CN108616475B - Subcarrier index differential modulation method for OFDM system - Google Patents

Abstract

The invention belongs to the technical field of wireless communication, and particularly relates to a differential subcarrier index modulation method for an OFDM system. In the proposed scheme, bit information is transmitted jointly by subcarrier index and PSK symbol, while channel information is no longer needed for detection by differential modulation of subcarrier index. From simulation results, the performance of the D-IM-OFDM system is superior to that of the traditional D-OFDM system, the performance loss is about 1.5dB compared with that of the SIM-OFDM system, and meanwhile the PAPR of the D-IM-OFDM system is lower than that of the OFDM and SIM-OFDM systems. The scheme simultaneously achieves the advantage that channel information is not needed for DSM detection while retaining the SIM-OFDM performance advantage.

Description

Subcarrier index differential modulation method for OFDM system

Technical Field

The invention belongs to the technical field of wireless communication, and particularly relates to a differential subcarrier index modulation method for an OFDM system; the present invention relates to techniques such as Differential Spatial Modulation (DSM) and OFDM (Multiple Input Multiple Output).

Background

The ofdm (orthogonal Frequency Division multiplexing) technology is a high-speed transmission technology for wireless communication, and its basic principle is to decompose a high-speed data stream into many low-rate sub-data streams, i.e. to divide a signal into many orthogonal sub-carriers, and to simultaneously transmit the sub-carriers by using the mutually orthogonal sub-carriers. The technology utilizes the subcarrier to modulate data, expands the pulse width of a Symbol, can effectively resist intersymbol Interference (ISI), and improves the performance of resisting multipath fading. In contrast to conventional Frequency Division Multiplexing (FDM), OFDM does not require a dedicated guard band. Although there is overlap between the frequency spectrums, the carriers are orthogonal to each other. According to the orthogonality principle, interference does not exist among all carriers, and therefore the utilization rate of a frequency spectrum is greatly improved.

Recently, a new multicarrier communication scheme, OFDM system based on Subcarrier Index Modulation (SIM), has been proposed. For the SIM-OFDM system, the steps are not different from the traditional OFDM system except for adding the SIM modulation module. The most core SIM modulation module adopts the idea of subcarrier blocking. The whole multi-carrier is firstly divided into a plurality of sub-blocks with the same size continuously, wherein a plurality of sub-carriers (called as active sub-carriers) are selected by index bits in each sub-block to transmit data, and the rest sub-carriers do not transmit data (called as silent sub-carriers). Since the index bits are not transmitted by themselves, but are implicit in the location information of the active subcarriers, the index bits do not occupy the spectrum resources. At the receiving end, the information of the index bit can be obtained by activating the position of the subcarrier. Compared with the traditional OFDM communication method, the existing SIM-OFDM communication method has many advantages, such as smaller peak-to-average power ratio of the SIM-OFDM system, better performance of resisting interference among subcarriers, lower error rate and the like, and can save transmitter energy by selecting different power allocation strategies. The disadvantage is that the SIM-OFDM system cannot simply insert pilots at fixed positions to complete channel estimation as in the OFDM system due to the block mode. And the pilot frequency position judgment error has great influence on the channel estimation performance of the SIM-OFDM system.

Disclosure of Invention

The present invention aims to solve the above problems, and provides a D-IM-OFDM (Differential index modulation OFDM, D-IM-OFDM) transmission scheme for performing Differential modulation on subcarrier indexes based on a conventional Differential OFDM (D-OFDM) system, which retains the characteristic that conventional D-OFDM detection does not require channel estimation and effectively improves the performance of the system.

For ease of understanding, the Differential Spatial Modulation (DSM) technique employed by the present invention is described as follows:

the differential spatial modulation technology is a novel MIMO technology which adopts incoherent detection based on the spatial modulation technology. And the transmitting end transmits information by adopting the activated transmission antenna matrix index and the constellation symbol. Therefore, the differential spatial modulation can accurately recover the transmitted information at the receiving end under the condition of no channel information by carrying out differential coding at the transmitting end while keeping the characteristic of the traditional spatial modulation single radio frequency. It has attracted extensive attention because it does not require channel information at both the transmitting and receiving ends and can retain the characteristics of a single radio frequency. The differential spatial modulation is introduced into the SIM-OFDM, namely the subcarrier indexes are subjected to differential modulation, and the advantage that channel information is not needed by DSM detection is obtained under the condition that the performance advantage of the SIM-OFDM is reserved.

The technical scheme of the invention is as follows:

a method for differential modulation of subcarrier index for OFDM system, comprising the steps of:

s1, dividing an OFDM symbol having N subcarriers into R OFDM-IM subblocks, each subblock including N/R subcarriers, each subblock being denoted by X (R) ([ X ])₁(r) X₂(r) … X_g(r) … X_G(r)]^TWhere R is 1,2, …, R, and only one X_g(r) is not 0; the right shift matrix E of gxg is defined as:

s2, for r ≦ G, each sub-block transmitting a reference symbol, namely X (r) E (: r), which represents the r-th column of the matrix E; deriving a set of length G DSM transmission vectors

a_kIs a number N_tA x 1-dimensional vector, and only one element is 1, and the other elements are 0;

s3, r>G, dividing the transmission information bit with the length of B into B₁And B₂Two parts; b is₁＝log₂(G) One bit is used to select the transmission vector a_k(q＝1,...,Q)；B₂＝log₂The (L) bits are used to modulate L-PSK constellation symbols(s)₁,s₂...,s_L) L represents the PSK modulation constellation size; the bits transmitted per sub-block are log₂Q+log₂L; the symbol vector of the r-th sub-block is obtained as follows: s (r) ═ a_q(r)s_l(r)，r>G；

S4, obtaining the transmitting vector of the r sub-block by differential coding

Comprises the following steps:

X(r)＝G[X(r-1)]S(r)，r>G

for arbitrary vectors

Function G [ alpha ]]The definition is as follows:

s5, after performing differential encoding on all sub-blocks, the OFDM symbols obtained by differential encoding are:

[X(1)^T … X(r)^T … X(R)^T]^T。

the invention has the beneficial effects that: bit information is transmitted together through subcarrier indexes and PSK symbols, and meanwhile, channel information is not needed any more during detection through differential modulation on the subcarrier indexes. From simulation results, the performance of the D-IM-OFDM system is superior to that of the traditional D-OFDM system, the performance loss is about 1.5dB compared with that of the SIM-OFDM system, and meanwhile the PAPR of the D-IM-OFDM system is lower than that of the OFDM and SIM-OFDM systems.

Drawings

FIG. 1 is a block diagram of a D-IM-OFDM system;

FIG. 2 is a diagram comparing the performance of D-IM-OFDM system with that of conventional D-OFDM and SIM-OFDM system when the number of sub-carriers is 1024;

fig. 3 is a diagram illustrating the comparison of the performance of the D-IM-OFDM system when the number of subcarriers is different.

Fig. 4 is a diagram comparing PAPR of D-IM-OFDM with OFDM and SIM-OFDM systems.

Detailed Description

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

for example, if G is 4 and BPSK modulation is performed, Q is 4. The bits transmitted per subcarrier block are log₂Q+log₂L＝3bit。

Step 1: the reference symbols transmitted by the first 4 sub-blocks are obtained,

i.e. matrix E.

Step 2: obtaining a length G RDSM transmission vector set: a is₁＝[1 0 0 0]^T，a₂＝[0 1 0 0]^T，a₃＝[0 0 1 0]^T，a₄＝[0 0 0 1]^T. Suppose that the ith-5 sub-block transmitted bits are

Transmission bit division into B₁＝log₂Q is 2 and B₂＝log₂And M is 1.

And step 3: front B ₁2 bits

For selecting a selected transmit antenna vector

Rear B₂With BPSK modulation of 1 bit (0) to +1, i.e. s₁(5) + 1. To obtain

And 4, step 4: due to the fact that

Then

Obtaining the ith-5 son by differential codingThe transmit vector for a block is:

suppose the bit of the 6 th sub-block is

Front B ₁2 bits

For selecting a selected transmit antenna vector

Rear B₂With BPSK modulation of-1, i.e. s, for 1 bit (1)₂(6) Is-1. Then

Obtaining a transmission vector of i ═ 6 subblocks by the same method from step 2 to step 4:

and 5: after all the sub-carrier blocks are differentially encoded, the OFDM symbol of the differential encoding is [ X (1) ]^T … X(r)^T … X(R)^T]^TI.e. by

Claims (1)

1. A method for differential modulation of subcarrier index for OFDM system, comprising the steps of:

s1, dividing an OFDM symbol having N subcarriers into R OFDM-IM subblocks, each subblock including N/R subcarriers, each subblock being denoted by X (R) ([ X ])₁(r) X₂(r)…X_g(r)…X_G(r)]^TWhere R is 1,2, …, R, and only one X_g(r) is not 0; the right shift matrix E of gxg is defined as:

s2, for r ≦ G, each sub-block sends a reference symbol, namely X (r) E (: r), and E (: r) represents the r-th column of the matrix E; deriving a set of length G DSM transmission vectors

a_qIs a G × 1-dimensional vector, Q is 1,2, …, Q, and only one element is 1, and the other elements are 0;

s3, r>G, dividing the transmission information bit with the length of B into B₁And B₂Two parts; b is₁＝log₂Q bits are used to select the transmission vector a_q(r)；B₂＝log₂The (L) bits are used to modulate L-PSK constellation symbols(s)₁,s₂...,s_L) L represents the PSK modulation constellation size; the bits transmitted per sub-block are log₂Q+log₂L; the symbol vector of the r-th sub-block is obtained as follows: s (r) ═ a_q(r)s_l(r)，r>G，l＝1,2,…,L；

S4, obtaining the transmitting vector of the r sub-block by differential coding

Comprises the following steps:

X(r)＝G[X(r-1)]S(r)，r>G

for arbitrary vectors

Function G [ alpha ]]The definition is as follows:

s5, after performing differential encoding on all sub-blocks, the OFDM symbols obtained by differential encoding are:

[X(1)^T…X(r)^T…X(R)^T]^T。

Images