CN1980209A - Random-cycle time-delay diversity orthogonal frequency division multiplexing transmission method - Google Patents

Abstract

Using different antenna branches to transmit OFDM (orthogonal frequency division multiplex) symbols time delayed at random each other obtain time and frequency selectivity of slow fading channel artificially to make gain of each carrier channel of OFDM produce quick-change so as to lower probability of occurring continuous error in symbols to reach purpose of increasing performance of system bit error rate. The method includes steps: coded modulating information of bit stream in order to generate data symbols, carrying IFFT modulation for generated data symbols so as to form transmitting signals for each antenna branch; generating time delay sample number for each antenna branch at random; based on time delay sample number, carrying out cycle shift for transmitting signals for each antenna branch; inserting cycle prefix to shifted each branch forms symbols of OFDM; sending symbols of OFDM to corresponding antenna end, and transmitting symbols to channel.

Description

Random-cycle time-delay diversity orthogonal frequency division multiplexing transmission method

Technical field

The present invention relates to the wireless messages transmission field, particularly adopt the information transmission system and the corresponding communication standard such as WLAN (wireless local area network), fixed wireless access, mobile communication, ground digital television broadcast of OFDM (OFDM) technology.More specifically, the present invention relates to a kind of random-cycle time-delay diversity OFDM transmission method.

Background technology

Along with the fusion gradually of wireless network, multimedia technology and internet, people are more and more higher to the requirement of the type of radio communication service and quality.For satisfying the requirement of radio multimedium and high speed data transfers, need the Development of New Generation wireless communication system.The novel radio system will extensively adopt some new technologies, as OFDM (OFDM), many antennas (MIMO) etc.

OFDM is divided into many orthogonal sub-channels to channel at frequency domain; whole wideband frequency selective channel is divided into the subchannel of relatively flat; simultaneously, insert Cyclic Prefix (CP) as protection interval (GI), reduced intersymbol interference (ISI) greatly at each OFDM intersymbol.Because OFDM has advantages such as ability of anti-multipath is strong, its application of in xDSL, DVB, DAB and systems such as WLAN, IEEE802.16, succeeding.At present, in the Long Term Evolution plan (LTE) of 3G (Third Generation) Moblie standardization body (3GPP), also it as down (on) the critical transmissions technology of row.

The advantage of OFDM is that it has the frequency diversity ability.This is that the channel gain difference of each carrier wave of OFDM is so it has the frequency diversity ability because in frequency selective fading channels.The time delay expansion of channel is long more, and then the channel gain difference of each carrier wave end of OFDM is big more, and data carried by data symbol independence is strong more on it, helps correct decoding more.But when channel delay was expanded short or flat fading, each carrier dropout of whole OFDM symbol was identical, so correlation is stronger between the data symbol of carrier wave end, this can bring long burst error, even adopt coding techniques also to be difficult to error correction.

For improving the antiburst error ability of ofdm system, document [1] proposes a kind of Cyclic Delay Diversity (Cycle Delay Diversity:CDD) ofdm system, and it carries out every width of cloth antenna branch signal to launch after the fixed delay cyclic shift, as shown in Figure 1.Receiving terminal is the stack of a plurality of different delay signals, and similar transmitting passed through multipath transmisstion.By CDD, increased the frequency selectivity of channel artificially, reduced the correlated bandwidth of channel and OFDM intercarrier correlation, make the OFDM in the slow fading channel obtain the frequency diversity ability.

In fact, CDD is a class transmit diversity techniques, and previously presented Space Time Coding (STC) OFDM also can improve system's performance of BER, but it need carry out Space Time Coding by the emission symbol to each antenna branch before IFFT.If it is used for existing ofdm communication technical standard,, need the system in the primary standard be changed, i.e. existence and the incompatible shortcoming of prior art standard as IEEE802.11a etc.And when CDD is applied to existing OFDM technical standard, need not primary standard is done than cataclysm, can compatible existing OFDM technical standard.Notice in the CDD ofdm system that has proposed, in each OFDM symbol time interval, each antenna branch has adopted the cyclic shift mode of fixed sample length, when channel variation in this time interval is slow, the frequency diversity gain that fixation of C DD mode can not utilize channel to improve fully, for further improving systematic function, the present invention proposes a kind of random-cycle time-delay diversity ofdm communication method, and it can improve the performance of BER of system in the time varying channel environment.

The list of references tabulation:

[1] A.Dammann and S.Kaiser are published in IEEE Globecom, November calendar year 2001, the article Standard conformable antenna diversitytechniques for OFDM systems andits application to the DVB-T system on the 3100-3105 page or leaf.

Summary of the invention

The objective of the invention is to improve the frequency diversity ability of CDD ofdm system under the slow fading channel environment.

To achieve these goals, according to the present invention, a kind of random-cycle time-delay diversity orthogonal frequency division multiplexing transmission method has been proposed, may further comprise the steps: message bit stream is carried out coded modulation, to generate data symbol, and the data symbol that generates is carried out IFFT modulate, form transmitting at each antenna branch; Produce time delay sample number at random at each antenna branch; According to the time delay sample number, transmitting of each antenna branch carried out cyclic shift; Each branch road displacement back signal is inserted Cyclic Prefix form OFDM symbol; And OFDM symbol sent into corresponding antenna end and be transmitted into channel.

Preferably, described method also comprises: before described cyclic shift step, transmitting of each antenna branch carried out contrary discrete Fourier transform (DFT).

Preferably, the data symbol that is about to carry out cyclic shift is divided into some groups, every group can comprise one or more data symbols, and with the base unit of each group of data symbols as cyclic shift.

Preferably, each group of data symbols comprises the data symbol of equal number.

Preferably, each group of data symbols comprises the data symbol of varying number.

According to the present invention, also proposed a kind of multiplexing-the random-cycle time-delay diversity orthogonal frequency division multiplexing combining transmission method, several antennas are divided into two groups on multiplex antenna and random-cycle time-delay diversity antenna, when transmitting different service quality professional, the business that will have the high quality-of-service requirement is mapped on the random-cycle time-delay diversity antenna branch, and according to transmitting as the described method of one of claim 1 to 5; And the business that will have low quality of service requirement is mapped on the multiplex antenna branch road, and transmits according to existing multiplex transmission method.

When an OFDM symbol experience is hanged down the slow fading channel of time delay expansion, its the channel fading difference that each carrier wave experienced is little, and intercarrier has strong correlation, and continuous mistake appears in symbol easily, shown in (a) among Fig. 2, decoder is difficult to carry out the bit error correction.Adopt CDD to increase the frequency selectivity of channel, reduced the correlated bandwidth of channel and OFDM intercarrier correlation, make OFDM obtain the frequency diversity ability.Like this, the symbol error randomization that becomes more helps decoder and corrects the bit of makeing mistakes, shown in (b) among Fig. 2.

In the general CDD ofdm system, each antenna branch adopts fixing cycle time-delay, as being x from first width of cloth antenna transmitting sequence vector ₁=s (k) is x from second width of cloth antenna transmitting sequence vector ₂=s (k-T) is x from the 3rd width of cloth antenna transmitting sequence vector ₃=s (k-2T), by that analogy, shown in (a) among Fig. 3.When two continuous OFDM intersymbol channels when becoming slowly, see that on the whole these two CDD OFDM symbols still have certain correlation.

For reducing correlation between different OFDM symbol carriers, the present invention proposes every day line branch road and adopts random-cycle time-delay diversity (Random CCD:RCDD), and promptly the vector from the sequence of first width of cloth antenna emission is x ₁=s (k) is x from the vector of the sequence of second width of cloth antenna emission ₂=s (k-T ₂), be x from the vector of the sequence of the 3rd width of cloth antenna emission ₃=s (k-T ₃), or the like, T here ₂, T ₃... be the random delay sample number, shown in (b) among Fig. 3.From receiving terminal, be similar to and make the fast change of slow fading channel generation artificially, the channel gain of each carrier wave of OFDM symbol is differed greatly, reduced the correlation of intercarrier, reduce the probability that the continuous symbol mistake occurs, shown in (c) among Fig. 2.

In the present invention, term " at random " should be understood to by the whole bag of tricks obtain at random or the pseudorandom situation.

If the signal after the IFFT modulation can be expressed as following formula (1)

$s\left(l\right)=\frac{1}{\sqrt{N}}\underset{k=0}{\overset{N-1}{\mathrm{\&Sigma;}}}S\left(k\right)\exp \left(\mathrm{<figure-callout\; id="2"\; label="j"\; filenames="A20051012950200121.png,A20051012950200221.png"\; state="\{\{state\}\}">j</figure-callout>}\frac{2\mathrm{\&pi;}}{N}\mathrm{kl}\right)---\left(1\right)$

S in the formula (k) is for frequency domain transmits, and N is a carrier number, and s (l) is the time-domain signal after the IFFT modulation.Then the CDD signal after the cyclic shift can be expressed as following formula (2)

As seen, the CDD signal is equivalent to phase diversity (PD) signal.In the fixation of C DD ofdm system, time delay δ _CrcIt is a steady state value.And in the RCDD ofdm system, time delay δ _CrcThe random integer value of ∈ [0, N], the RCDD ofdm signal can be expressed as following formula (3)

Compare with formula (2), difference is the δ among the CDD _CrcBecome the δ among the RCDD _Crc(t), promptly the time delay of each antenna branch signal changes with the OFDM symbol is different.

Description of drawings

Below with reference to accompanying drawings, the preferred embodiments of the present invention are described in detail, wherein:

Fig. 1 shows CDD OFDM example;

Fig. 2 is the schematic diagram that is used to explain the principle of RCDD OFDM;

Fig. 3 is the schematic diagram that is used for the different cyclic shift methods of comparison;

Fig. 4 shows the flow chart according to RCDD OFDM transmission method of the present invention;

Fig. 5 shows the block diagram of the communication system that can use RCDD OFDM transmission method of the present invention;

Fig. 6 is the key diagram that is used to explain the example of cyclic shift;

Fig. 7 is the schematic diagram that is used to explain the second embodiment of the present invention;

Fig. 8 is the schematic diagram that is used to explain the third embodiment of the present invention;

Fig. 9 shows the chart of multipath channel distribute power;

Figure 10 shows the schematic diagram of CDD OFDM emission symbol;

Figure 11 shows the schematic diagram according to RCDD OFDM emission symbol of the present invention;

Figure 12 is the schematic diagram that is used for the carrier gains of comparison the present invention and prior art; And

Figure 13 shows under the situation that 16QAM modulation and QPSK modulate, the curve chart of the simulation result of the BER performance of the present invention and prior art.

Embodiment

Below in conjunction with accompanying drawing the present invention is specified.Be noted that described embodiment only is for illustrative purposes, rather than limitation of the scope of the invention.Described various numerical value is not to be used to limit the present invention, and these numerical value can carry out any suitable modification according to those of ordinary skills' needs.

Fig. 4 shows the flow chart according to RCDD OFDM transmission method of the present invention.

At first,, message bit stream is carried out coded modulation,, and the data symbol that generates is carried out IFFT modulate with the generation data symbol in step 401.Then, in step 402, in [0, N] interval, produce time delay sample number at random at each antenna branch.In step 403,, each branch road transmitted carry out cyclic shift according to the time delay sample number.Afterwards, in step 404, each branch road displacement back signal is inserted CP form the OFDM symbol.At last, in step 405, the OFDM symbol is sent into corresponding antenna end and is transmitted into channel.

By different transmit antennas emission random delay OFDM symbol, further make slow fading channel produce fast the change artificially, the channel gain of each carrier wave of OFDM symbol is differed greatly, reduced the correlation of intercarrier, reduce the probability that the continuous symbol mistake occurs, thereby improved the performance of BER of system.

Fig. 5 shows the block diagram of the communication system that can use RCDD OFDM transmission method of the present invention.

The communication system that can use this bright RCDD OFDM transmission method comprises N _TWidth of cloth transmitting antenna and N _RAmplitude receiver antenna.At transmitting terminal, the encoded modulation module 501 of incoming bit stream is modulated to symbols streams (step 401).Insert pilot module 502 frequency pilot sign is inserted into data symbol stream, 503 pairs of pilot tones of IDFT module and data symbol carry out contrary discrete fourier transform.Transmitting terminal circular shift module 506～506 " according to different time delays, carry out cyclic shift (step 403), and resulting transmitting is sent to every width of cloth antenna end.Insert CP module 504～504 " the OFDM symbol of every width of cloth antenna end is inserted protection back (step 404) at interval, by antenna 505～505 " be transmitted into channel (step 405).

The OFDM symbol example to be launched that forms through cyclic shift as shown in Figure 6, the signal that the transmitting terminal base band produces and storage is modulated through IFFT, utilize each given antenna branch end displacement sample number then,, and send into each antenna branch end after inserting CP the displacement of IFFT output symbol.Particularly, with reference to Fig. 6, cyclic shift is according to time delay, with in the IFFT output symbol, be displaced to the front end (A) of symbol sebolic addressing with the corresponding symbols formerly of described time delay (A ').CP is in the symbol sebolic addressing after the cyclic shift, the copy of the symbols formerly of equal in length (CP ') with it.

At receiving terminal, shift out CP module 508～508 " each antenna branch 507～507 " the CP deletion of the OFDM symbol that receives, after merging module 509 merges all antenna receiving signals, after utilizing DFT module 510 that reception OFDM symbol is carried out the Fourier transform demodulation, balance module 511 carries out equilibrium to received signal and eliminates intersymbol interference, output bit flow behind demodulation coding 512.

First embodiment

Return Fig. 3,, the data symbol of IFFT modulation is done as a whole, utilize the random delay that produces, this data symbol is carried out cyclic shift with reference to (b) among the figure 3.Antenna 1 emission symbol is 1,2,3,4,5,6,7,8,9,10; Relative antenna 1,4 samples of emission symbol cyclic shift of antenna 2 are 7,8,9,10,1,2,3,4,5,6; Relative antenna 1,1 sample of emission symbol cyclic shift of antenna 3 is 10,1,2,3,4,5,6,7,8,9.This shows that the cyclic shift sample number of antenna 2,3 relative antennas 1 is at random.

Second embodiment

From transmitting terminal, the method for first embodiment is actually each antenna branch emission data is interweaved, and the weaving length difference of each each symbol of antenna (that is the length difference of cyclic shift).It is generalized to more generally under the situation, for example, the data symbol of IFFT modulation is divided into some groups, every group can comprise one or more data symbols, with the base unit of every group code as cyclic shift.

Fig. 7 is the schematic diagram that is used to explain the second embodiment of the present invention.As shown in Figure 7, per 2 OFDM carrier data are divided into one group, then, carry out random rotation displacement (interweaving), and launch.If the emission symbol of antenna 1 is (1,2), (3,4), (5,6), (7,8), (9,10); The emission symbol of antenna 2 is a resulting symbol (7,8) behind cyclic shift 2 group codes, (9,10), (1,2), (3,4), (5,6); The emission symbol of antenna 3 is a resulting symbol (1,2) behind cyclic shift 0 group code, (3,4), (5,6), (7,8), (9,10).

In the present embodiment, provided every group of example that comprises the data symbol of equal number, but should be understood that, every group of data symbol that can comprise varying number.For example, can be (1,2,3) (4), (5,6,7,8), (9,10) with the emission symbol packets of antenna 1.In this case, if with reference to said circumstances, the emission symbol of antenna 2 is a resulting symbol (5,6,7,8) behind cyclic shift 2 group codes, (9,10), (1,2,3), (4); The emission symbol of antenna 3 is a resulting symbol (1,2,3) behind cyclic shift 0 group code, (4), (5,6,7,8), (9,10).

The 3rd embodiment

Make full use of the characteristics that CDD OFDM is easy to realize, under the prerequisite that does not change the mimo system structure, proposed a kind of mixing circulation time delay diversity and multiplexing MIMO ofdm system, shown in (b) among Fig. 8.This system comprises multiplexing and CDD two parts, and multiplex antenna branch road 1 and CDD antenna branch 2,3,4 transmit different data streams respectively, improves system's speed.And each antenna branch 2,3,4 of CDD (can correspond respectively to the antenna branch 1,2,3 among Fig. 7) transmits identical data symbol, but each circuit-switched data is random interleaving in time, to improve system's performance of BER.

Like this, when transmitting different service quality QoS professional, can be mapped to the CDD antenna branch to multimedia service and transmit, and the business of low qos requirement is mapped to the transmission of multiplex antenna branch road with high qos requirement.Simultaneously, realize that this system need not the structure of former multiaerial system is carried out bigger change, shown in (a) among Fig. 8.

Emulation experiment

By emulation experiment, verify performance of the present invention, the system parameter of setting up departments is:

4 transmit antennas, 1 reception antenna

The total carrier number N=64 of each OFDM symbol

1/2 speed convolutional encoding

Multipath channel power is exponential distribution, as shown in Figure 9, every footpath power is carried out normalization to first footpath, and the power that obtains every footpath thus is followed successively by: [exp (0) exp (1) exp (2) exp (3) exp (4) exp (5) exp (6) exp (7)].

Figure 10 shows the schematic diagram of CDD OFDM emission symbol, in the expression CDD ofdm system, and the signal real part of every width of cloth antenna transmission (QPSK modulation).In Figure 10, be (a) from first width of cloth antenna transmission signal, (b) be shift signal from second width of cloth antenna transmission, (c) be shift signal from the 3rd width of cloth antenna transmission, (d) be shift signal from the 4th width of cloth antenna transmission.

Figure 11 shows the schematic diagram according to RCDD OFDM emission symbol of the present invention, in the expression RCDD ofdm system, and the signal real part of every width of cloth antenna transmission.In Figure 11, be (a) from first width of cloth antenna transmission signal, (b) be shift signal from second width of cloth antenna transmission, (c) be shift signal from the 3rd width of cloth antenna transmission, (d) be shift signal from the 4th width of cloth antenna transmission.Relatively Figure 10 and Figure 11 can find that each tributary signal of CDD OFDM is launched with regular time delay, and each tributary signal of RCDD OFDM is launched with random delay.

Figure 12 is the schematic diagram that is used for the carrier gains of comparison the present invention and prior art, wherein (a) is illustrated in each carrier channel gain of general ofdm system in certain time slot, in the zone that is shown by the dotted ellipse frame table, the change in gain of carrier wave is little, and there is stronger correlation in intercarrier; And (b) each carrier channel gain of expression RCDD ofdm system, in the same area of being shown by the dotted ellipse frame table, each carrier gains of RCDD OFDM changes greatly, and intercarrier independence strengthens, thereby can obtain the frequency diversity ability.

Figure 13 shows under the situation of 16QAM modulation and QPSK modulation, the curve chart of the present invention and the simulation result of the BER performance of prior art, and wherein (a) represents that 16QAM modulates, and channel delay is system's bit error rate (BER) performance under the 200ns condition; And (b) expression QPSK modulation, channel delay is the BER performance under the 100ns condition.Can see that under the different modulating mode, the BER performance of RCDD OFDM transmission method of the present invention all is better than general CDD OFDM transmission method.

Although illustrated and described the present invention at exemplary embodiments, will be understood by those skilled in the art that, under the situation that does not break away from the spirit and scope of the present invention, can carry out various other change, replacement and interpolations.Therefore, the present invention should not be understood that to be limited to above-mentioned particular instance, and should be limited by claims.

Claims (6)

1, a kind of random-cycle time-delay diversity orthogonal frequency division multiplexing transmission method may further comprise the steps:

Message bit stream is carried out coded modulation,, and the data symbol that generates is carried out IFFT modulate, form transmitting at each antenna branch with the generation data symbol;

Produce time delay sample number at random at each antenna branch;

According to the time delay sample number, transmitting of each antenna branch carried out cyclic shift;

Each branch road displacement back signal is inserted Cyclic Prefix form OFDM symbol; And

OFDM symbol is sent into corresponding antenna end and is transmitted into channel.

2, random-cycle time-delay diversity orthogonal frequency division multiplexing transmission method according to claim 1 is characterized in that also comprising: before described cyclic shift step, transmitting of each antenna branch carried out contrary discrete Fourier transform (DFT).

3, random-cycle time-delay diversity orthogonal frequency division multiplexing transmission method according to claim 1 and 2, the data symbol that it is characterized in that being about to carrying out cyclic shift is divided into some groups, every group can comprise one or more data symbols, and with the base unit of each group of data symbols as cyclic shift.

4, random-cycle time-delay diversity orthogonal frequency division multiplexing transmission method according to claim 3 is characterized in that each group of data symbols comprises the data symbol of equal number.

5, random-cycle time-delay diversity orthogonal frequency division multiplexing transmission method according to claim 3 is characterized in that each group of data symbols comprises the data symbol of varying number.

6, a kind of multiplexing-the random-cycle time-delay diversity orthogonal frequency division multiplexing combining transmission method, several antennas are divided into two groups on multiplex antenna and random-cycle time-delay diversity antenna, when transmitting different service quality professional, the business that will have the high quality-of-service requirement is mapped on the random-cycle time-delay diversity antenna branch, and according to transmitting as the described method of one of claim 1 to 5; And the business that will have low quality of service requirement is mapped on the multiplex antenna branch road, and transmits according to existing multiplex transmission method.

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