CN102647386B - Timely aligning method for use in multi-point cooperation OFDM (Orthogonal Frequency Division Multiplexing) system based on sub-band pre-coding - Google Patents

Abstract

The invention discloses a timely aligning method for use in a multi-point cooperation OFDM (Orthogonal Frequency Division Multiplexing) system based on sub-band pre-coding. The method comprises the following steps of: after a user receiving end intercepts an OFDM symbol, feeding an estimated channel state information molecular band back to a base station end; feeding back the average transmission time delay difference among moments from different base stations to each user; selecting users with closer time delay differences in each sub-band based on information of the transmission time delay difference at the base station end for performing data transmission; and performing phase rotation on data of different users in a frequency domain to compensate for the transmission time delay difference among different base stations, correct the transmission time delay difference and enhance the relevance among subcarriers. According to the method, remarkable system and rate gains can be obtained only by increasing a small calculation amount and feedback information.

Description

Based on timing alignment schemes in the multipoint cooperative ofdm system of subband precoding

Technical field

The invention belongs to and belong to wireless communication technology field, particularly a kind of based on timing alignment schemes in the multipoint cooperative ofdm system of subband precoding.

Background technology

In order to improve the service quality of Cell Edge User, reduce the interference of adjacent base station, one of cooperation between base stations key technology becoming future mobile communication system.

At present, 3GPP defines two kinds of coordinated multipoint transmission schemes: collaborative wave beam forming CB (Coordinated Beamforming) and Combined Treatment JP (Joint Processing).Wherein, the user data in CB scheme is transmitted by single cooperative base station, and between base station, combined dispatching reduces the interference between different districts, can obtain spatial multiplexing gain; User data in JP scheme, by multiple cooperative base station joint transmission, can obtain diversity gain.The present invention is directed to the multipoint cooperative transmission system of JP scheme.

Show with the research of combining precoding the multi-subscriber dispatching in arrowband falt fading channel, multipoint cooperative can obtain higher spatial multiplex gains and multi-user diversity gain.For frequency selective fading channels, channel can be changed into multiple parallel narrow band channel by OFDM, the most direct method carries out multi-subscriber dispatching and precoding respectively to every sub-channels, thus the processing method of narrow band channel is directly expanded in ofdm system, but each like this available subcarrier is wanted feedback channel information, will be done user scheduling and precoding respectively, and feedback quantity and computational complexity are directly proportional to available subcarrier number.

At real system as in LTE and WiMax, in order to reduce feedback quantity and computational complexity, according to the frequency domain correlation of channel, usually many for OFDM adjacent subcarriers are formed a subband, each subband only feeds back an average channel condition information, also carries out multi-subscriber dispatching and precoding by subband in base station end.But in frequency selective fading channels, all sub-carrier channels declines in subband are also incomplete same.Especially in the multipoint cooperative system of JP scheme, different base station is different to the propagation delay time of user, the delay inequality of minizone can be introduced, expand the equivalent multi-path delay spread that user experiences, thus the correlation that reduce further in subband between subcarrier, limit basic handling unit-subband-width.

The patent " compensating delay difference among cells in cooperative multi-point transmission system " of Xian Electronics Science and Technology University, the motion R1-091502 of Fujitsu " Pseudo Transmission Timing Control using Cyclic Shift for Downlink CoMP Joint Transmission " and the motion R1-090193 of Datang Telecom " Aspects of Joint Processing in Downlink CoMP " all give solution to the delay inequality problem in multipoint cooperative transmission system between different base station, but to suggest plans be all ofdm system in units of each subcarrier, for based on the user scheduling in the ofdm system of subband and delay inequality Correction Problems, do not provide concrete solution.

Summary of the invention

In order to overcome above-mentioned the deficiencies in the prior art, the object of the present invention is to provide a kind of based on timing alignment schemes in the multipoint cooperative ofdm system of subband precoding, to carrying out timing alignment based on the transmission symbol in the multipoint cooperative ofdm system of subband precoding between different base station, thus improve the frequency domain correlation of system, improve the rate gain of system.

To achieve these goals, the technical solution used in the present invention is:

Based on timing alignment schemes in the multipoint cooperative ofdm system of subband precoding, comprise the steps:

Step one, when multiple base station cooperates, user k the n-th received over subcarriers to signal be:

$y_{\mathrm{kn}}=\left(h_{\mathrm{kn}}{w}_{\mathrm{kl}}\right)x_{\mathrm{kn}}+\underset{i\∈U_l}{\underset{i\≠k}{\mathrm{\Σ}}}\left(h_{\mathrm{kn}}{w}_{\mathrm{il}}\right)x_{\mathrm{in}}+z_{\mathrm{kn}}$

Wherein: subscript l represents that subcarrier n belongs to l subband, h _knrepresent that base station is to the channel vector of user k on subcarrier n, w _klrepresent the beamforming vector of user k on l subband, x _knrepresent the data sending to user k on the n-th subcarrier, U _lchoose the user's set carrying out data transmission, w at l subband _il, represent the beamforming vector of other users, x _in(i ≠ k) represents the transmission data of other users, z _knrepresent additive white Gaussian noise;

Step 2, user side intercepts OFDM symbol, carries out channel estimating and obtains channel condition information and by subband feedback, feed back the mean transit delay difference τ of different base station to user simultaneously _k;

Step 3, base station end receives the channel condition information of user feedback and the mean transit delay difference τ of different base station _kafter, in each subband, by different user grouping, between the channel of the interior user of different group, correlation meets wherein user k, i belongs to different user's groups respectively, α is a constant determined according to orthogonality requirement, then form user from the top-quality user of selective channel in each group and collect U, then in U, select the actual user carrying out transmitting according to delay inequality, the interval that the delay inequality of selected user is formed is requirement the user selected further set is designated as U _l, to the transmission data acquisition ZF precoding choosing user, wherein the precoding vector of each user and the channel of other user mutually orthogonal, namely ${\tilde{h}}_{{\mathrm{ic}}_l}{w}_{\mathrm{kl}}=0,$ $\∀i\≠k,i\∈U_l;$

Step 4, base station end is poor according to the mean transit delay of the different base station of user feedback, sends after carrying out delay compensation to the data after the precoding of selected user.

In described step 2, user k intercepts OFDM symbol, then through FFT conversion, utilizes pilot frequency information to estimate to obtain channel status h by least-squares algorithm and least-mean-square error algorithm _kn.

Described by subband feedback, the average channel condition information by subcarriers all in subband is fed back, and specifically comprises average channel directional information and average channel quality information, and average channel direction is average channel quality wherein b represents the number of sub carrier wave in subband l, for reducing receiving terminal amount of calculation, adopts approximate: c _l=lB+ (B-1)/2, represent the channel status at subband center subcarrier place.

Wherein, for reducing receiving terminal amount of calculation, in the channel vector approximation subband at employing subband center subcarrier place during average channel vector, when transmitting terminal does delay compensation, should ensure that the channel vector of subband center is constant.

In described step 4, delay compensation realizes by adopting frequency domain phase rotating.

Compared with prior art, advantage of the present invention gives solution to the multi-subscriber dispatching adopted in the ofdm system of subband precoding and base station time delay difference Correction Problems, and relatively every subcarrier feedback reduces feedback overhead.

Accompanying drawing explanation

Fig. 1 is the multipoint cooperative downlink broadcast system schematic of two base stations, a K single-antenna subscriber.

Fig. 2 is realization flow schematic diagram of the present invention.

Fig. 3 is the present invention's system achievable rate analogous diagram when two cell cooperative transmission.

Embodiment

Below in conjunction with drawings and Examples, the present invention is described in further details.

Figure 1 shows the multipoint cooperative downlink broadcast system of two base stations, a K single-antenna subscriber, suppose that each base station has M/2 antenna, the antenna serial number of base station 1,2 is respectively (1,2,, M/2) and (M/2+1 ... M) wherein, K, M are positive integer and K is more than or equal to M usually, and system adopts the OFDM mode of N point FFT, and the useful subcarrier number wherein transmitting data is G, and being divided into L subband, every B continuous subcarrier is divided into a subband.

Different sub carrier in each subband adopts unified precoding vector, and the signal that the n-th subcarrier sends is:

$X_n=\underset{k\∈U_l}{\mathrm{\Σ}}{w}_{\mathrm{kl}}{x}_{\mathrm{kn}},$ lB≤n≤lB+B-1 (1)

Wherein U _lchoose the user's set carrying out data transmission, w at l subband _kl=(w _kl1, w _kl2... w _klM) ^tbe l subband correspond to user k M × 1 tie up precoding vector and || w _kl||=1, x _knthe data sending to a kth user on the n-th subcarrier.

Easy in order to describe below, belong to l subband if not specified otherwise then gives tacit consent to the n-th subcarrier.

A kth user the n-th received over subcarriers to signal be y _kn:

$y_{\mathrm{kn}}=\left(h_{\mathrm{kn}}{w}_{\mathrm{kl}}\right)x_{\mathrm{kn}}+\underset{i\∈U_l}{\underset{i\≠k}{\mathrm{\Σ}}}\left(h_{\mathrm{kn}}{w}_{\mathrm{il}}\right)x_{\mathrm{in}}+z_{\mathrm{kn}}---\left(2\right)$

Wherein Section 1 is the useful signal of transmission, and Section 2 is the interference of other users, and Section 3 is channel white Gaussian noise.H _kn=(h _kn1, h _kn2... h _knM) be channel response in the n-th subcarrier base station to user k, z _knfor additive white Gaussian noise, z _knobedience zero-mean, variance are multiple Gaussian Profile.

After user k intercepts OFDM symbol, through FFT conversion, estimate to obtain channel vector h _kn, then by subband feedback channel condition information (CSI), by the average channel vector feedback in each subband to base station, in reality, often have following formula to set up:

$\frac{1}{B}\underset{n=\mathrm{lB}}{\overset{\mathrm{lB}+B-1}{\mathrm{\Σ}}}{h}_{\mathrm{kn}}\≈h_{{\mathrm{kc}}_l}---\left(3\right)$

Wherein, c _l=lB+ (B-1)/2 represents the central subcarrier of subband, and concrete feedback quantity comprises channel direction information (CDI) and channel quality information (CQI), and the sub-band averaging channel direction of feedback is sub-band averaging channel quality is in addition, user also feeds back mean transit delay difference τ between two cooperative base station _k=τ _2k-τ _1k, wherein τ _1k, τ _2krepresent that base station 1, base station 2 are to the average delay of user k respectively.

Base station, according to the channel condition information of each subband feedback, adopts accurate orthogonal users to dispatch and ZF precoding (SUS-ZFBF) algorithm.For each subband, from K user, selective channel standard is orthogonal, channel quality better and the user that base-station transmission delay inequality is close, and note user set is U _l, U _lin channel relevancy between any two user k and i very little, namely space correlation coefficient meets: wherein α is a very little normal number.To the data acquisition ZF precoding that the user that chooses sends, wherein the precoding vector of each user and the channel of other user mutually orthogonal, namely

Before user data sends, the transmission time delay difference of base station end reply different base station corrects, the data delivery time that can adjust different user in time domain corrects delay inequality, consider realization complexity and according to the time delay of Fourier transform time domain be equivalent to frequency domain rotate character, adopt frequency domain phase rotating poor with compensation delay, operand is little and calculating is simple.Concrete grammar is after precoding, and before IFFT conversion, corrected by phase rotating at frequency domain the delay inequality of different antennae, other process is substantially constant, and the signal sent at l subband is:

$X_l=\underset{k\∈U_l}{\mathrm{\Σ}}{\mathrm{\Φ}}_{\mathrm{kn}}{w}_{\mathrm{kl}}{x}_{\mathrm{kn}}---\left(4\right)$

Wherein Φ _knbeing the antenna delay correction matrix of user k on the n-th subcarrier, is the diagonal matrix of M × M, matrix Φ _knm diagonal element be φ _knm:

${\mathrm{\φ}}_{\mathrm{knm}}=e^{j\frac{2\mathrm{\π}{\stackrel{\‾}{\mathrm{\τ}}}_{\mathrm{km}}}{N}(n-\mathrm{lB}-\frac{B-1}{2})}---\left(5\right)$

Wherein, represent the average eguivalent multipath additional delay of m root antenna to user k, there is following form:

${\stackrel{\‾}{\mathrm{\τ}}}_{\mathrm{km}}=\left\{\begin{array}{cc}-\frac{{\mathrm{\τ}}_k}{2},& m=\mathrm{1,2},\·\·\·,\frac{M}{2}\\ \frac{{\mathrm{\τ}}_k}{2},& m=\frac{M}{2}+1,\·\·\·,M\end{array}\right.---\left(6\right)$

After time delay adjustment, the equivalent channel of each subband center position (n=lB+ (B-1)/2) remains unchanged compared with when not doing time delay adjustment, and now user scheduling and precoding still can according to fed back average channel vector carry out.

After time delay adjustment, useful signal equivalent channel h _knΦ _knequivalent base delay inequality reduce to 0, and interference signal equivalent channel h _knΦ _inequivalent base delay inequality become τ _k-τ _iif do not consider the delay inequality of user during user scheduling, time delay adjustment may make the equivalent delay difference of interference signal between user increase, and causes the achievable rate of user to decline.For this reason when user scheduling, the user's delay inequality selected in same subband should relatively, below be described:

In a subband, by multi-subscriber dispatching, the delay inequality of selected user is positioned at between, for certain user k, the delay inequality mean value of other interference users is after time delay adjustment, the interference signal average delay difference of user k is if reduce the equivalent delay difference of each user's interference signal, then need namely like this through time delay adjustment not only useful signal get a promotion, inter-user interference is also inhibited, thus improves transmission rate more significantly, therefore when user scheduling, delay inequality user relatively should be selected to transmit on identical running time-frequency resource.

This programme needs the average delay difference τ fed back between base station _k, this value and user, base station spacing are relevant, and when user's low speed moves, delay inequality change is comparatively slow, and the speed of feedback is lower, so this programme only need increase less operand and feedback information can obtain obvious system and rate gain.

Effect of the present invention can be further illustrated by emulation, and simulated environment is in table 1.

Table 1 simulated environment

Based on the simulation parameter that table 1 is arranged, and the delay inequality of selected user is uniformly distributed between [15,25] individual sampled point, namely Δ τ=5, the simulation result obtained as shown in Figure 3, as can be seen from the figure, along with the raising of signal to noise ratio, the system not doing delay inequality correction becomes interference-limited, and achievable rate almost no longer increases, and after delay inequality corrects, the achievable rate of system significantly improves, and the root mean square of channel (RMS) delay spread is less, it is more that speed improves.

Claims (4)

1., based on timing alignment schemes in the multipoint cooperative ofdm system of subband precoding, comprise the steps:

Step one, when multiple base station cooperates, user k the n-th received over subcarriers to signal be:

$y_{\mathrm{kn}}=\left(h_{\mathrm{kn}}{w}_{\mathrm{kl}}\right)x_{\mathrm{kn}}+\underset{\underset{{i\∈U}_l}{i\≠k}}{\mathrm{\Σ}}\left(h_{\mathrm{kn}}{w}_{\mathrm{il}}\right)x_{\mathrm{in}}+z_{\mathrm{kn}}$

Wherein: subscript l represents that subcarrier n belongs to l subband, h _knrepresent that base station is to the channel vector of user k on subcarrier n, w _klrepresent the beamforming vector of user k on l subband, x _knrepresent the data sending to user k on the n-th subcarrier, U _lchoose the user's set carrying out data transmission, w at l subband _ilrepresent the beamforming vector of other users, x _in(i ≠ k) represents the transmission data of other users, z _knrepresent additive white Gaussian noise;

Step 2, user side intercepts OFDM symbol, carries out channel estimating and obtains channel condition information and by subband feedback, feed back the mean transit delay difference τ of different base station to user simultaneously _k;

Step 3, base station end receives the channel condition information of user feedback and the mean transit delay difference τ of different base station _kafter, in each subband, by different user grouping, between the channel of the interior user of different group, correlation meets wherein user k, i belongs to different user's groups respectively, α is a constant determined according to orthogonality requirement, then form user from the top-quality user of selective channel in each group and collect U, then in U, select the actual user carrying out transmitting according to delay inequality, the interval that the delay inequality of selected user is formed is requirement user's set of further selection, is U _l, to the transmission data acquisition ZF precoding choosing user, wherein the precoding vector of each user and the channel of other user mutually orthogonal, namely

Step 4, base station end is poor according to the mean transit delay of the different base station of user feedback, sends after carrying out delay compensation to the data after the precoding of selected user;

Described by subband feedback, the average channel condition information by subcarriers all in subband is fed back, and specifically comprise average channel directional information and average channel quality information feedback, average channel direction is ${\tilde{h}}_{{\mathrm{kl}}_a}=h_{{\mathrm{kl}}_a}/\left|\right|h_{{\mathrm{kl}}_a}\left|\right|,$ Average channel quality $q_{\mathrm{kl}}=\left|\right|h_{{\mathrm{kl}}_a}\left|\right|,$ Wherein $\frac{1}{B}\underset{n=\mathrm{lB}}{\overset{\mathrm{lB}+B-1}{\mathrm{\Σ}}}{h}_{\mathrm{kn}}=h_{{\mathrm{kl}}_a},$ B represents the number of sub carrier wave in subband l, for reducing receiving terminal amount of calculation, adopts approximate: c _l=lB+ (B-1)/2, represent the channel status at subband center subcarrier place, c _l=lB+ (B-1)/2 represents the central subcarrier of subband, represent that the average delay of all users of selected user's collection is poor; Δ τ represents the deviation of user's delay inequality that selected user collects and its average delay difference; represent that a represents average, and the sub-band averaging channel direction of feedback is at the average channel vector of l sub-banded base stations to user k

2. timing alignment schemes according to claim 1, is characterized in that: in described step 2, user k intercepts OFDM symbol, then through FFT conversion, utilizes pilot frequency information to estimate to obtain channel status h by least-squares algorithm and least-mean-square error algorithm _kn.

3. timing alignment schemes according to claim 1, it is characterized in that: for reducing receiving terminal amount of calculation, in the channel vector approximation subband at employing subband center subcarrier place during average channel vector, when transmitting terminal does delay compensation, should ensure that the channel vector of subband center is constant.

4. timing alignment schemes according to claim 1, is characterized in that: in described step 4, delay compensation realizes by adopting frequency domain phase rotating.