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In the joint radar and communication system, using orthogonal frequency division multiplexing (OFDM) signals, cyclic prefix (CP) and pilots lead to the problem of high peak at the sidelobe (PSL) level in autocorrelation function (ACF), which deteriorates the radar detection performance seriously. To solve this problem, first, a new RadCom signal based on time-domain synchronization OFDM (TDS-OFDM) was proposed. TDS-OFDM adopts training sequence (TS) for guard interval, as well as synchronization and channel estimation, so that CP and pilots can be avoided. And then, ambiguity function (AF) of TDS-OFDM RadCom signal was analyzed. Finally, TS are optimized to suppress PSL of TDS-OFDM signal and maintain the autocorrelation properties of TS simultaneously. The results show that the autocorrelation performance of designed TDS-OFDM RadCom signal is much better than that of CP-OFDM RadCom signal. Considering the importance of radar target detection, TDS-OFDM is more appropriate than CP-OFDM for the RadCom system.

Joint radar and communication (RadCom) is proposed as a technology using a single signal to accomplish both radar and communication functions, which can not only effectively reduce the load of the platform, energy consumption, and electromagnetic interference, but also greatly improve the utilization of energy and spectrum, thus receiving much attention from military and civil fields [

Orthogonal frequency division multiplexing (OFDM) is a multicarrier data transmission technology which has been widely used in communication system. Due to the flexibility in waveform design, OFDM has also been gradually utilized in radar [

The application of CP suppresses the intercarrier interference (ICI) and improves the orthogonality of subcarriers. However, when CP-OFDM signal is utilized in the RadCom system, CP, the replication of the later section of OFDM data block, will inevitably cause the high level sidelobes of autocorrelation function (ACF). Moreover, the CP sidelobe level increases with the length ratio of CP and OFDM data block. In addition, to achieve communication synchronization and channel estimation, pilots must be used in CP-OFDM RadCom signal, but the pilot symbols will also lead to the problem of pilot sidelobes. Those sidelobes deteriorate the performance of radar target detection; thus the application of OFDM radar-communication integration will be severely restricted.

To address the problem, a solution is put forward in [

Being matured in communication, TDS-OFDM is a key technology in the standard of digital television terrestrial broadcasting (DTTB), whose application is successful in China, Cuba, Cambodia, etc. [

The rest of the paper is organized as follows. The TDS-OFDM RadCom signal model is presented in Section

As shown in Figure

Structure of TDS-OFDM RadCom signals.

Ambiguity function demonstrates the delay-Doppler characteristics of the radar signal, making it an important tool for studying radar signal and waveform design. In this section, AF of TDS-OFDM RadCom signal is deduced in detail.

For general point targets, the following narrowband AF can be used for analysis:

In fact, increasing the time-bandwidth product of OFDM pulse can effectively reduce the variance of

First, we deduce the AF of OFDM data blocks. According to the definition of AF, (

① when

② when

Equation (

Ambiguity function of TDS-OFDM RadCom signal (

According to the definition, substituting (

As the amplitude of

Substituting (

PSL of radar signal is an important factor affecting radar target detection. In this section, PSL of average ACF of TDS-OFDM signals, denoted as PSL_{0}, is used as one of the objective functions

On the other hand, to realize robust communication synchronization and channel estimation, TS itself should be well autocorrelated. Let

Thus, the final optimizing criterion is minimizing the weighted sum of average peak sidelobe of TDS-OFDM signal and maximal peak sidelobe of TS, which is presented as the following fitness function:

Flow chart of GA.

Initialize the population

Calculate fitness value according to the fitness function represented by (_{0} and

According to the individual fitness, select superior individuals and eliminate inferior individuals by roulette selection.

Recombine individuals with one-point crossover method.

Mutate individual gene values with 0.1 mutation probability.

When the iterations number reaches its maximum, the genetic algorithm completes and the optimization operation can be terminated. If the termination condition is not satisfied, go back to Step

Besides, the time complexity of GA is linearly increasing with iterative times and input size. When using the termination strategy above, the time complexity of GA can be approximately described as

In the following simulations, GA parameters are as follows: the number of individuals is 100, maximum number of generations is 120, generation gap is 0.9, and weight

We first optimize the TDS-OFDM RadCom signal with high time-bandwidth product whose parameters are shown in Table

Parameters of TDS-OFDM signal.

Parameters | Value |
---|---|

Number of OFDM symbols ( | 4 |

TS length ( | 63 |

Number of sub-carriers ( | 256 |

Bandwidth ( | 20MHz |

Duration of an OFDM data block ( | 12.8 |

Duration of guard interval ( | 3.15 |

Pulse width | 63.8 |

Time-bandwidth product | 1276 |

Fitness change of GA algorithm.

ACF of 4 training sequences.

By using the designed TS-string, average ACF of TDS-OFDM signal is shown in Figure _{0} of the signal is 32.46 dB, which means the designed signal on average is able to detect weak targets.

Average ACF of TDS-OFDM signal.

In the ACF of TDS-OFDM signals, there are accompanying noise-like floors which are caused by the random OFDM data. Therefore, the instantaneous PSL of random TDS-OFDM signal will be lower than PSL_{0}. The noise-like effect of the random components in the transmitted signal is described as correlation noise in [_{0}.

To verify this analysis, in this subsection, after phase-coherent accumulation of

Figure _{0}=28.52. As shown in the figure, correlation noise leads to a significant fluctuation of ACF, and PSL of a single pulse is only about 20dB. With the growth of accumulation pulse number, the fluctuation of ACF decreases gradually. When P is 64, PSL is about 28dB, which is very close to PSL_{0}.

ACF of TDS-OFDM signal (

Figure _{0}=32.46dB). As the time-bandwidth product is high, the fluctuation of sidelobe decreases relatively. PSL reaches about 32dB after phase-coherent accumulation of just 16 pulses. In addition, by comparing Figures

ACF of TDS-OFDM signal (

This simulation indicates that, as the technology of phase-coherent accumulation is generally used in modern radar, even if the time-bandwidth product of the signal is not high enough, ACF can also approach its expectation by using more pulses in phase-coherent accumulation, which means the signal design method based on optimizing average PSL is effective.

Furthermore, simulation of range ACF of CP-OFDM RadCom signal after phase-coherent accumulation of 16 pulses is also conducted in the same condition in Table

ACF of CP-OFDM signal (

As pseudorandom noise (PN) sequences are usually used in conventional TDS-OFDM communication signal, in this subsection, we compare the optimized sequence with m sequence and Gold sequence about the performance of _{0.} The data of three different sequence lengths is shown in Table _{0} is low, making it only suitable for communication. On the contrary, PSL_{0} of Gold sequence is higher because of the good cross-correlation, but

Performance comparison of different TS.

Length | Sequence | | PSL_{0} (dB) |
---|---|---|---|

31 | m sequence | 14.26 | 23.38 |

Gold sequence | 10.74 | 27.72 | |

optimized sequence | 14.26 | 30.74 | |

| |||

63 | m sequence | 15.99 | 17.36 |

Gold sequence | 10.41 | 21.22 | |

optimized sequence | 15.99 | 32.46 | |

| |||

127 | m sequence | 19.80 | 11.91 |

Gold sequence | 15.63 | 19.66 | |

optimized sequence | 17.47 | 34.47 |

The elapsed time of the GA program is shown in Table

Elapsed time of GA (

Length of TS | 20 | 40 | 60 | 80 | 100 | 120 |
---|---|---|---|---|---|---|

Elapsed time (sec) | 6.79 | 7.62 | 8.02 | 9.34 | 9.72 | 10.1 |

To address the problem of CP and pilot sidelobes in conventional CP-OFDM RadCom signal, a novel RadCom signal based on TDS-OFDM is proposed in this paper. AF of proposed signal was analyzed in detail. And then, TS are optimized to suppress PSL of TDS-OFDM signal and maintain the autocorrelation properties of TS simultaneously, thus achieving good performance in both radar and communication. Although, the TS and the OFDM data block cause mutual interference to each other, thus more calculation has to be costed to achieve reliable time-domain channel estimation and frequency-domain data detection in TDS-OFDM systems. Generally, considering the importance of autocorrelation of the signal to radar target detection, TDS-OFDM is more appropriate than CP-OFDM for the RadCom system.

The data used to support the findings of this study are available from the corresponding author upon request.

The authors declare that they have no conflicts of interest.