CN109600333B - Method for reducing cyclic prefix - Google Patents

Abstract

The invention belongs to the technical field of wireless communication, and particularly relates to a method for reducing cyclic prefixes, which is used for an OFDM-NOMA system. The method of the invention is to adopt a new transmitting end structure on the traditional OFDM-IDMA system, namely, when OFDM modulation is carried out, CP is only inserted before even symbol frames, while odd symbol frames are not processed, and interference reconstruction is carried out on a receiving end to recover signals. Compared with the traditional method, the method improves the transmission spectrum efficiency while not obviously reducing the bit error rate performance.

Description

Method for reducing cyclic prefix

Technical Field

The invention belongs to the technical field of wireless communication, and particularly relates to a method for reducing Cyclic Prefix (CP). The present invention relates to a Non-Orthogonal Multiple Access (NOMA) technique based on Orthogonal Frequency Division Multiplexing (OFDM).

Background

With the development of mobile communication, transmission efficiency is very important in 5G, and how to save time domain and frequency domain resources more effectively is a problem to be faced. Therefore, based on this, Non-Orthogonal Multiple Access (NOMA) is proposed, and time domain and frequency domain resources on a sub-band are not allocated to only one user but are shared by a plurality of users, so that the frequency spectrum efficiency and the user Access number are improved. The interleaved-Division Multiple Access (IDMA) is a type of NOMA, and is also being investigated for high spectral efficiency, improved performance and low receiver complexity. This scheme relies on interleaving as the only means of distinguishing signals from different users, IDMA inherits many of the advantages of Code Division Multiple Access (CDMA), particularly with respect to diversity against fading and mitigating the worst-case other-cell user interference problems. Furthermore, it allows a very simple layer-by-layer iterative Multi-user Detection (MUD) strategy. The normalized MUD cost (per user) is independent of the number of users.

Existing NOMA is mainly based on multiple carriers, such as Orthogonal Frequency Division Multiplexing (OFDM), and has been widely adopted in many broadband mobile communications due to its inherent robustness to Inter-symbol interference (ISI). In OFDM, to overcome the problem of the sensitivity of a wideband channel to frequency selective fading, the entire channel is divided into a series of narrowband channels that experience flat fading, which are transmitted in parallel to maintain a high data rate, while at the same time, the symbol duration is increased to combat inter-symbol interference. The OFDM-IDMA system combines the advantages of OFDM and IDMA. It can eliminate intersymbol interference through the OFDM technique and suppress multiple access interference through the IDMA technique. In addition, OFDM-IDMA systems use low complexity iterative multi-user detection methods, where the complexity of a single user is independent of the number of multipaths and the number of users.

However, OFDM introduces a Cyclic Prefix (CP), which is a periodic extension of the last part of a symbol to cancel Inter-carrier interference (ICI) and Inter-symbol interference (ISI), which is longer than the Channel Impulse Response (CIR). Although the cyclic prefix introduced by OFDM is useful for mitigating ISI and ICI, this redundancy results in a reduction in the number of channels available for signal transmission, which violates criteria for more efficient communication systems.

Disclosure of Invention

To improve spectral efficiency, this section proposes an OFDM-NOMA scheme that can reduce CP, which uses a cyclic prefix only before even symbol frames, unlike the conventional method that uses a cyclic prefix before each symbol frame. First, in the proposed scheme, half of the spectrum for carrying the cyclic prefix can be saved to transmit useful data. Secondly, this scheme is advantageous to reduce the delay of the system. Thirdly, as can be seen from the simulation results, the scheme can achieve the bit error rate close to that of the traditional OFDM-NOMA system.

The technical scheme of the invention is as follows:

taking an IDMA system as an example, combining the IDMA and the OFDM, and inserting a cyclic prefix only in front of an even symbol frame and not inserting the cyclic prefix in an odd symbol frame at a transmitting end; and at the receiving end, carrying out interference reconstruction to recover the signal. The method specifically comprises the following steps: a transmitting end:

suppose that the OFDM-IDMA system contains K users in total.

S1, for the k-th user, first, data sequence d_kEncoded by a Forward Error Correction (FEC) code, such as a convolutional, turbo, or LDPC code, to generate an encoded code sequence b_k. The coding sequence b is then further extended_kTo further reduce the code rate, thereby producing a low code rate spreading sequence c_k；

S2, spreading sequence c_kInterleaver pi feeding to the kth user_kForming a scrambling sequence X_kWherein the interleaver { pi_kIs different for different users;

s3, scrambling the sequence X_kQAM modulating to obtain S_k；

S4, modulating the signal S by Inverse Fast Fourier Transform (IFFT)_kTransform to time domain and place cyclic prefix only in front of even frame symbol vector.

Receiving end:

s5, detecting even frame signals, firstly removing cyclic prefix, and transforming the signals after removing the cyclic prefix to a frequency domain through Fast Fourier Transform (FFT); then adopting Turbo iterative detection to recover even frame user signal

The Turbo iterative detection structure comprises an Elementary Signal Estimator (ESE) and K Decoders (DEC) except for a de-interleaver and an interleaver. The function of the ESE estimator is mainly to estimate the soft information of all users' transmitted signals, when only the multiple access interference between users is considered and the coding constraints are not considered. The DEC decoder for each subscriber is an FEC encoder and spreader corresponding to the transmitting end, and performs corresponding despreading and channel decoding to generate soft information for the chip sequence. The ESE estimator and the K DEC decoders are cascaded through a de-interleaver and an interleaver, and soft information of user signals is mutually transmitted between the ESE estimator and the K DEC decoders. The receiver of the IDMA system can greatly reduce the complexity of signal detection by separately considering multiple access interference and coding constraints. When the receiver reaches the iteration times set by budget, the DEC decoder makes hard decision on the data bit of each user and outputs the final decision result.

S6, use

Reconstructing ISI^(2i→2i+1)；

Wherein ISI^(2i→2i+1)Representing the interference of the 2i even frame to the 2i +1 odd frame.

S61, converting the even frame user signal frequency domain signal

Into a time domain signal by Inverse Fast Fourier Transform (IFFT)

S62 reconstruction ISI^(2i→2i+1)；

Where N denotes the frame length, N_gIs the length of the cyclic prefix and,

the channel impulse response of the k-th user of the even frame is shown, and L represents the multi-path time delay of the channel.

S7 reconstruction ISI^{(2i+1→2i+2)}；

Wherein ISI^{(2i+1→2i+2)}Representing the interference of the 2i +1 odd frame to the 2i +2 even frame.

S8 reconstruction

Wherein the content of the first and second substances,

a received signal representing the reconstructed odd frame, r²ⁱ⁺¹Representing the received signal of an odd frame.

S9, detection of even frame-like signals by Fast Fourier Transform (FFT)

Transforming to frequency domain, then adopting Turbo iterative detection to recover odd frame user signal

S10, finally, combining the odd frame signals

And even frame signal

And (6) outputting.

The technical scheme of the invention provides a scheme capable of reducing the use of cyclic prefix aiming at the traditional OFDM-IDMA system. The invention has the beneficial effects that: the bit error rate performance of the system is not obviously reduced, and the transmission spectrum efficiency is improved.

Drawings

Fig. 1 is a basic model of a system transmitter according to the present invention;

fig. 2 is a basic model of a system receiver proposed by the present invention;

fig. 3 is a simulation diagram comparing the error rate performance of the proposed system and the conventional OFDM-IDMA system.

Detailed Description

The technical scheme of the invention is described in detail in the following with reference to the accompanying drawings and embodiments:

in this example, the number of users is 4, a convolutional code with a code rate of 1/2 is used, the spreading sequence is { -1, +1, -1, +1}, the modulation scheme is QPSK modulation, the number of subcarriers is 128, and the number of multipath channels is 16. The example adopts the following steps:

step 1: generating initial data of 4 groups of users, respectively generating 32bits, respectively and sequentially coding, spreading, interleaving, modulating by QPSK and then sending to a Fourier transform processor;

step 2: performing fast Fourier transform on the data in a Fourier transform processor, adding a cyclic prefix only before an even frame, and sending out the odd frame without changing;

and step 3: sending the data to a receiving end after passing through a multipath channel, carrying out ESE detection, de-interleaving, de-spreading and decoding on the even-numbered frames, completing iterative cycle, and recovering the even-numbered frame data;

and 4, step 4: interference ISI reconstruction with data of even frames for odd frames^(2i→2i+1)And ISI^{(2i+1→2i+2)}And then reconstruct

Then carrying out ESE detection, de-interleaving, de-spreading and decoding, completing an iterative cycle, and recovering odd frame data;

and 5: and finally, recombining data output of the odd frame and the even frame and comparing the data output with initial data to obtain BER.

As can be seen from fig. 3, an OFDM-IDMA system proposed herein can reduce the usage of cyclic prefix, and improve the transmission spectrum efficiency without significantly degrading the bit error rate performance of the system.

Claims (1)

1. A method for reducing cyclic prefix is used for OFDM-NOMA system, the system adopts QPSK modulation, the number of users is K, the method is characterized by comprising the following steps:

a transmitting end:

s1, for the k-th user, first, data sequence d_kEncoded by a forward error correction code to generate an encoded code sequence b_kThen, the coding sequence b is further extended_kTo further reduce the code rate, resulting in a low code rate spreading sequence c_k；

S2, spreading sequence c_kInterleaver pi feeding to the kth user_kForming a scrambling sequence X_kWherein the interleaver { pi_kIs different for different users;

s3, scrambling the sequence X_kQAM modulating to obtain S_k；

S4, converting the modulated signal S through fast Fourier inverse transformation_kTransforming to time domain, and placing cyclic prefix only in front of even frame symbol vector;

receiving end:

s5, detecting even frame signals, firstly removing cyclic prefix, and transforming the signals after removing the cyclic prefix to a frequency domain through fast Fourier transform; then adopting Turbo iterative detection to recover even frame user signal

S6, use

Reconstructing ISI^(2i→2i+1)；

Wherein ISI^(2i→2i+1)Representing the interference of the 2i even frame to the 2i +1 odd frame; the method specifically comprises the following steps:

s61, converting the even frame user signal frequency domain signal

Into a time-domain signal by inverse fast Fourier transform

S62 reconstruction ISI^(2i→2i+1)：

Where N denotes the frame length, N_gIs the length of the cyclic prefix and,

representing the channel impulse response of the kth user of an even frame, and L representing the multi-path time delay of the channel;

s7 reconstruction ISI^{(2i+1→2i+2)}：

Wherein ISI^{(2i+1→2i+2)}Representing the interference of the 2i +1 odd frame to the 2i +2 even frame;

s8 reconstruction

Wherein the content of the first and second substances,

a received signal representing the reconstructed odd frame, r²ⁱ⁺¹A received signal representing an odd frame;

s9, classDetection of the even-like frame signal by fast Fourier transform

Transforming to frequency domain, then adopting Turbo iterative detection to recover odd frame user signal

S10 combination odd frame message

And even frame signal

And (6) outputting.

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