CN102118762A - One-way coverage scheme of wireless communication network for high-speed railway - Google Patents

Abstract

The invention discloses a one-way coverage scheme of a wireless communication network for a high-speed railway. A mode combining base band units (BBUs) and radio remote units (RRUs) is adopted; the BBUs and RRUs are arranged along the high-speed railway in a linear coverage manner; one BBU corresponds to a logic cell, is connected with multiple RRUs through optical fibers and covers the logic cell by use of the RRUs; and the RRU in each logic cell adopts one-way coverage, i.e. the RRU transmits signals toward one direction and the transmission direction is consistent. According to the invention, the environmental characteristics of the high-speed railway are taken into full consideration, and the proposed one-way coverage scheme can effectively reduce the influence caused by the Doppler effect during high-speed movement, reduce the workload of the system compensating the Doppler frequency offset, and lower the complexity in channel estimation, thereby providing a guarantee for reliable communication.

Description

The unidirectional covering scheme of a kind of cordless communication network at high-speed railway

Technical field

The invention belongs to the mobile communication technology field, relate to a kind of cordless communication network and cover scheme at high-speed railway.Specifically, the high-speed railway cordless communication network covers the performance that design for scheme directly influences whole network, need take into full account the influence that high-speed mobile is brought, for reliable communication provides safeguard.

Background technology

In recent years, the high-speed railway of China was greatly developed, and mileage number and speed have all reached new height.At present, the average speed per hour of national high-speed railway is 200-300km/h.The communication of high-speed railway has following characteristics: (1) new type train compartment closure is good, and speed per hour is fast, and signal attenuation is big; (2) user moving speed is fast, and Doppler effect is obvious; (3) covering is wire, on the main high-speed railway of concentrating; (4) telephone traffic is concentrated relatively, and train is through out-of-date traffic burst.

Be the big problem of solution signal attenuation, and many at present employing baseband processing units (Base Band Unit, BBU)+(use of RRU has strengthened signal quality to Remote Radio Unit, has enlarged cell range for RadioRemote Unit, RRU) pattern.

Doppler effect is wave source and observer when relative motion is arranged, and the observer receives frequency and the phenomenon inequality that wave frequency and wave source send.If f _cBe carrier frequency (Hz) that v is user moving speed (m/s), c is the light velocity (m/s), and θ (t) is the angle of arrival bearing and user's moving direction, f _dFor the variation of frequency is a maximum doppler frequency, then have

User's translational speed is fast more, and the Doppler frequency shift of generation is big more, needs covering scheme reasonable in design to reduce the expansion of Doppler frequency.

For solving the frequency offset issues that Doppler effect brings, need carry out frequency offset estimating, thereby adopt effective Doppler frequency deviation compensation technique.Here introduce frequency offset estimation technique based on Cyclic Prefix.

To under the stable condition, use N arbitrarily _pExpression arrives the multipath number of receiving terminal, Φ _N0Be initial phase, z _n(t) be the angle of arrival of n paths, the phase place in n path of receiving terminal can be provided by following formula:

Φ(t)＝Φ _n0+2πΔftcosz _n(t)

Wherein Δ f is the Doppler frequency skew.

According to the Jakes model, energy that Doppler frequency spectrum showed and the decline of band limit are:

$S\left(f_n\right)=\frac{{\left|h_n\right|}^2}{{\mathrm{\πf}}_D}\frac{1}{\sqrt{1-{(f_n/\mathrm{\Δf})}^2}},$ |f _n|≤Δf

Under the mobile cellular environment, ofdm signal is through a frequency selective fading and time selective fading channels, and channel has comprised practical channel impulse response and transmission filter

Therefore, the compound baseband signal r (k) that receives is subjected to the influence of multipath and additive white Gaussian noise, has following form on discrete sampling point:

$r\left(k\right)=\underset{n=1}{\overset{N_p}{\mathrm{\Σ}}}{h}_n\left(k\right)z(k-{\mathrm{\τ}}_n)+n\left(k\right)=r_i\left(k\right)+{\mathrm{jr}}_q\left(k\right)=(x\left(k\right)+n_i\left(k\right))+j(y\left(k\right)+n_q\left(k\right))$

Wherein z (k) sends signal, and x (k) and y (k) are respectively in the same way and quadrature component; N (k)=n _i(k)+jn _q(k) represent compound white Gaussian noise, its power is E[n _i ²(k)]=E[n _q ²(k)]=N ₀B/2.At receiving terminal, send the average power E[x of signal ²(k)]=E[y ²(k)]=σ ²/ 2.Additive noise signal n (k) is separate with transmission signal z (k).By central-limit theorem, the auto-correlation function of received signal can be expressed as:

$C_{r_i{r}_i}\left(\mathrm{\&tau;}\right)=C_{r_q{r}_q}\left(\mathrm{\&tau;}\right)=E\left[r_i\left(k\right)r_i(k+\mathrm{\&tau;})\right]=\frac{{\mathrm{\&sigma;}}^2{J}_0\left(2\mathrm{\&pi;\&Delta;f\&tau;}\right)}{2}+\frac{{\mathrm{<figure-callout\; id="0"\; label="N"\; filenames="HSA00000464976000022.png"\; state="\{\{state\}\}">N</figure-callout>}}_0\mathrm{B\&delta;}\left(\mathrm{\&tau;}\right)}{2}$

A complete OFDM symbol is made up of useful signal and cyclic prefix CP, and the length that is provided with information is T _b=NT _s, the length of CP is T _g=LT _s, the Doppler frequency deviation that can utilize CP to carry out ofdm system is estimated.Suppose to obtain snr value comparatively accurately, then Δ f can estimate with following formula:

$\widehat{\mathrm{\&rho;}}\left({\mathrm{NT}}_s\right)=\frac{1}{1+N_{0}B/{\mathrm{\&sigma;}}^2}{J}_0\left(2\mathrm{\&pi;\&Delta;f}(N+L)T_s\right)+\frac{1}{1+{\mathrm{<figure-callout\; id="0"\; label="N"\; filenames="HSA00000464976000022.png"\; state="\{\{state\}\}">N</figure-callout>}}_0{\mathrm{\&sigma;}}^2/B}\mathrm{\&delta;}\left({\mathrm{NT}}_s\right)$

Thereby draw corresponding maximum Doppler frequency offset f _dValue, finish frequency offset estimating.

But along with the raising of user moving speed, Doppler effect is more and more serious, and maximum frequency deviation increases, and existing compensate of frequency deviation technology is formed challenge.On the other hand, along with the raising of speed, the time of leaving base station processing frequency deviation for is less and less, needs covering scheme reasonable in design to reduce the complexity of compensate of frequency deviation algorithm.

Building mobile communications network needs in advance channel to be made rational estimation, the algorithm of channel estimating has least-squares estimation (Least Square at present, LS), linear minimum mean-squared error estimate (Linear Minimum Mean Square Error, LMMSE), based on the channel estimating of basic extended model etc.Wherein under the high velocity environment, many employings are based on the channel estimation methods of basic extended model.This method is approached the channel variation curve by the linear weighted function of orthogonal basis, and concrete grammar is as follows:

By nyquist sampling theorem as can be known, if the bandwidth of primary signal is W, then want to recover primary signal, sampling frequency is at least 2W undistortedly.If the duration of signal is T, then the dimension of signal is defined as N=2WT.The energy of real signal can be provided by following formula:

${\mathrm{\&epsiv;}}_x={\&Integral;}_{-\&infin;}^{+\&infin;}{x}^2\left(t\right)\mathrm{dt}={\&Integral;}_{-\&infin;}^{+\&infin;}{\left|X\left(f\right)\right|}^2\mathrm{df}$

Consider the signal that time domain is limited and frequency band is concentrated, establishing signal duration is [T/2, T/2], and most of concentration of energy only is less than η ε in bandwidth [W, W] _xBe in outside the band, that is:

$\frac{1}{{\mathrm{\&epsiv;}}_x}{\&Integral;}_{-\&infin;}^{+\&infin;}{\left|X\left(f\right)\right|}^2\mathrm{df}\&GreaterEqual;1-\mathrm{\&eta;}$

Know by dimension theorem, have N orthogonal signalling { φ _j(t), 1≤j≤N}, wherein t ∈ [T/2, T/2] satisfies:

By top dimension theorem, we can use N mutually orthogonal signal psi _j(t) linear combination comes approximate representation x (t), thereby realizes channel estimating.Make and carry out channel estimating in this way and can reduce on the one hand and estimate the noise component(s) introduced, thereby can reduce the bigger data volume of number of pilots transmission of use on the other hand.But user's high-speed mobile can be brought huge channel evaluation data, therefore needs covering scheme reasonable in design to reduce the complexity of channel estimating.

The present invention takes into full account above factor, proposes the unidirectional covering scheme of a kind of cordless communication network at high-speed railway, guarantees the performance of mobile communication system.

Summary of the invention

The covering scheme that the purpose of this invention is to provide a kind of cordless communication network at high-speed railway is used for guaranteeing mobile subscriber's communication under the high-speed railway environment.

For achieving the above object, the invention provides the unidirectional covering scheme of a kind of cordless communication network at high-speed railway, comprise following content:

1. adopt the pattern of baseband processing unit (BBU)+Remote Radio Unit (RRU), BBU and RRU are linear covering along high speed railway construction.

2. the corresponding logic district of BBU links to each other with a plurality of RRU by optical fiber, utilizes RRU that this logic district is covered;

3. the RRU in the logic district adopts unidirectional covering, and promptly RRU only transmits towards a direction, and the transmit direction unanimity.

The covering scheme that the present invention proposes has following advantage:

1. build BBU+RRU along the railway, can carry out linear covering effectively, the use of RRU can effectively enlarge cell range;

2. unidirectional covering can effectively reduce the spread spectrum that Doppler effect brings;

3. unidirectional covering can effectively reduce the complexity of channel estimation method;

4. unidirectional covering can effectively reduce the complexity of Doppler frequency deviation backoff algorithm;

Description of drawings

Fig. 1 is the unidirectional covering scenario-frame of a high-speed mobile communications network of the present invention schematic diagram.

Fig. 2 is the unidirectional covering scheme of a present invention RRU part enlarged drawing.

Fig. 3 is a Doppler effect schematic diagram under the two-way coverage condition.

Fig. 4 is that Doppler frequency shift changes schematic diagram under the two-way coverage condition.

Fig. 5 is a Doppler effect schematic diagram under the unidirectional coverage condition.

Fig. 6 is that Doppler frequency shift changes schematic diagram under the unidirectional coverage condition.

Embodiment

For making technical scheme of the present invention and advantage clearer, the present invention is described in further detail below in conjunction with drawings and the specific embodiments.

Referring to shown in Figure 1, the present invention adopts the pattern of baseband processing unit (BBU)+RF processing unit (RRU), and BBU and RRU are linear covering along high speed railway construction.The corresponding logic district of BBU utilizes a plurality of RRU that this logic district is covered.The limited coverage area of BBU itself, but cell range is expanded by the effect of RRU.

Referring to shown in Figure 2, the interior RRU of logic district adopts unidirectional covering, and promptly RRU only transmits towards a direction, and the transmit direction unanimity.

Below by the variation of Doppler effect under more two-way covering and the unidirectional coverage condition, the advantage of the unidirectional covering scheme of labor.

Doppler effect is meant the phenomenon that the wave frequency that with respect to the motion of propagation medium the observer received because of wave source or observer changes.The variation of Doppler frequency shift can be provided by following formula:

f _d(t)＝f _maxcosθ(t)

F wherein _dBe maximum doppler frequency, f _Max=f _c.v/c; f _cBe carrier frequency (Hz); V is train speed (m/s), and c is the light velocity (m/s).

Referring to shown in Figure 3, under the two-way coverage condition, angle θ (t) is calculated by following formula:

$\cos \left(\mathrm{\&theta;}\left(t\right)\right)=\frac{d_0-\mathrm{vt}}{\sqrt{{d_B}^2+{(d_0-\mathrm{vt})}^2}},0\&le;t\&le;{2\mathrm{<figure-callout\; id="0"\; label="d"\; filenames="HSA00000464976000022.png"\; state="\{\{state\}\}">d</figure-callout>}}_0/v$

D wherein ₀Be the initial distance that train is ordered to O, O is the intersection point that the vertical line of rail is arrived in the base station, d _BBe the distance of base station to rail.

By f=f _c+ f _d(t) can get the frequency variation curve that influenced by Doppler effect under two coverage conditions, referring to shown in Figure 4, the frequency under the two-way coverage condition is increase and decrease on the separator bar that with BBU is basic point, and promptly frequency is at preceding half section relative f _cIncrease, at relative f of second half section _cReduce, total amplitude of variation is:

$\mathrm{\&Delta;f}=2{\mathrm{<figure-callout\; id="0"\; label="f\; c\; vd"\; filenames="HSA00000464976000022.png"\; state="\{\{state\}\}">f</figure-callout>}}_c{\mathrm{vd}}_{}{}_0/(c\&CenterDot;\sqrt{{{\mathrm{<figure-callout\; id="0"\; label="d"\; filenames="HSA00000464976000022.png"\; state="\{\{state\}\}">d</figure-callout>}}_0}^2+{d_B}^2})$

Referring to shown in Figure 5, under the unidirectional coverage condition, the initial distance that train is ordered to O is d ₀', satisfy d ₀＜d ₀'≤2d ₀Then the computing formula of angle θ (t) becomes:

$\cos \left(\mathrm{\&theta;}\left(t\right)\right)=\frac{{{\mathrm{<figure-callout\; id="0"\; label="d"\; filenames="HSA00000464976000022.png"\; state="\{\{state\}\}">d</figure-callout>}}_0}^{\&prime;}-\mathrm{vt}}{\sqrt{{d_B}^2+{({{\mathrm{<figure-callout\; id="0"\; label="d"\; filenames="HSA00000464976000022.png"\; state="\{\{state\}\}">d</figure-callout>}}_0}^{\&prime;}-\mathrm{vt})}^2}},0\&le;t\&le;{{\mathrm{<figure-callout\; id="0"\; label="d"\; filenames="HSA00000464976000022.png"\; state="\{\{state\}\}">d</figure-callout>}}_0}^{\&prime;}/v$

Can get the frequency variation curve that influenced by Doppler effect under the unidirectional coverage condition, referring to shown in Figure 6, the frequency under the unidirectional coverage condition is monotone variation on the separator bar that with BBU is basic point, and amplitude of variation is:

${\mathrm{\&Delta;f}}^{\&prime;}={\mathrm{<figure-callout\; id="0"\; label="f\; c\; vd"\; filenames="HSA00000464976000022.png"\; state="\{\{state\}\}">f</figure-callout>}}_c{{\mathrm{vd}}_{}}^{}{{}_0}^{\&prime;}/(c\&CenterDot;\sqrt{{{\mathrm{<figure-callout\; id="0"\; label="d"\; filenames="HSA00000464976000022.png"\; state="\{\{state\}\}">d</figure-callout>}}_0}^{\&prime;2}+{d_B}^2})$

Δ f＞Δ f ' is obviously arranged, and promptly unidirectional covering can effectively reduce the spread spectrum that Doppler effect brings.On the other hand, subtract when increasing during frequency during two-way covering, and the variation of unidirectional coverage condition lower frequency is dull (is frequency always increase or always reduce) that therefore unidirectional covering scheme more helps the realization of compensate of frequency deviation, helps to simplify the compensate of frequency deviation algorithm.

To bandwidth is W, and the duration is the signal of T, when channel estimating, can approach the channel variation curve by the linear weighted function of orthogonal basis, and the complexity of this method of estimation is proportional to the signal dimension

Because the unidirectional covering scheme that the present invention proposes can effectively reduce the spread spectrum that Doppler effect causes, thereby reduces the signal dimension, reduce the complexity of channel estimating.

The Cyclic Prefix that utilizes the OFDM symbol is when skew is estimated to Doppler frequency, and the length of Cyclic Prefix has certain influence to the value of frequency deviation.When value one timing of frequency deviation, unidirectional covering scheme can be selected short Cyclic Prefix, makes the frequency offset estimating algorithm obtain simplifying.

To sum up, the unidirectional covering scheme of the cordless communication network at high-speed railway that the present invention proposes takes into full account the environmental quality of high-speed railway, the spread spectrum that Doppler effect brings when effectively reducing high-speed mobile, reduce the complexity of Doppler frequency deviation backoff algorithm and channel estimation method, for reliable communication provides safeguard.

Claims (2)

1. the unidirectional covering scheme of the cordless communication network at high-speed railway is characterized in that, adopts the pattern of baseband processing unit (BBU)+RF processing unit (RRU), and BBU and RRU are linear covering along high speed railway construction; The corresponding logic district of BBU links to each other with a plurality of RRU by optical fiber, utilizes RRU that this logic district is covered.

2. method according to claim 1 is characterized in that, the RRU in the logic district adopts unidirectional covering, and promptly RRU only transmits towards a direction, and the transmit direction unanimity.

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