CN103501186A

CN103501186A - Mutual-complementing code CDMA (Code Division Multiple Access) system of time-frequency mixing separation sub-code structure

Info

Publication number: CN103501186A
Application number: CN201310493002.4A
Authority: CN
Inventors: 孟维晓; 孙思月; 陈晓华
Original assignee: Harbin Institute of Technology
Current assignee: Harbin Institute of Technology
Priority date: 2013-10-21
Filing date: 2013-10-21
Publication date: 2014-01-08
Anticipated expiration: 2033-10-21
Also published as: CN103501186B

Abstract

The invention relates to a mutual-complementing code CDMA (Code Division Multiple Access) system of a time-frequency mixing separation sub-code structure, relates to a mutual-complementing code CDMA system, and aims to reduce multipath interference in a sub-band of each subcarrier in a communication system. The signal transmission process comprises the following steps of replicating data of a user k into W parts, W subcarrier spread spectrum modules respectively adopting a set of corresponding sub-codes to spread the data of the user k and respectively modulating onto W subcarriers, and then superposing W subcarriers and sending the subcarriers to a wireless channel. The signal reception process comprises the following steps that a receiver respectively demodulates the data on W sub-carriers, W sub-carrier despreading modules respectively despread the signals spreaded by each sub-code on each path by using the corresponding sub-code, and decision is carried out to output after three combinations. The invention is suitable for wireless communication occasions.

Description

Complementary code CDMA system of time-frequency mixed sub-carrier code structure

Technical Field

The present invention relates to a complementary code CDMA system.

Background

Among the cellular wireless communication related technologies, the multiple access technology is one of the most core physical layer technologies, which fundamentally determines the way in which multiple users and multiple channels share radio resources in a communication system, and the choice of the way also finally significantly affects the quality of service provided to end users. Therefore, the development of multiple access techniques marks the evolution of cellular mobile communication systems from generation to generation: from the first generation of Frequency Division Multiple Access (FDMA) analog mobile communication systems, to the second generation of Time Division Multiple Access (TDMA) GSM systems and narrowband Code Division Multiple Access (CDMA) IS-95 systems, to the third generation of systems that widely use wideband CDMA technology, to the fourth generation of orthogonal frequency Division Multiple Access (ofdma) mobile communication systems.

Although CDMA has many advantages such as high security, high frequency reuse factor, resistance to narrowband interference and multipath fading, compared with frequency division and time division multiple access, all CDMA systems are interference limited, especially when considering that the communication system has problems such as multipath interference and asynchronous transmission. Therefore, the CDMA multiple access scheme is facing the embarrassment of gradually fading out the stage of cellular mobile communication.

As the name implies, CDMA relies on the orthogonality of the address/signature codes (SignatureCodes) it employs to distinguish between different users/different channels, as compared to FDMA and TDMA. Thus, the choice of code fundamentally determines the performance of a CDMA system. The direct reason for the limited interference of CDMA systems is that the correlation properties of the spreading codes used are not ideal, i.e.: 1) the autocorrelation function of the spreading sequence has a small side peak at other shifts besides one peak at zero shift. In a multipath transmission environment, the side peaks are collected in a correlator at a receiving end, so that multipath interference is caused; 2) the cross-correlation function of the two spreading sequences has a non-zero value. In multipath or inter-user asynchronous transmission environment, these non-zero correlation values will be collected by the correlator at the receiving end, thereby causing multiple access interference and near-far effect. Ultimately, even if a series of auxiliary subsystems or techniques are used to solve these problems, such as complex power control, multi-user detection techniques, etc., the CDMA system still cannot provide competitive system performance.

In order to accelerate the innovation of code division technology and to promote the code division technology to return to the stage, a new solution of next generation CDMA based on a novel class of spreading Codes, Complementary Codes (CCs), is proposed. The complementary codes adopt a code design concept combined with a real communication environment, and can really realize ideal autocorrelation and cross-correlation characteristics, thereby endowing the CDMA technology with the capability of getting rid of interference limitation. The fundamental reason why the complementary codes can achieve ideal correlation characteristics is the structure of multiple subcodes, i.e. each user needs to spread with multiple subcodes contained in one complementary code to achieve the desired ideal correlation characteristics. However, the particularity of the complementary code structure brings new problems to the design of the corresponding CDMA system. In order to support the complementary code based spreading structure and implement the complementary correlation based detection procedure, a complementary code based CDMA (CC-CDMA) system needs to satisfy: effectively separating four conditions of sub-code, sub-code sequence correspondence, sub-code displacement synchronization, correlation value and other gain combination. To meet the above requirements, CC-CDMA system structures can be classified into two types according to the way of separating complementary code subcodes: 1) a serial time division CC-CDMA system; 2) a parallel frequency division CC-CDMA system. The CC-CDMA system with serial time division structure adopts different time slices to serially send signals after different subcode spread, and has the advantages of completely eliminating multipath interference when the guard interval is not less than the maximum time delay spread of a multipath channel, thereby obtaining excellent multipath diversity gain. The disadvantage is that the communication between multiple users in a serial time division CC-CDMA system needs to be synchronous or quasi-synchronous in order to ensure effective separation of the subcodes in the CC-CDMA system. That is, the relative time delays of different users arriving at the receiver due to synchronization errors and propagation delays must be within a certain range, which facilitates the determination of the guard interval. The introduction of the guard interval also reduces the spectral efficiency of the system. In contrast, the CC-CDMA system using parallel frequency division transmits signals spread by different subcodes in parallel using different carriers. A group of subcarriers may be either subcarriers with or without orthogonality. The parallel frequency division CC-CDMA system can completely eliminate multiple access interference in synchronous and asynchronous communication, and further, the near-far effect problem in the traditional CDMA system can not exist. Therefore, frequency division CC-CDMA systems have become more popular with researchers in recent years than time division schemes. However, the performance of a parallel frequency division CC-CDMA system is sensitive to frequency selective fading caused by multipath. This is because frequency selective fading will cause different sub-codes to experience different fading, resulting in the inability to achieve equal gain combining in the complementary correlation definition. Especially in uplink communication, different carriers of various users experience different fading, and "equal gain combining" cannot be recovered at the receiving end by any combining criterion. Although, some novel detection algorithms are proposed to improve the performance of the parallel frequency division CC-CDMA system under the frequency selective fading channel. These schemes are only applicable in cases where frequency selective fading is present over the entire bandwidth, but flat fading is present within each subcarrier. When selective fading also occurs in the bandwidth of each subcarrier, i.e. the coherence bandwidth of the channel is smaller than the bandwidth of each subcarrier, these detection algorithms are no longer applicable and the performance of the corresponding CC-CDMA system will also be degraded by the influence of multiple access and multipath interference.

Disclosure of Invention

The present invention provides a complementary code CDMA system of a time-frequency hybrid split sub-code structure in order to reduce multi-path interference within a sub-band of each sub-carrier in a communication system.

A complementary code CDMA system of a time-frequency hybrid split-ion code structure,

for user k, the signal transmission process of the transmitter:

step one, data b of a user k is acquired^(k)Copying W parts, wherein W is a positive integer, and respectively transmitting the W parts to W subcarrier spreading modules, and each subcarrier spreading module respectively corresponds to Q subcodes;

step two, W subcarrier spread spectrum modules respectively adopt a group of corresponding subcodes to spread spectrum to the data of the user k to obtain a signal after spread spectrum

The specific process of the spread spectrum of each subcarrier spread spectrum module is as follows:

firstly, spreading each data in a subcarrier by adopting Q sub-codes respectively, wherein Q is a positive integer;

then, respectively carrying out serial/parallel conversion on the spread data, splicing the data together in sequence, and inserting a guard interval with the length of G chips between two adjacent subcode spread signals, wherein G is a positive integer;

finally, the spliced data is subjected to parallel/serial conversion to obtain an output signal of the subcarrier spread spectrum module

w=1,2,...,W；

Step three, the spread signal obtained in the step two is used

Modulated to W sub-carriers f respectively₁，f₂，…，f_WThen, the W sub-carriers are superposed and sent to a wireless channel; finishing signal transmission of the transmitter;

for user g, the signal receiving process of the receiver:

step four, independently modeling the sub-channels of each sub-carrier, wherein the channels of the W sub-carriers are modeled into distinguishable paths with the same number L and corresponding to the time delay tau of each path₁,τ₂,…,τ_LThe same independent and identically distributed tapped-line delay model; receiver separately demodulates W sub-carriers f₁，f₂，…，f_WData on, resulting W baseband signals

Respectively transmitting the data to W subcarrier despreading modules, wherein each subcarrier despreading module respectively corresponds to Q subcodes;

step five, the W subcarrier frequency-removing module respectively adopts corresponding subcodes to de-spread and combine the signals spread by each subcode on each path to obtain W subcarrier de-spread signals;

step six, the signals despread by the W subcarriers obtained in the step five are weighted

Merging to obtain merged resultOutputting after judgment;

completing the signal reception of the receiver.

In the second step, the method for spreading each data in the subcarriers by using Q subcarriers is the same for the 1 st subcarrier, specifically:

the signal after spreading with the qth sub-code is:

wherein:represents the j data sent by user k in a data block, j belongs to {1,2, …, x }, wherein x represents the number of bits sent by each subcarrier, and N is the code length of each subcode; t is_cIs one code chipThe duration of (d); b is the length of the data block; t is time; p is a radical of_kFor transmit power, Q ∈ {1,2, …, Q },

the spreading waveform for user k, corresponding to sub-carrier 1 qth sub-code, is expressed as:

wherein, (t) is a chip transmission pulse waveform N ∈ {1,2, …, N };

let q (t) be a pulse shape, then:

in the step five, the method for despreading and combining the signals spread by the W subcarrier despreading modules on each path by using the corresponding subcodes respectively uses the 1 st subcarrier to be the same, and specifically comprises the following steps:

fifthly, aligning the signals of all paths by adopting a first group of delays, namely: delay tau_l,gThe corresponding subsequent processing is to detect the signal of the ith path in the propagation channel of the user g; and performing alignment operation on the signal spread by the sub-code by using a second group of delays, namely: the subsequent processing corresponding to the delay (q-1) delta is to adopt the q-th sub-code for de-spreading;

step five, respectively carrying out matched filtering on the L paths and the Q sub-codes, namely despreading to obtain Q despread signals u on each path_l,1,u_l,2,…u_l,Q，l=1,2,...,L；

Assuming that the receiver and the signal of the user g realize ideal carrier, bit and chip synchronization, the specific process of despreading the jth data of the user g by the qth sub-code in the ith path is as follows:

<math> <mrow> <msubsup> <mi>u</mi> <mrow> <mi>q</mi> <mo>,</mo> <mi>l</mi> </mrow> <mrow> <mo>(</mo> <mi>g</mi> <mo>)</mo> </mrow> </msubsup> <mrow> <mo>(</mo> <mi>j</mi> <mo>)</mo> </mrow> <mo>=</mo> <msub> <mrow> <msubsup> <mo>&Integral;</mo> <mn>0</mn> <msub> <mi>NT</mi> <mi>c</mi> </msub> </msubsup> <mi>r</mi> </mrow> <mn>1</mn> </msub> <mrow> <mo>(</mo> <mi>t</mi> <mo>+</mo> <mi>j</mi> <msub> <mi>T</mi> <mi>b</mi> </msub> <mo>+</mo> <msub> <mi>τ</mi> <mrow> <mi>l</mi> <mo>,</mo> <mi>g</mi> </mrow> </msub> <mo>+</mo> <mrow> <mo>(</mo> <mi>q</mi> <mo>-</mo> <mn>1</mn> <mo>)</mo> </mrow> <mi>Δ</mi> <mo>)</mo> </mrow> <msubsup> <mi>C</mi> <mrow> <mn>1</mn> <mo>,</mo> <mi>q</mi> </mrow> <mrow> <mo>(</mo> <mi>g</mi> <mo>)</mo> </mrow> </msubsup> <mrow> <mo>(</mo> <mi>t</mi> <mo>)</mo> </mrow> <mi>dt</mi> </mrow> </math>

wherein L is equal to {1,2, …, L }, Q is equal to {1,2, …, Q }, j is equal to {1,2, …, x }, T is equal to {1,2, …, x }, and T is equal to_b=(xN+G)Tc，△=GT_c，

A spreading waveform for user g corresponding to sub-carrier 1 qth sub-code;

step five and three, obtaining Q despread signals u aiming at each path_l,1,u_l,2,…u_l,QAnd performing equal gain combination to complete the complementary correlation process of the complementary codes, wherein L is equal to {1,2, L …, L },

namely:

step five, combining the obtained detection signals on each path by adopting a maximum ratio criterion, namely:

wherein,

a complex attenuation factor of the l path experienced by the 1 st subcarrier of the user g is represented by calculating a conjugate; the despreading process for subcarrier 1 is completed.

Compared with the traditional CC-CDMA with parallel frequency division, the time-frequency mixed molecular code scheme provided by the invention can realize ideal autocorrelation characteristics by utilizing a plurality of sub-codes on each sub-carrier, thereby effectively resisting multipath interference in the sub-band of each sub-carrier. Under the same three-path channel (channel fading factor variance is [0.7, 0.2, 0.1 ]]Time delay of [0, 2, 4 ]]) The bit error rate of the two systems is shown in fig. 6 under the simulation condition that 4 users exist in the system and all adopt 4 subcarriers. Presence informationWhen the noise ratio is 10dB, the error rate of the system can reach 3 multiplied by 10^-4Whereas the traditional parallel frequency division system only reaches 6 x 10^-3。

Drawings

FIG. 1 is a schematic diagram of a transmitter in the system of the present invention;

fig. 2 is a schematic structural diagram of a subcarrier spreading module taking subcarrier 1 as an example;

fig. 3 is a schematic diagram of a signal structure after spreading by a subcarrier spreading module, taking subcarrier 1 as an example;

FIG. 4 is a schematic diagram of a receiver in the system of the present invention;

fig. 5 is a schematic structural diagram of a subcarrier despreading module taking subcarrier 1 as an example;

FIG. 6 is a schematic diagram of comparing the bit error rate of the hybrid molecular code structure CC-CDMA system of the present invention with that of the conventional parallel frequency division CC-CDMA system.

Detailed Description

Detailed description of the preferred embodimentsthe present embodiment, a complementary code CDMA system of a time-frequency hybrid CDMA structure,

the signal transmission process of the transmitter comprises the following steps:

taking the transmission process of user k as an example, the transmitter structure is shown in fig. 1 and 2, and the transmission process mainly includes the following steps:

step one, data b of user k^(k)After copying W parts, the signals respectively enter W subcarrier spreading modules corresponding to the W groups of subcodes, as shown in fig. 1.

Step two, W sub-carrier spreading module respectively adopts a group of corresponding sub-codes to spread the data of user k, and the first sub-carrier spreading moduleFor example, the process is shown in fig. 2, and the structure of the signal after spreading is shown in fig. 3.

Represents the jth data transmitted by user k in a data block, j ∈ {1,2, …, x }, where x represents the number of bits transmitted per data block, determined by the time-varying characteristics of the channel. T is_cIs a duration of one chip, T_gThe length of the guard interval between data spread by different subcodes is determined by a channel estimation module according to transmission delay, maximum delay spread of a channel and synchronization error.

The specific process is that firstly, each data in the data block is spread by Q sub-codes respectively, and in the sub-carrier 1, the signal spread by the Q sub-code is

Wherein Q ∈ {1,2, …, Q }, pk is the transmission power,

where q (t) is a chip transmission pulse waveform. For simplicity, we can assume q (t) to be a rectangular pulse, i.e.:

<math> <mrow> <mi>q</mi> <mrow> <mo>(</mo> <mi>t</mi> <mo>)</mo> </mrow> <mo>=</mo> <mfenced open='{' close='' separators=''> <mtable> <mtr> <mtd> <mfrac> <mn>1</mn> <msqrt> <msub> <mi>T</mi> <mi>c</mi> </msub> </msqrt> </mfrac> </mtd> <mtd> <mn>0</mn> <mo>≤</mo> <mi>t</mi> <mo>≤</mo> <mi>T</mi> </mtd> </mtr> <mtr> <mtd> <mn>0</mn> </mtd> <mtd> <mi>elsewhere</mi> </mtd> </mtr> </mtable> <mrow> <mo>-</mo> <mo>-</mo> <mo>-</mo> <mrow> <mo>(</mo> <mn>3</mn> <mo>)</mo> </mrow> </mrow> </mfenced> </mrow> </math>

then, the spread data are respectively serial/parallel converted, then spliced together in sequence, and a guard interval with the length of G chips, namely T, is inserted between two adjacent subcode spread signals_g=GT_c. The spliced data is subjected to parallel/serial conversion to obtain an output signal of the subcarrier spread spectrum module

The structure is shown in fig. 3.

Length T of guard interval_gThe maximum delay spread of the sub-channels and the maximum value of the synchronization error among multiple users are determined, namely: t is_g≥|τ_l,k-τ_z,g+θ_k-θ_gL, where τ is_l,kAnd τ_z,gPropagation delay, theta, of any path of any user, respectively_kAnd theta_gRespectively, the time delay of the signal of any two users arriving at the receiver.

Step three, as shown in figure 1, WSignal output by subcarrier spread spectrum module

Modulated to W sub-carriers f respectively₁，f₂，…，f_WAnd finally, the W sub-carriers are superposed and then are sent to a wireless channel.

Signal receiving process of the receiver:

the invention is suitable for frequency selective fading channels, and the coherent bandwidth of the channel is less than the bandwidth of one subcarrier. And independently modeling the sub-channel of each sub-carrier. Since the bandwidth of each subcarrier is the same, the channel of W subcarriers can be modeled as having the same number of resolvable paths (L paths) and the same time delay (τ) for each path₁,τ₂,…,τ_L) Independent and identically distributed tapped-line delay models. Assuming that the w-th subcarrier is a slow fading channel of L path, its equivalent low-pass channel can be described by the following time-varying impulse response:

wherein,

complex attenuation factor, τ, for the l path experienced by the w sub-carrier of user k_l,k(t) is the propagation delay of the path. Assuming that the channel is a slowly varying channel, i.e. assuming that

And τ_l,k(t) is constant for at least x bit transmission times.

In the system, K users communicate simultaneously, taking the receiving process of user g as an example, the receiver structure is shown in fig. 4 and 5, and the signal detection process of the receiver mainly includes the following steps:

step four, as shown in FIG. 4The receiver demodulates W sub-carriers f respectively₁，f₂，…，f_WData on, resulting W baseband signals

Respectively enters W subcarrier despreading modules corresponding to W groups of subcodes.

And step five, the W subcarrier frequency-demodulating module respectively adopts corresponding subcodes to despread and combine the signal spread by each subcode on each path. Taking the first sub-carrier despreading block as an example, the process is shown in fig. 5. The method comprises the following substeps:

in step fifthly, fig. 5, the first set of delays is used to align the signals of the paths, i.e., delay τ_l,gThe corresponding subsequent processing is to detect the signal of the l-th path in the propagation channel of user g. The second set of delays is used to align the signal spread by the sub-codes, i.e. the subsequent processing corresponding to delay (q-1) Δ is despreading with the qth sub-code.

And step five, performing matched filtering, namely despreading on the L paths and the Q sub-codes respectively. Assuming that the receiver and the signal of the user g realize ideal carrier, bit and chip synchronization, the specific process of despreading the jth data of the user g by the qth sub-code in the ith path is as follows:

<math> <mrow> <msubsup> <mi>u</mi> <mrow> <mi>q</mi> <mo>,</mo> <mi>l</mi> </mrow> <mrow> <mo>(</mo> <mi>g</mi> <mo>)</mo> </mrow> </msubsup> <mrow> <mo>(</mo> <mi>j</mi> <mo>)</mo> </mrow> <mo>=</mo> <msub> <mrow> <msubsup> <mo>&Integral;</mo> <mn>0</mn> <msub> <mi>NT</mi> <mi>c</mi> </msub> </msubsup> <mi>r</mi> </mrow> <mn>1</mn> </msub> <mrow> <mo>(</mo> <mi>t</mi> <mo>+</mo> <mi>j</mi> <msub> <mi>T</mi> <mi>b</mi> </msub> <mo>+</mo> <msub> <mi>τ</mi> <mrow> <mi>l</mi> <mo>,</mo> <mi>g</mi> </mrow> </msub> <mo>+</mo> <mrow> <mo>(</mo> <mi>q</mi> <mo>-</mo> <mn>1</mn> <mo>)</mo> </mrow> <mi>Δ</mi> <mo>)</mo> </mrow> <msubsup> <mi>C</mi> <mrow> <mn>1</mn> <mo>,</mo> <mi>q</mi> </mrow> <mrow> <mo>(</mo> <mi>g</mi> <mo>)</mo> </mrow> </msubsup> <mrow> <mo>(</mo> <mi>t</mi> <mo>)</mo> </mrow> <mi>dt</mi> <mo>-</mo> <mo>-</mo> <mo>-</mo> <mrow> <mo>(</mo> <mn>5</mn> <mo>)</mo> </mrow> </mrow> </math>

wherein L is equal to {1,2, …, L }, Q is equal to {1,2, …, Q }, j is equal to {1,2, …, x }, T is equal to {1,2, …, x }, and T is equal to_b=(xN+G)T_c，△=GT_c，

The spreading waveform for user g corresponding to sub-carrier 1 qth sub-code is shown in equation (2).

Step five and three, obtaining Q despread signals u aiming at each path_l,1,u_l,2,…u_l,QAnd L ∈ {1,2, L } is subjected to equal gain combination to complete the complementary correlation process of the complementary codes, that is:

step five four, as shown in fig. 5, the obtained detection signals on each path are combined by adopting a maximum ratio criterion, that is:

whereinThe complex attenuation factor for the l-th path experienced by the 1 st subcarrier of user g indicates the conjugation. This completes the despreading process for subcarrier 1.

Step six, as shown in fig. 4, the signals despread by the W subcarriers obtained in the step two are weighted

Merging:

wherein, mu_wThe combining weight coefficient corresponding to the w-th sub-carrier signal can be obtained by a maximum ratio combining criterion, a minimum mean square error combining criterion or an adaptive combining manner. Taking the minimum mean square error merging criterion as an example, the weight coefficients are obtained as follows:

wherein N is_uTo activate the number of users, E_bIs bit energy, N₀Is the power spectral density of channel gaussian white noise.Final combined result

The incoming decider completes the receiving process of the present invention.

A CC-CDMA system uses a family of complementary codes C (K, M, N) as the address codes for K users, where M is the number of subcodes that a complementary code contains and N is the code length of each subcode, i.e., the number of chips it contains. Assume that the complementary code assigned to user k is

<math> <mrow> <msup> <mi>C</mi> <mrow> <mo>(</mo> <mi>k</mi> <mo>)</mo> </mrow> </msup> <mo>=</mo> <mo>{</mo> <msubsup> <mi>c</mi> <mi>m</mi> <mrow> <mo>(</mo> <mi>k</mi> <mo>)</mo> </mrow> </msubsup> <msubsup> <mo>}</mo> <mrow> <mi>m</mi> <mo>=</mo> <mn>1</mn> </mrow> <mi>M</mi> </msubsup> <mo>&Element;</mo> <mi>C</mi> <mrow> <mo>(</mo> <mi>K</mi> <mo>,</mo> <mi>M</mi> <mo>,</mo> <mi>N</mi> <mo>)</mo> </mrow> <mo>,</mo> <mi>k</mi> <mo>&Element;</mo> <mo>{</mo> <mn>1,2</mn> <mo>,</mo> <mo>.</mo> <mo>.</mo> <mo>.</mo> <mi>K</mi> <mo>}</mo> <mo>.</mo> </mrow> </math>

Wherein,

c_{m}^{(k)} = [c_{m, 1}^{(k)}, c_{m, 2}^{(k)}, . . ., c_{m, N}^{(k)}]

is the mth subcode (subsequence), M is e to {1,2, L, M },n belongs to {1,2, L, N }. In the CC-CDMA system of the time-frequency mixed sub-code structure of the present invention, K sub-codes allocated to users are divided into W = M/Q groups, i.e.

S_{2}^{(k)} = {c_{m}^{(k)}}_{m = Q + 1}^{2 Q} . . . S_{W}^{(k)} = {c_{m}^{(k)}}_{m = M - Q + 1}^{M} .

The complementary code has the following two characteristics.

Characteristic 1: the complementary codes have ideal complementary aperiodic correlation characteristics

<math> <mrow> <mi>ρ</mi> <mrow> <mo>(</mo> <msup> <mi>C</mi> <mrow> <mo>(</mo> <mi>k</mi> <mo>)</mo> </mrow> </msup> <mo>,</mo> <msup> <mi>C</mi> <mrow> <mo>(</mo> <mi>g</mi> <mo>)</mo> </mrow> </msup> <mo>,</mo> <mi>δ</mi> <mo>)</mo> </mrow> <mo>=</mo> <munderover> <mi>Σ</mi> <mrow> <mi>m</mi> <mo>=</mo> <mn>1</mn> </mrow> <mrow> <mi>M</mi> <mo>-</mo> <mn>1</mn> </mrow> </munderover> <munderover> <mi>Σ</mi> <mrow> <mi>n</mi> <mo>=</mo> <mn>1</mn> </mrow> <mrow> <mi>N</mi> <mo>-</mo> <mi>δ</mi> </mrow> </munderover> <msubsup> <mi>c</mi> <mrow> <mi>m</mi> <mo>,</mo> <mi>n</mi> </mrow> <mrow> <mo>(</mo> <mi>k</mi> <mo>)</mo> </mrow> </msubsup> <msubsup> <mi>c</mi> <mrow> <mi>m</mi> <mo>,</mo> <mi>n</mi> <mo>+</mo> <mi>δ</mi> </mrow> <mrow> <mo>(</mo> <mi>g</mi> <mo>)</mo> </mrow> </msubsup> <mo>=</mo> <mfenced open='{' close=''> <mtable> <mtr> <mtd> <mi>MN</mi> <mo>,</mo> </mtd> <mtd> <mi>δ</mi> <mo>=</mo> <mn>0</mn> <mo>,</mo> <mi>k</mi> <mo>=</mo> <mi>g</mi> </mtd> </mtr> <mtr> <mtd> <mn>0</mn> <mo>,</mo> </mtd> <mtd> <mi>elsewhere</mi> </mtd> </mtr> </mtable> </mfenced> </mrow> </math>

Where K ∈ {1,2, …, K }, and δ is the relative chip shift.

Characteristic 2: w groups of subcodes of complementary codes

Has ideal complementary non-periodic autocorrelation characteristics

<math> <mrow> <mi>ρ</mi> <mrow> <mo>(</mo> <msubsup> <mi>S</mi> <mi>w</mi> <mrow> <mo>(</mo> <mi>k</mi> <mo>)</mo> </mrow> </msubsup> <mo>,</mo> <msubsup> <mi>S</mi> <mi>w</mi> <mrow> <mo>(</mo> <mi>k</mi> <mo>)</mo> </mrow> </msubsup> <mo>;</mo> <mi>δ</mi> <mo>)</mo> </mrow> <munderover> <mi>Σ</mi> <mrow> <mi>q</mi> <mo>=</mo> <mrow> <mo>(</mo> <mi>w</mi> <mo>-</mo> <mn>1</mn> <mo>)</mo> </mrow> <mi>Q</mi> <mo>+</mo> <mn>1</mn> </mrow> <mi>wQ</mi> </munderover> <munderover> <mi>Σ</mi> <mrow> <mi>n</mi> <mo>-</mo> <mn>1</mn> </mrow> <mrow> <mi>N</mi> <mo>-</mo> <mi>δ</mi> </mrow> </munderover> <msubsup> <mi>c</mi> <mrow> <mi>q</mi> <mo>,</mo> <mi>n</mi> </mrow> <mrow> <mo>(</mo> <mi>k</mi> <mo>)</mo> </mrow> </msubsup> <msubsup> <mi>c</mi> <mrow> <mi>q</mi> <mo>,</mo> <mi>n</mi> <mo>+</mo> <mi>δ</mi> </mrow> <mrow> <mo>(</mo> <mi>k</mi> <mo>)</mo> </mrow> </msubsup> <mo>=</mo> <mfenced open='{' close=''> <mtable> <mtr> <mtd> <mi>QN</mi> </mtd> <mtd> <mi>δ</mi> <mo>=</mo> <mn>0</mn> </mtd> </mtr> <mtr> <mtd> <mn>0</mn> </mtd> <mtd> <mi>elsewhere</mi> </mtd> </mtr> </mtable> </mfenced> </mrow> </math>

Where K is {1,2, …, K }, W is {1,2, …, W }, and δ is the relative chip shift.

Compared with the traditional CC-CDMA with parallel frequency division, the time-frequency mixed molecular code scheme provided by the invention can realize ideal autocorrelation characteristics by utilizing a plurality of sub-codes on each sub-carrier, thereby effectively resisting multipath interference in the sub-band of each sub-carrier. Under the same three-path channel (channel fading factor variance is [0.7, 0.2, 0.1 ]]Time delay of [0, 2, 4 ]]) The bit error rate of the two systems is shown in fig. 6 under the simulation condition that 4 users exist in the system and all adopt 4 subcarriers. When the signal-to-noise ratio is 10dB, the bit error rate of the system can reach 3 multiplied by 10^-4Whereas the traditional parallel frequency division system only reaches 6 x 10^-3。

Claims

1. A complementary code CDMA system of a time-frequency hybrid split sub-code structure is characterized in that:

for user k, the signal transmission process of the transmitter:

finally, the spliced data is subjected to parallel/serial conversion to obtain an output signal of the subcarrier spread spectrum modulew=1,2,...,W；

Step three, the spread signal obtained in the step two is usedModulated to W sub-carriers f respectively₁，f₂，…，f_WThen, the W sub-carriers are superposed and sent to a wireless channel; finishing signal transmission of the transmitter;

for user g, the signal receiving process of the receiver:

Merging to obtain merged result

Outputting after judgment;

completing the signal reception of the receiver.

2. The system according to claim 1, wherein in step five, the W subcarrier despreading modules respectively despread and combine the signals spread by each subcarrier on each path by using the corresponding subcodes, and the methods of despreading and combining the signals by using the 1 st subcarrier are the same, specifically:

Step five and three, obtaining Q despread signals u aiming at each path_l,1,u_l,2,…u_l,QAnd performing equal gain combination to complete the complementary correlation process of the complementary codes, wherein L belongs to {1,2, L, L },

namely:

wherein,