Random access preamble design and timing advance estimation method of time delay-Doppler domain communication system

Abstract

The invention discloses a random access preamble design and timing advance estimation method of a time delay-Doppler domain communication system, which takes a chirp signal as a random access preamble sequence (RAP) and realizes access user detection and timing advance (TA) estimation according to cross-correlation operation after fractional Fourier transform (FrFT). Comprising the following steps: taking a chirp sequence as an RAP root sequence, firstly performing FrFT conversion on the RAP root sequence, and storing the RAP root sequence; performing cyclic shift on the root sequence by integral multiples of a fixed length to obtain an available RAP set; the user randomly selects one RAP from the available RAP set and maps the selected RAP to a delay-Doppler (DD) domain; converting the DD domain RAP signal to a time domain and then sending the DD domain RAP signal; converting the received signal to a DD domain and carrying out Doppler diversity combination; a chirp signal is extracted after cyclic shift of the combined sequence, and FrFT is executed; performing cross-correlation on the FrFT of the RAP root sequence and the FrFT of the received signal to obtain a detection function; and sending RAP detection and TA estimation are realized according to the number of cyclic shift points when a detection function is greater than a threshold.

Description

Technical Field

[0001] The present invention belongs to the field of wireless communications, and relates to a random access preamble design and a timing advance estimation method for an uplink of a communication system, and in particular to a random access sequence design based on a chirp signal and a random access user detection and timing advance estimation method based on a fractional Fourier transform in a delay-Doppler domain communication system. Background Art

[0002] With the rapid development of high-speed rail, drones, low-orbit satellites and other systems, providing stable and reliable mobile Internet services in high-speed mobile scenarios has become an inevitable trend in the development of future B5G (Beyond 5G) and 6G mobile communication systems. In addition, with the explosive growth of mobile traffic demand, the shortage of spectrum resources has become increasingly prominent, prompting mobile providers to use higher carrier frequency bands. However, the Doppler effect caused by the relative motion between the transmitter and the receiver poses a huge challenge to communications in high-speed mobile scenarios and high carrier frequencies. In order to solve the problem of reliable communication under severe Doppler shift conditions, Orthogonal Time Frequency Space (OTFS) technology came into being. OTFS introduces the Delay-Doppler (DD) domain, converting the dual-selective fading channel in the time-frequency domain into a quasi-static channel in the DD domain, thereby overcoming the severe multipath effect and the influence of the Doppler effect, and has broad application prospects in high-speed mobile scenarios with high carrier frequencies.

[0003] In cellular mobile communication systems based on OTFS, uplink random access is an essential part of establishing a reliable communication link between base stations and users. However, due to the differences in principle between OTFS technology and the single-carrier frequency division multiple access (SC-FDMA) technology used for the uplink of 4G and 5G systems, the random access preamble (RAP) and timing advance (TA) estimation scheme used for SC-FDMA cannot be applied to OTFS systems. Therefore, exploring the RAP design and TA estimation scheme for the uplink of OTFS communication systems is the key to the application of OTFS systems in future high-carrier frequency and high-speed mobile scenarios. Summary of the invention

[0004] In order to solve the deficiencies of the prior art, the present invention provides a RAP design scheme based on chirp signals for the OTFS communication system, and implements a random access user detection and timing advance estimation method based on fractional Fourier transform (FrFT). The technical problem to be solved by the present invention is achieved by the following technical solutions:

[0005] The sending end RAP generation includes the following steps:

[0006] (1) Set the length to N c The chirp sequence is used as the RAP root sequence, and the sequence is transformed by FrFT and saved, which is recorded as X(u);

[0007] (2) The root sequence is vN in length. cs cyclic shift of to obtain the available RAP set;

[0008] (3) User q randomly selects one from the available RAP set and maps it to the DD domain;

[0009] (4) Performing inverse symplectic finite fourier transform (ISFFT) and Heisenberg transform on the generated DD domain RAP signal to obtain a time domain RAP frame and send it;

[0010] The receiving end RAP detection and TA estimation includes the following steps:

[0011] (5) Convert the signals received from multiple users to the DD domain and merge the first N signals on the Doppler axis. D A sequence with a large average power;

[0012] (6) The combined sequence is cyclically shifted by d sampling points and the chirp signal is extracted, denoted as y(n);

[0013] (7) Perform FrFT on y(n) to obtain Y d (u), X(u) and Y d (u) Perform cross-correlation operation to obtain the detection function M(d);

[0014] (8) When the value of the detection function M(d) is greater than the threshold λ, the RAP is sent according to the corresponding d detection and the TA is estimated.

[0015] In the step (1), the chirp-based RAP root sequence is:

[0016]

[0017] Where n = 0, 1, ..., Nc -1,f 0 is the initial frequency of the chirp signal, μ is the modulation frequency of the chirp signal. 0 Or μ to get a new RAP root sequence.

[0018] In step (1), the FrFT transform of the signal x(t) is defined as:

[0019]

[0020] Where K α (t, u) is the kernel function, defined as:

[0021]

[0022] In step (2), the available RAP set generated by the root sequence is:

[0023] x v (n) = x((n+vN cs )modN c ), v = 0, ..., N p -1

[0024] Where N cs is the cyclic shift offset, The number of available RAPs.

[0025] In step (3), the RAP generated by the chirp sequence is mapped to the center of the DD domain and placed along the delay axis, which is expressed as:

[0026]

[0027] Where M and N are the number of grids of the delay axis and Doppler axis of the OTFS frame, k and l are the indexes of the delay axis and Doppler axis in the DD domain, and G is the guard interval of the delay axis.

[0028] In step (7), the detection function M(d) is defined as:

[0029]

[0030] In step (8), the detection threshold λ is set. If M(d q )>λ, the RAP detection and TA estimation results are:

[0031]

[0032]

[0033] The benefits of the present invention are embodied in:

[0034] (1) The present invention uses chirp signals for the RAP of the system in high-carrier frequency and high-speed mobile scenarios, which can resist the influence of severe Doppler frequency shift and achieve superior access user detection and TA estimation performance.

[0035] (2) The present invention performs cross-correlation operation on the received signal in the fractional Fourier domain to obtain a detection function, which can fully utilize the energy aggregation characteristics of the chirp signal FrFT and resist the influence of noise.

[0036] (3) The present invention implements the uplink random access physical layer core algorithm of the cellular mobile communication system based on the OTFS system, laying a foundation for the practical application of the OTFS technology in high-carrier frequency and high-speed mobile scenarios. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Figure 1 Flow chart of random access preamble sequence design and timing advance estimation scheme proposed by the present invention

[0038] Figure 2 Delay-Doppler domain schematic diagram of the random access preamble sequence waveform designed for the present invention

[0039] Figure 3 Comparison diagram of missed detection probability of random access preamble sequence detection method proposed by the present invention

[0040] Figure 4 RMSE performance comparison diagram of the timing advance estimation method proposed in this invention DETAILED DESCRIPTION

[0041] The method of the present invention is described in detail with reference to the accompanying drawings and embodiments.

[0042] The present invention proposes a random access preamble design and timing advance estimation method for the uplink of the OTFS system based on the chirp signal. The chirp signal is used as the random access preamble sequence, and the access user detection and timing advance estimation are realized according to the cross-correlation operation after FrFT. Taking the OTFS communication system with M=512 and N=5 as an example, the algorithm flow chart is as follows Figure 1 As shown, the specific implementation is as follows:

[0043] The sending end RAP generation includes the following steps:

[0044] (1) Set the length to N c The chirp sequence is used as the RAP root sequence, and the sequence is transformed by FrFT and saved, which is recorded as X(u);

[0045] Referring to the RAP setting method of the LTE system, when the bandwidth B and subcarrier spacing Δf are given, the length N of the chirp signal isc is the largest prime number not greater than B / Δf. In this embodiment, B = 1.08 MHz, Δf = 3 kHz, N c =359. The generated chirp signal is:

[0046]

[0047] where f 0 =5MHz, μ=1.643e9. The optimal order of the FrFT of the chirp signal is: p=arccot(-k) / (π / 2). Perform a p-order FrFT on x(n), recorded as X(u).

[0048] (2) The root sequence is vN in length. cs cyclic shift of to obtain the available RAP set;

[0049] N cs The value of should not be greater than the number of sampling points of the maximum round-trip transmission delay. In this embodiment, N cs = 20. The number of available RAPs is: The available RAP set is represented as:

[0050] x v (n) = x((n+vN cs )modN c ), v = 0, ..., N p -1

[0051] (3) User q randomly selects one from the available RAP set and maps it to the DD domain;

[0052] The specific location of Chirp signal mapping to the DD domain is as follows Figure 2 As shown, it is expressed as:

[0053]

[0054] Where M and N are the number of grids of the delay axis and Doppler axis of the OTFS frame, k and l are the indexes of the delay axis and Doppler axis in the DD domain, and G is the guard interval of the delay axis.

[0055] (4) performing ISFFT and Heisenberg transform on the generated DD domain RAP signal to obtain a time domain RAP frame and send it;

[0056] The receiving end RAP detection and TA estimation includes the following steps:

[0057] (5) Convert the signals received from multiple users to the DD domain and merge the first N signals on the Doppler axis. D A sequence with a large average power;

[0058] In this embodiment, N D =N=5, that is, the signals are superimposed along the Doppler axis and merged into an M×1 sequence.

[0059] (6) The combined sequence is cyclically shifted by d sampling points and the chirp signal is extracted, denoted as y(n);

[0060] (7) Perform FrFT on y(n) to obtain Y d (u), X(u) and Y d (u) Perform cross-correlation operation to obtain the detection function M(d);

[0061] The detection function M(d) is defined as:

[0062]

[0063] (8) When the value of the detection function M(d) is greater than the threshold λ, the RAP is sent according to the corresponding d detection and the TA is estimated.

[0064] If M(d q )>λ, the RAP detection and TA estimation results are:

[0065]

[0066]

[0067] will satisfy M(d q )>λ for all d q After the detection is completed, RAP detection and corresponding TA estimation of all access users are realized.

[0068] Simulation results:

[0069] The performance of the present invention is analyzed in conjunction with simulation below.

[0070] The simulation considers a single-cell multi-user cellular communication system, where both the base station and the user are equipped with a single antenna, M = 512, N = 5, and the subcarrier spacing is 3kHz. The parameters used by the Chirp root sequence are: 0 =1.4MHz, μ=1.64e9.

[0071] The channel model is 3GPP EVA channel. There are 9 multipath transmission paths, the path delays are 0us, 0.03us, 0.15us, 0.31us, 0.37us, 0.71us, 1.09us, 1.73us, 2.51us, and the path gains are 0dB, -1.5dB, -1.4dB, -3.6dB, -0.6dB, -9.1dB, -7dB, -12dB, -16.9dB; at the same time, the signal is affected by additive white Gaussian noise, and the SNR ranges from -15dB to 5dB.

[0072] The proposed method is simulated under different Doppler frequency shift conditions. The maximum Doppler frequency shift is set to 300Hz, 600Hz, and 1200Hz respectively. The number of users accessed at the same time is set to 2, and the situations under different SNRs are simulated. Figure 3 This is a comparison chart of missed detection probabilities of the random access preamble sequence detection method proposed in the present invention. Figure 3 It can be seen that the missed detection probability can be reduced to 10 when the maximum Doppler shift is 300Hz and 600Hz. -5 Below; when the maximum Doppler frequency shift is 1200Hz, the missed detection probability can be reduced to 10 -2 Therefore, the random access preamble sequence detection method proposed in the present invention is applicable to the OTFS system in a high carrier frequency and high-speed mobile scenario. Figure 4 This is a RMSE comparison chart of the timing advance estimation method proposed in the present invention. Figure 4 It can be seen that the RMSE of the proposed timing advance estimation method can be reduced to below 0.8 sampling points, and high-precision TA estimation can be achieved even in high-speed mobile scenarios.

[0073] The above is only a further explanation of the present invention and is not intended to limit the implementation and application of this patent. Any equivalent implementation of the present invention should be included in the scope of the claims of this patent.

Claims

1. A random access preamble design and timing advance estimation method for a delay-Doppler domain communication system. It is characterized in that Using the chirp signal as the random access preamble sequence, access user detection and timing advance estimation are implemented according to the cross-correlation operation after FrFT, including the following steps: (1) Set the length to N c The chirp sequence is used as the RAP root sequence, and the sequence is transformed by FrFT and saved, which is recorded as X(u); (2) The root sequence is vN in length. cs cyclic shift of to obtain the available RAP set; (3) User q randomly selects one from the available RAP set and maps it to the DD domain; (4) performing ISFFT and Heisenberg transform on the generated DD domain RAP signal to obtain a time domain RAP frame and send it; (5) Convert the signals received from multiple users to the DD domain and merge the first ND sequences with larger average power on the Doppler axis; (6) The combined sequence is cyclically shifted by d sampling points and the chirp signal is extracted, denoted as y(n); (7) Perform FrFT on y(n) to obtain Y d (u), X(u) and Y d (u) Perform cross-correlation operation to obtain the detection function M(d); (8) When the value of the detection function M(d) is greater than the threshold λ, the RAP is sent according to the corresponding d detection and the TA is estimated.

2. The random access preamble design and timing advance estimation method of the delay-Doppler domain communication system according to claim 1, It is characterized in that In the step (1), the chirp-based RAP root sequence is: Among them, f 0 is the initial frequency of the chirp signal, μ is the modulation frequency of the chirp signal. 0 Or μ to get a new RAP root sequence.

3. The random access preamble design and timing advance estimation method of the delay-Doppler domain communication system according to claim 1, It is characterized in that In step (2), the available RAP set generated by the root sequence is: v (n) = x((n+vN cs )modN c ), v = 0, ..., N p -1. Where N cs is the cyclic shift offset, is the number of available RAPs.

4. The random access preamble design and timing advance estimation method of the delay-Doppler domain communication system according to claim 1, It is characterized in that In step (3), RAP is mapped to the center of the DD domain and placed along the delay axis, which is expressed as: Where M and N are the number of grids of the delay axis and Doppler axis of the OTFS frame, k and l are the indexes of the delay axis and Doppler axis in the DD domain, and G is the guard interval of the delay axis.

5. The random access preamble design and timing advance estimation method of the delay-Doppler domain communication system according to claim 1, It is characterized in that In step (7), the detection function M(d) is defined as:

6. The random access preamble design and timing advance estimation method of the delay-Doppler domain communication system according to claim 1, It is characterized in that In step (8), the detection threshold λ is set. If M(d q )>λ, the RAP detection and TA estimation results are:

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