles:

1. The impact of the Doppler effect caused by the moving speed

In a communication system based on phase modulation, due to the inconsistency of the receiver and transmitter data due to channel random modulation, a certain channel estimation algorithm is used to compensate for these phase differences in order to recover the sent data correctly. Obviously within T, the channel phase change θ cannot exceed π (because θ has ambiguity at this time: θ and 2*π-θ, generally we take θ∈[0, π]), let the frequency shift along the transmitting direction is ω _d , then:

ω _d *T<π

Then: ω _d < π/T

For example: if the duration of each time slot is: 0.01/15 seconds, there are 40 symbols in total, 4 of which are pilot symbols, then T=0.01/15*36/40=0.0006 seconds, assuming that the transmission carrier frequency is 2GHz (Wavelength is: $\mathrm{\&lambda;}=\frac{\mathrm{<figure-callout\; id="2"\; label="c"\; filenames="C0010070000111.png"\; state="\{\{state\}\}">c</figure-callout>}}{2*{10}^9}=3/20,$ c is the speed of light: 300,000,000m/s), then requires:

_ωd <π/5.925926*10 ⁴

That is, the speed of the mobile station does not exceed:

Vmax＝ _ωd *λ(2*π)＝3/(40*5.925926)*10 ⁴ ＝126.5625m/s＝455.625km/h

2. The influence of moving speed on fading rate

The average fading rate of the mobile channel can be given by the following formula:

$\mathrm{<span\; class="notranslate">\; A</span>}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; v</span>}}{\mathrm{<span\; class="notranslate">\; \&lambda;</span>}<span\; class="notranslate">\; /</span>\mathrm{<span\; class="notranslate">\; 2</span>}}$

Among them: v represents the moving speed of the mobile station (m/s), and λ represents the carrier wavelength of the transmitted signal (m).

If a fading occurs within T, it will bring great difficulties to channel estimation, and on the other hand, it will also have a very adverse effect on power control. It can be seen that T should satisfy:

T＜λ/(2*v)

Namely: v＜λ/(2*T) (1)

In the foregoing example, T=0.6 milliseconds, then the speed of the mobile station is required not to exceed: 125 m/s, ie: 450 km/h.

From the above two angles, when the speed of the mobile station exceeds 450 km/h, accurate channel estimation is impossible. Of course, all of these are estimates without considering the influence of noise. Computer simulation shows that when the signal-to-noise ratio is 5dB, the speed can only reach about 150km/h when the bit error rate is less than 15% under the condition of no coding and interleaving. See Figure 2. In addition, using the pilot information of several adjacent time slots to perform channel estimation on the data part of a time slot can improve the speed of the mobile station to a certain extent, but generally it cannot exceed 400km/h.

The purpose of the present invention is to effectively overcome the problems in the channel estimation caused by the Doppler frequency shift and Rayleigh fading of the mobile station under high-speed mobile conditions by using a simple channel estimation method, improve the accuracy of wireless reception, and be able to The mobile station can still give accurate channel estimation at a relatively high speed.

The present invention can maximize the utilization rate of the channel under the time-division pilot structure, and at the same time maximize the maximum allowable moving speed. The dynamic change of pilot information density in a time slot can adaptively ensure the good communication of the mobile station at different land moving speeds.

The present invention is realized by the following scheme, the dynamic adjustment method of the time slot structure in the channel estimation of the present invention, adaptively adjusts the pilot symbol in the time slot, comprises the following steps:

(a) measuring the moving speed of the mobile station;

(b) When the measured moving speed of the mobile station increases, increase the ratio of pilot symbols in all symbols in each time slot, and reduce the pilot symbols in each time slot when the measured moving speed of the mobile station decreases Ratios among all symbols;

(c) According to the measured moving speed of the mobile station, the number and distribution position of the pilot symbols in each time slot are adjusted.

In the above step (c), the adjustment process to the number of pilot symbols in each time slot is: when the moving speed of the measured mobile station increases, increase the number of pilot symbols in each time slot; When the moving speed of the mobile station decreases, the number of pilot symbols in each slot is reduced.

In the above step (c), the adjustment to the distribution of pilot symbols in each time slot is to distribute multiple groups of pilot symbols composed of multiple pilot symbols at intervals of certain data symbols in each time slot.

When the measured moving speed of the mobile station increases, the data symbols between the above-mentioned multiple groups of pilot symbols are reduced; when the measured moving speed of the mobile station decreases, the data symbols between the above-mentioned multiple groups of pilot symbols are increased .

The above-mentioned increase and decrease of the number of pilot symbols is a continuous value.

When the measured speed of the mobile station is lower than 450 kmh, the number of pilot symbols in one slot is 10% of all symbols.

When the measured speed of the mobile station is above 450 km/h and below 1000 km/h, the number of pilot symbols in one slot is 20% of all symbols.

The present invention will be further described by way of embodiments below in conjunction with the accompanying drawings.

Fig. 1 is the existing time-division pilot structure;

Figure 2 is the bit error rate at different mobile station speeds;

Fig. 3 is an example of the slot diagram when the content of pilot symbols is 10%;

Figure 4 is an example of a slot diagram when the pilot symbol content is 20%

From the above formula (1), it can be found that:

1. The speed V _max can be increased by increasing the wavelength of the carrier frequency, but because the carrier frequency cannot be increased infinitely and the resources are limited, the carrier frequency is generally fixed.

2. On the premise that the carrier frequency remains unchanged, the speed can be increased by reducing the pilot symbol interval time T. V _max For example: when taking T=T'/k, V _max '=K*V _max can be obtained. Therefore, under the condition that the carrier frequency remains unchanged, there are two ways to achieve the purpose of reducing the interval time T of the pilot symbols:

(1) Increase the chip rate. Increasing the chip rate can reduce the time for sending a time slot, thereby reducing the pilot interval time T. However, due to reasons such as hardware, the chip rate cannot be increased indefinitely, and the increase of the chip rate will cause an increase in intersymbol interference.

(2) Increase the code rate of pilot symbols: that is, the method of reducing the interval time of pilot symbols by increasing the number of pilot symbols in one time slot. In specific implementation, the pilot symbols may be evenly distributed in a time slot, or the method of inserting the pilot symbols into time slots in blocks may be used for denoising.

The basic idea of the present invention is based on the state of the channel, specifically by detecting the approximate speed of the mobile station relative to the base station (wherein the detection of the speed does not need to be too precise, because the speed change of the mobile station is continuous, it can be regularly and intermittently detection), adaptively adjust the code rate of pilot symbols, and adjust the code rate of service data accordingly, because we assume that the total chip rate is fixed (ie: the duration of a slot, each The number of symbols contained in a slot is fixed). When the detected mobile station speed is high, the code rate of pilot symbols is increased accordingly, and the service code rate of transmission is also reduced (that is, the number of pilot symbols is increased in the time slot, and the number of service data symbols is reduced); Increasing the channel information density is beneficial to the receiver's channel estimation and improves the receiving accuracy. Conversely, when the detected speed of the mobile station is low, the pilot symbol rate can be appropriately reduced (ie: reduce the number of pilot symbols in the time slot, increase the number of service symbols), increase the service rate, and thus increase the utilization of the channel . In a word, by dynamically adjusting the transmission rate of pilot symbols and service data, the maximum allowable speed V _max can be maximized, and at the same time, the utilization rate of the wireless frequency band can be maximized.

For example: when the receiver detects that the bit error rate caused by the high moving speed of the mobile station is too high, consider an extreme situation at this time, and take the pilot symbol equal to the data symbol in a time slot (that is, the data symbol before each data symbol have a pilot symbol), in the above example there is:

T＝0.01/(15*20) seconds

The maximum moving speed of the mobile station can reach 2250 m/s, that is, 8100 km/h, so it can fundamentally solve the difficulty in channel estimation caused by the land movement of the mobile station.

During implementation, the pilot symbol rate is adaptively adjusted according to the detected moving speed, and at an appropriate position, such as the frame header information part of each frame, the transmitted time slot structure type (which can be continuous or Discretely changing), at the receiving side, data demodulation is performed according to the state of the received time slot structure.

Provide concrete embodiment respectively below:

(1) Discrete variable time slot structure type

Several types of time slot structure may be given first, and which type of time slot structure to adopt is determined by the detected moving speed of the mobile station.

(a) When the content of pilot symbols in the time slot is 10%, as shown in Figure 3, take 40 symbols per time slot as an example, assuming that in each time slot, the first 2 symbols and the last 2 symbols are pilot symbols, and the middle 36 symbols are data symbols, that is, the content of pilot symbols is 10%. According to the above formula (1), when the duration Ts=0.01/15 (second) of a time slot, the time interval T=36/40*Ts between pilot symbols, under the situation of ω _dmax =π/T,

_Vmax ＝ _ωdmax *3/20*1/2π

   = 3/20*1/2T = 3/(40*T) = 125 m/s = 450 km/h

Therefore, when the speed is <450 km/h, 10% of the pilot symbol types are used.

(b) When the content of pilot symbols in the time slot is 20%, as shown in Figure 4, for 40 symbols in each time slot, it is assumed that in each time slot, the first 4 symbols and the middle 4 symbols are For the pilot symbols, the 16 symbols between the two groups of pilot symbols are data symbols, that is, the content of the pilot symbols is 20%. At this time, the time slot cycle T=16/40*Ts=16/40*0.01/15, f _dmax =π/T. According to the above formula (1)

Vmax＝f _dmax *3/20*1/2π＝281.25 m/s＝1012.5 km/h

Therefore, when the speed of the mobile station is lower than 1000 km/h, a structure in which the content of the pilot symbols is 20% of the pilot symbols can be adopted.

In the case of (a) and (b) above, for noise removal, four symbols per block are used, and each block is evenly distributed in the time slot.

(2) Continuously changing slot structure type

According to the detected moving speed of the mobile station, the content of the pilot symbols is changed "continuously". The number of pilot symbols can be any number from 1 to (total number of symbols-1), according to the How much, adopt the method that the pilot symbol is divided into blocks (the size of a block can be one symbol) and evenly inserted into the whole time slot to form a time slot structure.

Detection of moving speed of mobile station

The moving speed of the mobile station can be roughly estimated by the variation speed of each path delay given by the multipath search and tracking module, and the present invention does not require an accurate estimation of the moving speed. And when the speed of the mobile station is overestimated, it will not cause the increase of the receiving bit error rate, so an estimate of the maximum moving speed of the mobile station can be given.

It should be noted that the indication of the state change of the time slot structure can be sent according to the signal-to-noise ratio of the received signal or the bit error rate given by the decoding module. Because when the signal-to-noise ratio is large, increasing the number of pilot symbols can increase the estimation accuracy of the channel parameters at the pilot symbols. The bit error rate given by the decoding module indicates the severity of the channel, and increasing the number of pilot symbols can improve the accuracy of channel estimation, thereby improving the reception effect.

Of course, the receiver can also make a request to the sender through the upper layer protocol as needed, requiring the sender to send a frame of a certain type of time slot structure, but the frame header information of all frames must indicate the type of time slot structure of the frame, so that correct demodulation at the receiver.

Claims (6)

1. a dynamic adjustment method of time slot structure in channel estimation, the pilot symbol in the adaptive adjustment time slot is characterized in that, comprises the following steps: (a) measuring the moving speed of the mobile station; (b) When the measured moving speed of the mobile station increases, increase the ratio of pilot symbols in all symbols in each time slot, and decrease the pilot frequency in each time slot when the measured moving speed of the mobile station decreases The ratio of symbols among all symbols; (c) Adjust the number and distribution position of the pilot symbols in each time slot according to the measured moving speed of the mobile station. 2. the dynamic adjustment method of time slot structure in the channel estimation according to claim 1, it is characterized in that, in above-mentioned step (c), the adjustment process to pilot symbol quantity in each time slot is: when measured When the moving speed of the mobile station increases, the number of pilot symbols in each time slot is increased; and when the measured moving speed of the mobile station decreases, the number of pilot symbols in each time slot is decreased. 3. the dynamic adjustment method of time slot structure in the channel estimation according to claim 1, it is characterized in that, in above-mentioned step (c), the adjustment to the pilot symbol distribution in each time slot is in each time slot In the slot, multiple groups of pilot symbols composed of multiple pilot symbols are distributed at intervals of certain data symbols. 4. The method for dynamically adjusting time slot structure in channel estimation according to claim 3, characterized in that, when the moving speed of the measured mobile station increases, the data symbols between the above-mentioned multiple groups of pilot symbols are reduced; When the measured moving speed of the mobile station decreases, the data symbols between the plurality of sets of pilot symbols are increased. 5. The method for dynamically adjusting the time slot structure in channel estimation according to claim 2, characterized in that the increase and decrease of the number of pilot symbols is a continuous value. 6. the dynamic adjustment method of time slot structure in the channel estimation according to claim 1 is characterized in that, when the speed of the mobile station of measurement is lower than 450 kilometers/hour, the quantity of pilot symbol in a time slot is all 10% of the sign.

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