CN101237251A - Direct frequency spreading-OFDM modulation and demodulation method under high-speed mobile environment - Google Patents

Abstract

The invention discloses a direct spread-orthogonal frequency division multiplexing modulation and demodulation method which can be used in high-speed mobile environments. The invention comprises the following steps: firstly, modeling is performed under the environment of a radio channel, and a spread spectrum/de-spread sequence is selected according to a spread spectrum/de-spread sequence selection method; secondly, binary data is acquired from a source and then mapped into a QPSK signal, and is subject to spread spectrum through a spread spectrum device; thirdly, the signal after spread spectrum is taken as a frequency domain sub-carrier data for performing inverse Fourier transform and forming an orthogonal frequency division multiplexing signal which is then transmitted on a communication channel; fourthly, the transmitted signal is received and then subject to orthogonal frequency division multiplexing demodulation; fifthly, the signal after orthogonal frequency division multiplexing demodulation passes through a de-spread device and is subject to de-spread by the de-spread sequence, then the transmitted signal is recovered; sixthly, the prior transmitted binary source data is de-mapped and recovered. The direct spread-orthogonal frequency division multiplexing modulation and demodulation method can effectively inhibit the interference between orthogonal frequency division multiplexing signal sub-carriers and the signal-interference-noise ratio is made to be the maximum, thereby the method can be operated under the high-speed mobile environments.

Description

Can be used for the straight expansion-OFDM modulation-demo-demodulation method under the high-speed mobile environment

Technical field

The present invention relates to digital communication, relate in particular to a kind of straight expansion-OFDM modulation-demo-demodulation method that can be used under the high-speed mobile environment.

Background technology

OFDM (OFDM) is a kind of mode of wide-band modulation efficiently.It with the data allocations that transmitted to a plurality of parallel subchannels; make the symbol width on each subchannel expand time delay greater than multipath; and between per two OFDM symbols, insert the protection interval of certain width, can suppress the intersymbol interference (ISI) that multipath effect is brought so substantially.And, because data are positioned at frequency domain, make channel equalization to finish with the filter of a single tap, greatly reduce the complexity of demodulating end equilibrium.Ofdm system is modal multi-carrier modulation technology, and its main advantage is can transmit the data of two-forty on broad-band channel and only need seldom balanced operation.The existing description also of such circuit progressively is widely used.But when having frequency shift (FS) between the carrier wave of sending and receiving end, the orthogonality between the subcarrier is damaged, and can produce inter-carrier interference (ICI), and this ICI has limited the performance of ofdm system, causes the error floor phenomenon (error floor) in the ofdm system.

Summary of the invention

The purpose of this invention is to provide a kind of straight expansion-OFDM modulation-demo-demodulation method that can be used under the high-speed mobile environment.

Comprise the steps:

1), chooses spread spectrum/despreading sequence according to spread spectrum/despreading sequence selection method and be stored in frequency multiplier and despreader to the wireless channel environment modeling;

2) obtain binary data from the source, binary data is mapped to the QPSK symbol, carry out spread spectrum with the good frequency expansion sequence of storage in advance through frequency multiplier;

3) with symbol behind the spread spectrum as the subcarrier in frequency domain data, carry out inverse-Fourier transform, form the orthogonal frequency division multiplexed signal, on communication port, send signal;

4) receive the signal that is sent, carry out the orthogonal frequency division multiplexed demodulation to the received signal;

5) signal after the orthogonal frequency division multiplexed demodulation goes despreading through despreader with the despreading sequence that storage in advance is good, recovers the symbol of transmission;

6) separate the original binary source data that mapping recovers transmission.

Described to the wireless channel environment modeling, choosing spread spectrum/despreading sequence according to spread spectrum/despreading sequence selection method, to be stored in the step of frequency multiplier and despreader as follows:

1) determines the probability distribution of each channel parameter, the multipath time varying channel is carried out modeling with the Jake model.For time varying channel, establishing time varying channel has the L footpath, and its impulse response is $h(n,\mathrm{\τ})=\underset{l=0}{\overset{L-1}{\mathrm{\Σ}}}{h}_{n,l}\mathrm{\δ}(\mathrm{\τ}-{\mathrm{\τ}}_l),$ τ wherein _lBe l ^ThThe delay in footpath, h _{N, l}Be the multiple gaussian variable of a zero-mean, represent n complex gain that becomes when go up in the 1st footpath constantly, each h _{N, l}To different paths is separate, and is subject to Doppler frequency f _dΔ ε=f _dT _sBe normalized Doppler frequency deviation;

2) expand the transmission signal by multipath time varying channel model by the receiving terminal demodulation, the data after the receiving terminal despreading are: $Y_m=\underset{s=0}{\overset{Q-1}{\mathrm{\Σ}}}{c}_s^{\′}\underset{n=0}{\overset{N-1}{\mathrm{\Σ}}}{y}_n{e}^{-j2\mathrm{\πn}(\mathrm{mQ}+s)/N}=Y_m^s+Y_m^I+W_m,$ Wherein $Y_m^s=\underset{s=0}{\overset{Q-1}{\mathrm{\Σ}}}{c}_s^{\′}\underset{t=0}{\overset{Q-1}{\mathrm{\Σ}}}{X}_m{c}_t{a}_{t-s}$ Be the useful signal of wishing, $Y_m^I=\underset{p\≠m}{\overset{P-1}{\underset{p=0}{\mathrm{\Σ}}}}\underset{s=0}{\overset{Q-1}{\mathrm{\Σ}}}{c}_s^{\′}\underset{t=0}{\overset{Q-1}{\mathrm{\Σ}}}{X}_m{c}_t{a}_{(p-m)Q+t-s}$ Be the inter-carrier interference that causes owing to frequency deviation, $W_m=\underset{s=0}{\overset{Q-1}{\mathrm{\Σ}}}{c}_s^{\′}{w}_{\mathrm{mQ}+s}$ It is white noise. $a_r=\frac{1}{N}\underset{n=0}{\overset{N-1}{\mathrm{\Σ}}}\underset{l=0}{\overset{L-1}{\mathrm{\Σ}}}{h}_{n,l}{e}^{-j2\mathrm{\π}(\mathrm{kQ}+t)l/N}{e}^{-j2\mathrm{\πnr}/N};$

3) adopt signal-interference-noise ratio $\mathrm{SINR}=\frac{E\left[{\left|Y_m^S\right|}^2\right]}{E\left[{\left|Y_m^I\right|}^2\right]+E\left[{\left|W_m\right|}^2\right]}=\frac{P_S}{P_{\mathrm{ICI}}+P_N}$ Weigh the influence of carrier-in-interference and noise to useful signal;

4) signal-interference-noise ratio formal representation of matrix,

$\mathrm{SINR}=\frac{P_S}{P_{\mathrm{ICI}}+P_N}=\frac{{\mathrm{\σ}}_x^2{c}^{{\′}^H}{\hat{S}}^{\′}{c}^{\′}}{{\mathrm{\σ}}_x^2{c}^{{\′}^H}\hat{I}{c}^{\′}+{\mathrm{\σ}}_n^2{c}^{{\′}^H}{I}_Q{c}^{\′}}=\frac{c^{{\′}^H}{\hat{S}}^{\′}{c}^{\′}}{c^{{\′}^H}\hat{I}{c}^{\′}+c^{{\′}^H}{I}_Q{c}^{\′}/\mathrm{SNR}},$ Wherein c is a sequence spreading, and c ' is the despreading sequence.Matrix

For comprising c and c ' binomial type, $\mathrm{SNR}=\frac{{\mathrm{\σ}}_x^2}{{\mathrm{\σ}}_n^2}$ Be signal to noise ratio.

Use iterative algorithm alternately to find out to make the frequency expansion sequence and the despreading sequence of signal-interference-noise ratio maximum by computer, as the coefficient of frequency expansion sequence in frequency multiplier and the despreader and despreading sequence.Alternately the main thought of iterative algorithm be fixedly among c and the c ' to ask for another so that the SINR maximum is changed the position of c and c ' then and is done same work, can prove, should replace iteration and reach convergence the most at last.

The described binary data that obtains from the source is mapped to the QPSK symbol with binary data, and is as follows with the step that the good frequency expansion sequence of storage in advance carries out spread spectrum through frequency multiplier:

The binary data source of 1) storing in the data source is that unit is read with 2 bits, and is high-order to the corresponding successively u of low level _k ¹, u _k ²

2) press the QPSK planisphere, to per 2 bit-binary data u _k ¹, u _k ²Shine upon, become QPSK symbol X _k, k=0 wherein ..., P-1;

3) read the frequency expansion sequence c that has stored in the frequency multiplier _t, t=0 wherein ..., Q-1 is with QPSK symbol X _kMultiply each other respectively with each coefficient in the frequency expansion sequence, finish spread spectrum, with symbol S behind P spread spectrum _KQ+tParallel arranged, as the subcarrier in frequency domain data, k=0 wherein ..., P-1, t=0 ..., Q-1.

Described with symbol behind the spread spectrum as the subcarrier in frequency domain data, carry out inverse-Fourier transform, form the orthogonal frequency division multiplexed signal, the step that sends signal on communication port is as follows:

1) symbol S behind the spread spectrum _KQ+tCarry out N point inverse-Fourier transform, form orthogonal frequency-division multiplex singal;

$x_n=\frac{1}{N}\underset{k=0}{\overset{P-1}{\mathrm{\Σ}}}\underset{t=0}{\overset{Q-1}{\mathrm{\Σ}}}{S}_{\mathrm{kQ}+t}{e}^{j2\mathrm{\πn}(\mathrm{kQ}+t)/N}=\frac{1}{N}\underset{k=0}{\overset{P-1}{\mathrm{\Σ}}}{X}_k\underset{t=0}{\overset{Q-1}{\mathrm{\Σ}}}{c}_t{e}^{j2\mathrm{\πn}(\mathrm{kQ}+t)/N},$ N=P * Q wherein;

2) orthogonal frequency-division multiplex singal characterizes with moulding pulse g (t), adopts the filter of raised cosine roll off frequency characteristic,

Get α=0.25 in the formula;

The signal that described reception sent, the step of carrying out the OFDM demodulation to the received signal is as follows:

1) receives the signal y that is sent _n, remove Cyclic Prefix to the received signal;

2) convert the signal into frequency domain, carry out following Fourier transform:

$Y_m^{\′}=\underset{n=0}{\overset{N-1}{\mathrm{\Σ}}}{y}_n{e}^{-j2\mathrm{\πnm}/N},$ M=0 wherein ..., Q-1.

Signal after the described orthogonal frequency division multiplexed demodulation goes despreading through despreader with the despreading sequence that storage is good in advance, and the step of symbol that recovers transmission is as follows:

1) reads the despreading sequence c that has stored in the despreader _m', m=0 wherein ..., Q-1;

2) with OFDM data after demodulating Y _m' with the despreading sequence in the summation of multiplying each other respectively again of each coefficient, finish despreading, obtain following former data symbol after the despreading:

$Y_m=\underset{s=0}{\overset{Q-1}{\mathrm{\Σ}}}{c}_s^{\′}\underset{n=0}{\overset{N-1}{\mathrm{\Σ}}}{y}_n{e}^{-j2\mathrm{\πn}(\mathrm{mQ}+s)/N},$ M=0 wherein ..., Q-1.

Describedly separate the original binary source data that mapping recovers transmission and be: press the QPSK planisphere, according to quadrant to symbol Y _mCarry out hard decision, it reverts to 2 bit-binary data to each symbol K=0 wherein ..., P-1.

The present invention has reduced because the performance loss that the inter-carrier interference that carrier wave frequency deviation brings causes, even under the situation of high-speed mobile, the orthogonality of Doppler frequency deviation heavy damage OFDM symbol subcarrier, this system can make signal-interference-noise ratio maintain the acceptable degree, thus normally demodulation; Therefore the despreading sequence that is used for the frequency expansion sequence of spread spectrum and despreading does not simultaneously increase any true burden of calculating to passing through the computer calculated in advance.

Description of drawings

Fig. 1 is the block diagram according to a kind of example communications system of embodiments of the invention;

Fig. 2 is the circuit block-diagram according to the frequency multiplier module in the embodiments of the invention;

Fig. 3 is the circuit block-diagram according to the OFDM modulator block in the embodiments of the invention;

Fig. 4 is the circuit block-diagram according to the ofdm demodulator module in the embodiments of the invention;

Fig. 5 is the circuit block-diagram according to the despreaders block in the embodiments of the invention;

Fig. 6 is according to the spread spectrum in the embodiments of the invention/despreading sequence selection method flow chart.

Embodiment

Straight expansion-OFDM the modulation-demo-demodulation method that can be used under the high-speed mobile environment comprises the steps:

1), chooses spread spectrum/despreading sequence according to spread spectrum/despreading sequence selection method and be stored in frequency multiplier and despreader to the wireless channel environment modeling;

2) obtain binary data from the source, binary data is mapped to the QPSK symbol, carry out spread spectrum with the good frequency expansion sequence of storage in advance through frequency multiplier;

3) with symbol behind the spread spectrum as the subcarrier in frequency domain data, carry out inverse-Fourier transform, form the orthogonal frequency division multiplexed signal, on communication port, send signal;

4) receive the signal that is sent, carry out the orthogonal frequency division multiplexed demodulation to the received signal;

5) signal after the orthogonal frequency division multiplexed demodulation goes despreading through despreader with the despreading sequence that storage in advance is good, recovers the symbol of transmission;

6) separate the original binary source data that mapping recovers transmission.

Described to the wireless channel environment modeling, choosing spread spectrum/despreading sequence according to spread spectrum/despreading sequence selection method, to be stored in the step of frequency multiplier and despreader as follows:

1) determines the probability distribution of each channel parameter, the multipath time varying channel is carried out modeling with the Jake model;

2) expand the transmission signal by multipath time varying channel model by the receiving terminal demodulation, the data decomposition after the receiving terminal despreading is useful signal, carrier-in-interference signal and white noise signal;

3) adopt the influence of signal-interference-noise ratio measurement carrier-in-interference and noise to useful signal;

4) signal-interference-noise ratio is with the formal representation of matrix, uses iterative algorithm alternately to find out to make the frequency expansion sequence and the despreading sequence of signal-interference-noise ratio maximum by computer, as the coefficient of frequency expansion sequence in frequency multiplier and the despreader and despreading sequence.

The described binary data that obtains from the source is mapped to the QPSK symbol with binary data, and is as follows with the step that the good frequency expansion sequence of storage in advance carries out spread spectrum through frequency multiplier:

The binary data source of 1) storing in the data source is that unit is read with 2 bits, and is high-order to the corresponding successively u of low level _k ¹, u _k ²

2) press the QPSK planisphere, to per 2 bit-binary data u _k ¹, u _k ²Shine upon, become QPSK symbol X _k, k=0 wherein ..., P-1;

3) read the frequency expansion sequence c that has stored in the frequency multiplier _t, t=0 wherein ..., Q-1 is with QPSK symbol X _kMultiply each other respectively with each coefficient in the frequency expansion sequence, finish spread spectrum, with symbol S behind P spread spectrum _KQ+tParallel arranged, as the subcarrier in frequency domain data, k=0 wherein ..., P-1, t=0 ..., Q-1.

Described with symbol behind the spread spectrum as the subcarrier in frequency domain data, carry out inverse-Fourier transform, form the orthogonal frequency division multiplexed signal, the step that sends signal on communication port is as follows:

1) symbol S behind the spread spectrum _KQ+tCarry out N point inverse-Fourier transform, form orthogonal frequency-division multiplex singal;

$x_n=\frac{1}{N}\underset{k=0}{\overset{P-1}{\mathrm{\Σ}}}\underset{t=0}{\overset{Q-1}{\mathrm{\Σ}}}{S}_{\mathrm{kQ}+t}{e}^{j2\mathrm{\πn}(\mathrm{kQ}+t)/N}=\frac{1}{N}\underset{k=0}{\overset{P-1}{\mathrm{\Σ}}}{X}_k\underset{t=0}{\overset{Q-1}{\mathrm{\Σ}}}{c}_t{e}^{j2\mathrm{\πn}(\mathrm{kQ}+t)/N},$ N=P * Q wherein;

2) orthogonal frequency-division multiplex singal characterizes with moulding pulse g (t), adopts the filter of raised cosine roll off frequency characteristic,

Get α=0.25 in the formula.

The signal that described reception sent, the step of carrying out the OFDM demodulation to the received signal is as follows:

1) receives the signal y that is sent _n, remove Cyclic Prefix to the received signal;

2) convert the signal into frequency domain, carry out following Fourier transform:

$Y_m^{\′}=\underset{n=0}{\overset{N-1}{\mathrm{\Σ}}}{y}_n{e}^{-j2\mathrm{\πnm}/N},$ M=0 wherein ..., Q-1.

Signal after the described orthogonal frequency division multiplexed demodulation goes despreading through despreader with the despreading sequence that storage is good in advance, and the step of symbol that recovers transmission is as follows:

1) reads the despreading sequence c that has stored in the despreader _m', m=0 wherein ..., Q-1;

2) with OFDM data after demodulating Y _m' with the despreading sequence in the summation of multiplying each other respectively again of each coefficient, finish despreading, obtain following former data symbol after the despreading:

$Y_m=\underset{s=0}{\overset{Q-1}{\mathrm{\Σ}}}{c}_s^{\′}\underset{n=0}{\overset{N-1}{\mathrm{\Σ}}}{y}_n{e}^{-j2\mathrm{\πn}(\mathrm{mQ}+s)/N},$ M=0 wherein ..., Q-1.

Describedly separate the original binary source data that mapping recovers transmission and be: press the QPSK planisphere, according to quadrant to symbol Y _mCarry out hard decision, it reverts to 2 bit-binary data to each symbol

K=0 wherein ..., P-1

Embodiment

The present invention has realized a kind of DSSS-OFDM modem line that can be used under the high-speed mobile environment.

The DSSS-OFDM signal is with pulse q (t), or moulding pulse g (t) characterizes accordingly, and relation between the two is as follows:

$q\left(t\right)={\∫}_{-\∞}^{t}g\left(u\right)\mathrm{du}$

Adopt the filter of raised cosine roll off frequency characteristic in typical case,

β is constant (0＜β≤1/2T in the formula _s), α=β/2T _sBe rolloff-factor (0＜α≤1) that it has determined the precipitous degree of channel redundancy bandwidth and edge.Get α=0.25.

Utilize above information, just can determine the DSSS-OFDM signal that is sent out.

Referring now to Fig. 1,, shows a kind of example communications system of embodiment of the present invention.Data source module 101 is joined with mapping block 102, mapping block 102 joins with frequency multiplier module 103, finish the subcarrier in frequency domain spread spectrum, frequency multiplier module 103 is joined with OFDM modulator block 104, finish the generation function of spread spectrum-ofdm signal, signal is finished demodulation by communication channel module 105 in ofdm demodulator module 106, finish despreading in despreaders block 107, separate and be mapped as binary data separating mapping block 108 at last.

Among the present invention, at moment kQT, mapping block 102 reads in 2bits binary source data according to the order of sequence from data source module 101; The 2bits source data is sent into mapping block 102, it is mapped to the QPSK symbol obtains exporting X _kIn the time interval, frequency multiplier module 103 is with symbol X at QT _kWith frequency expansion sequence { c _t, t=0 ..., Q-1} is extended to the long spread symbol of QT, gives OFDM modulator block 104; Modulation produces the transmission signal to OFDM modulator block 104 through OFDM; Send signal in 105 transmission of communication channel module; Ofdm demodulator module 106 receives signal, carries out the OFDM demodulation; The subcarrier in frequency domain number is by despreaders block 107, with despreading sequence { c _m', m=0 ..., the symbol into QPSK is adjudicated in the Q-1} despreading; QPSK symbol after the demodulation is sent into and is separated mapping block 108 and separate mapping, thereby restores the binary data of transmission.

Fig. 2 shows frequency multiplier module 103.QPSK symbol X _kEarlier through serial to parallel conversion, each symbol all with { c _t, t=0 ..., Q-1} multiplies each other, thereby has formed the subcarrier in frequency domain data for the treatment of the OFDM modulation.

Fig. 3 shows OFDM modulator block 104.The subcarrier in frequency domain data of frequency multiplier module 102 outputs transform to time domain by inverse-Fourier transform module 301, through parallel/serial converter module 302 data flow is become serial data stream after the conversion, add Cyclic Prefix in 303 pairs of data flow of Cyclic Prefix inserter module, do shaping filtering by forming filter module 304 at last, send signal.

Fig. 4 shows ofdm demodulator module 106.The signal that receives is converted to parallel data stream through serial/parallel converter module 401 earlier, then the Cyclic Prefix that transmitting terminal is added by cyclic prefix remover module 402 removes to eliminate the intersymbol interference that multipath brings, and in Fourier transform module 403 time domain data is carried out Fourier transform.

Fig. 5 shows despreaders block 107.Subcarrier in frequency domain data and despreading sequence { c _m', m=0 ..., Q-1} walks abreast and multiplies each other and merge, and obtains the frequency domain data after the despreading, changes back serial data stream by parallel/serial converter again and gives and separate mapping block.

Fig. 6 shows spread spectrum/despreading sequence selection method flow chart.It has determined the frequency expansion sequence and the despreading sequence of use in frequency multiplier module 103 and despreaders block 107.At first time-variant multipath channel is carried out modeling, use the Jake model, and determine the distribution of each channel parameter.Then the expression formula of SINR is derived.Use alternately iterative algorithm to find the solution frequency expansion sequence and despreading sequence vector so that the SINR maximum if not restraining then to return does not continue to find the solution, is spread spectrum/despreading sequence otherwise then export the result.

The present invention is the modulation demodulation system that a kind of digital communication multicarrier combines with spread spectrum.Method described here and circuit; single component about the OFDM modulation can be traditional fully; and the DSSS modulating part also is the different sequence of spread spectrum and despreading use, and we require their combination and spread spectrum/despreading sequence choosing method are protected as invention.The above only is the embodiment of certain applications, but true spirit of the present invention and scope are not limited to this, any those of ordinary skill in the art can revise the concrete grammar of single component, realize the DSSS-OFDM modulator-demodulator of different application occasion.The present invention is only limited by appended claim book and equivalence techniques scheme thereof, and we require these are protected as the present invention.

Claims (7)

1. the straight expansion-OFDM modulation-demo-demodulation method that can be used under the high-speed mobile environment is characterized in that comprising the steps:

1), chooses spread spectrum/despreading sequence according to spread spectrum/despreading sequence selection method and be stored in frequency multiplier and despreader to the wireless channel environment modeling;

2) obtain binary data from the source, binary data is mapped to the QPSK symbol, carry out spread spectrum with the good frequency expansion sequence of storage in advance through frequency multiplier;

3) with symbol behind the spread spectrum as the subcarrier in frequency domain data, carry out inverse-Fourier transform, form the orthogonal frequency division multiplexed signal, on communication port, send signal;

4) receive the signal that is sent, carry out the orthogonal frequency division multiplexed demodulation to the received signal;

5) signal after the orthogonal frequency division multiplexed demodulation goes despreading through despreader with the despreading sequence that storage in advance is good, recovers the symbol of transmission;

6) separate the original binary source data that mapping recovers transmission.

2. a kind of straight expansion-OFDM modulation-demo-demodulation method that can be used under the high-speed mobile environment according to claim 1, it is characterized in that, described to the wireless channel environment modeling, choosing spread spectrum/despreading sequence according to spread spectrum/despreading sequence selection method, to be stored in the step of frequency multiplier and despreader as follows:

1) determines the probability distribution of each channel parameter, the multipath time varying channel is carried out modeling with the Jake model;

2) expand the transmission signal by multipath time varying channel model by the receiving terminal demodulation, the data decomposition after the receiving terminal despreading is useful signal, carrier-in-interference signal and white noise signal;

3) adopt the influence of signal-interference-noise ratio measurement carrier-in-interference and noise to useful signal;

4) signal-interference-noise ratio is with the formal representation of matrix, uses iterative algorithm alternately to find out to make the frequency expansion sequence and the despreading sequence of signal-interference-noise ratio maximum by computer, as the coefficient of frequency expansion sequence in frequency multiplier and the despreader and despreading sequence.

3. a kind of straight expansion-OFDM modulation-demo-demodulation method that can be used under the high-speed mobile environment according to claim 1, it is characterized in that, the described binary data that obtains from the source, binary data is mapped to the QPSK symbol, as follows with the step that the good frequency expansion sequence of storage in advance carries out spread spectrum through frequency multiplier:

The binary data source of 1) storing in the data source is that unit is read with 2 bits, and is high-order to the corresponding successively u of low level _k ¹, u _k ²

2) press the QPSK planisphere, to per 2 bit-binary data u _k ¹, u _k ²Shine upon, become QPSK symbol X _k, k=0 wherein ..., P-1;

3) read the frequency expansion sequence c that has stored in the frequency multiplier _t, t=0 wherein ..., Q-1 is with QPSK symbol X _kMultiply each other respectively with each coefficient in the frequency expansion sequence, finish spread spectrum, with symbol S behind P spread spectrum _KQ+tParallel arranged, as the subcarrier in frequency domain data, k=0 wherein ..., P-1, t=0 ..., Q-1.

4. a kind of straight expansion-OFDM modulation-demo-demodulation method that can be used under the high-speed mobile environment according to claim 1, it is characterized in that, described with symbol behind the spread spectrum as the subcarrier in frequency domain data, carry out inverse-Fourier transform, form the orthogonal frequency division multiplexed signal, the step that sends signal on communication port is as follows:

1) symbol s behind the spread spectrum _KQ+tCarry out N point inverse-Fourier transform, form orthogonal frequency-division multiplex singal:

$x_n=\frac{1}{N}\underset{k=0}{\overset{P-1}{\mathrm{\Σ}}}\underset{t=0}{\overset{Q-1}{\mathrm{\Σ}}}{S}_{\mathrm{kQ}+t}{e}^{j2\mathrm{\πn}(\mathrm{kQ}+t)/N}=\frac{1}{N}\underset{k=0}{\overset{P-1}{\mathrm{\Σ}}}{X}_k\underset{t=0}{\overset{Q-1}{\mathrm{\Σ}}}{c}_t{e}^{j2\mathrm{\πn}(\mathrm{kQ}+t)/N},$ N=P * Q wherein;

2) orthogonal frequency-division multiplex singal characterizes with moulding pulse g (t), adopts the filter of raised cosine roll off frequency characteristic,

Get α=0.25 in the formula.

5. a kind of straight expansion-OFDM modulation-demo-demodulation method that can be used under the high-speed mobile environment according to claim 1 is characterized in that, the signal that described reception sent, and the step of carrying out the OFDM demodulation to the received signal is as follows:

1) receives the signal y that is sent _n, remove Cyclic Prefix to the received signal;

2) convert the signal into frequency domain, carry out following Fourier transform:

$Y_m^{\′}=\underset{n=0}{\overset{N-1}{\mathrm{\Σ}}}{y}_n{e}^{-j2\mathrm{\πnm}/N},$ M=0 wherein ..., N-1.

6. a kind of straight expansion-OFDM modulation-demo-demodulation method that can be used under the high-speed mobile environment according to claim 1, it is characterized in that, signal after the described orthogonal frequency division multiplexed demodulation is through despreader, go despreading with the despreading sequence that prior storage is good, the step of symbol that recovers transmission is as follows:

1) reads the despreading sequence c that has stored in the despreader _m', m=0 wherein ..., Q-1;

2) with OFDM data after demodulating Y _m' with the despreading sequence in the summation of multiplying each other respectively again of each coefficient, finish despreading, obtain following former data symbol after the despreading:

$Y_m=\underset{s=0}{\overset{Q-1}{\mathrm{\Σ}}}{c}_s^{\′}\underset{n=0}{\overset{N-1}{\mathrm{\Σ}}}{y}_n{e}^{-j2\mathrm{\πn}(\mathrm{mQ}+s)/N},$ M=0 wherein ..., Q-1.

7. a kind of straight expansion-OFDM modulation-demo-demodulation method that can be used under the high-speed mobile environment according to claim 1 is characterized in that, describedly separates the original binary source data that mapping recovers transmission and is:

Press the QPSK planisphere, according to quadrant to symbol Y _mCarry out hard decision, it reverts to 2 bit-binary data to each symbol

K=0 wherein ..., P-1.

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