OFDM Communication Method Based on Optimal Frequency Hopping Diagram

Abstract

This paper proposes an OFDM communication method based on optimal frequency hopping diagrams. It applies optimal frequency hopping diagrams to OFDM communication systems and provides a design scheme for user frequency hopping diagrams specific to cellular mobile communication system architectures. In OFDM communication systems using optimal frequency hopping diagrams, the subcarrier frequencies are no longer constant but fluctuate within different time slots. The fluctuation pattern is controlled by a pre-designed optimal frequency hopping diagram. This system uses the optimal frequency hopping diagram to control carrier frequency hopping. Due to the ideal autocorrelation characteristics of the optimal frequency hopping diagram, it can effectively reduce or eliminate inter-carrier interference caused by time delay and Doppler shift. Furthermore, because the optimal frequency hopping diagram has ideal cross-correlation, it can effectively eliminate interference between users.

Description

Technical Field

[0001] This invention relates to the field of communications, and more specifically to an OFDM communication method based on an optimal frequency hopping diagram. Background Technology

[0002] To achieve high communication rates while overcoming frequency-selective fading, multi-carrier modulation technology emerged. As a representative of multi-carrier modulation technology, Orthogonal Frequency Division Multiplexing (OFDM) has become one of the fundamental modulation technologies of modern digital communication after more than 60 years of development since the 1960s, and is widely used in communication systems such as wireless local area networks and digital communications.

[0003] The main problem caused by frequency-selective fading is severe inter-symbol interference (ISI), which increases the bit error rate. To address the potential ISI problem between OFDM symbols, a time-domain solution is to add a guard interval between OFDM symbols. As long as the length of the guard interval is greater than the maximum delay spread of the channel, even if an OFDM symbol arrives at the receiver later due to multipath propagation, it will not interfere with the next symbol. Regarding the specific content transmitted during the guard interval, if it is all zero, although this saves transmit power, it will destroy the orthogonality between subcarriers within the symbol, causing inter-carrier interference (ICI). This is because to ensure the orthogonality between subcarriers, the inner product operation must contain an integer number of waveform periods within the integration interval. Failure to meet this requirement will result in a non-zero inner product between different subcarriers within the OFDM symbol, which will affect the demodulation of a particular subcarrier due to ICI.

[0004] Therefore, the common practice is to add a cyclic prefix (CP), which copies the last part of the OFDM symbol to the guard interval before the symbol. Even with multipath delay, the portion of the CP shifted into the demodulation window ensures that the integral interval contains an integer number of waveform periods for each element involved in the inner product operation. Similar to the guard interval, the length of the CP must be extended relative to the maximum channel delay. However, the CP content is essentially useless information. To reduce the overhead of adding the CP, the OFDM symbol period can be increased, thus reducing the proportion of useless information in the total length. However, a longer OFDM symbol period results in a smaller subcarrier spacing, making the system more susceptible to frequency offset. Furthermore, if the channel's coherence time is exceeded, fast fading can occur. Therefore, the selection of the CP length must consider multiple factors.

[0005] In static communication systems where Doppler shift is not a concern, OFDM systems employing CP technology already possess a good ability to reduce ICI problems caused by delay spread, and there is no need to use equalization technology at the receiver, thus simplifying the receiver.

[0006] However, in mobile communication systems, both relative motion between the transmitter and receiver, and reflections from moving objects, can cause signal interference due to Doppler frequency shift. Traditional OFDM technology, with its fixed subcarrier frequency, cannot avoid Doppler frequency shift interference and therefore cannot solve the ICI (Inter-Carrier Interference) caused by Doppler frequency shift. Thus, traditional OFDM technology cannot be applied to mobile communication systems. In this case, interference can only be avoided by hopping the OFDM subcarrier frequencies in different time slots. Designing an optimal frequency hopping diagram with ideal two-dimensional autocorrelation and cross-correlation is key to solving the interference between OFDM subcarriers caused by time delay and Doppler frequency shift using frequency hopping. The ideal two-dimensional autocorrelation in the optimal frequency hopping diagram counteracts interference between OFDM subcarriers caused by time delay and Doppler frequency shift, while the ideal two-dimensional cross-correlation prevents interference between users. Summary of the Invention

[0007] To address the problems existing in the above-mentioned background technology, this application proposes an OFDM communication method based on the optimal frequency hopping diagram. It is the first time that the optimal frequency hopping diagram has been applied to the OFDM communication system, and provides a design scheme for the user frequency hopping diagram for the cellular mobile communication system architecture. By hopping frequency according to the frequency hopping diagram, users can eliminate or reduce inter-carrier interference and inter-user interference caused by time delay and Doppler frequency shift in the OFDM system.

[0008] The OFDM communication method based on the optimal frequency hopping diagram includes the following steps:

[0009] Step 1: Determine the number of optimal frequency hopping diagram families F based on the number of cells in the cellular system; determine the number of optimal frequency hopping diagrams F1 in each cell based on the maximum number of users in the cell; for the structure of the cellular cell system, cellular mobile communication systems using optimal frequency hopping diagrams use the same frequency hopping diagram reuse, that is, if the distance between two users is greater than the protection distance of the same optimal frequency hopping diagram, the same optimal frequency hopping diagram is used.

[0010] Step 2: Based on the maximum Doppler frequency shift and frequency hopping gap in the system, obtain the normalized and rounded maximum Doppler frequency shift D. max This allows us to determine the minimum Doppler distance d between groups;

[0011] Step 3: Based on the maximum multipath delay and frequency hopping time slot in the system, obtain the normalized and rounded maximum multipath delay τ. maxThis allows us to determine the minimum time delay distance d1 between the optimal frequency hopping maps within the same optimal frequency hopping map family;

[0012] Step 4: Based on the obtained minimum Doppler distance d and minimum time delay distance d1, combined with the optimal number of frequency hopping pattern families F and the optimal number of frequency hopping patterns F1, determine the order n of the Welch Costas sequence and construct the Welch Costas sequence C.

[0013] Step 5: Design the frequency hopping diagram of the head of the optimal frequency hopping diagram family; add a blank line to the (n+1)th row of the sequence C obtained in Step 1 to obtain the sequence C1; perform a vertical cyclic shift on the sequence C1 according to the minimum Doppler distance d to obtain the placement function of the corresponding sequence. The new sequence is the head frequency hopping diagram of each optimal frequency hopping diagram family.

[0014] Step 6: Design the frequency hopping diagram within each family; based on the minimum delay distance d1, perform a horizontal cyclic shift on the frequency hopping diagram of the family head obtained in Step 2 to obtain the placement function of the corresponding sequence;

[0015] Step 7: Following the optimal frequency hopping map design method described above, obtain the placement function of the first frequency hopping map of the F optimal frequency hopping map families, and the nth frequency hopping map in the j-th cell. j The placement function of the optimal frequency hopping diagram;

[0016] Step 8: Apply the optimal frequency hopping diagram to the transmitting end of the OFDM communication system. Assume the transmitted signal length is N. At the transmitting end, N optimal frequency hopping diagram generators control N frequency synthesizers to output N subcarriers. These subcarriers are multiplied by N baseband signals, and then summed to obtain one signal. At the receiving end, each signal has an optimal frequency hopping diagram generator that generates the optimal frequency hopping diagram. The optimal frequency hopping diagram then controls the frequency synthesizer to output a frequency-hopping carrier, which is then phase-shifted by a phase shifter to obtain a shifted carrier. The OFDM communication system first down-converts the received signal, then performs serial-to-parallel conversion, dividing the signal into N signals. The real and imaginary parts of the received signal in each time slot of the N signals are multiplied by the frequency-hopping carrier and the shifted carrier, respectively, and summed to obtain a total signal, thus completing the OFDM communication.

[0017] Furthermore, in step 1, when designing the optimal frequency hopping diagram for use in an OFDM cellular mobile communication system, it is only necessary to design the optimal frequency hopping diagram for all users within a single cell group. The reuse distance D of the optimal frequency hopping diagram is expressed as:

[0018]

[0019] In equation (9), R is the cell radius, N is the number of cells in a cell cluster, and N = i 2 +im+m2 , where i and m are non-negative integers that are not both 0.

[0020] Furthermore, in step 1, for a cell cluster, the number of optimal frequency hopping map families should not be less than the number of cells in the cluster; each cell should be assigned at least one optimal frequency hopping map family, and any extra optimal frequency hopping map families should be reserved.

[0021] If multiple cells are small in size, they can share a single optimal frequency hopping pattern family; or if a cell is too large in size, it can be assigned multiple optimal frequency hopping pattern families.

[0022] In a cell, each user is assigned at least one optimal frequency hopping map; if a cell uses a family of optimal frequency hopping maps, the number of optimal frequency hopping maps in the family should not be less than the number of users.

[0023] Furthermore, in step 2, let the non-negative integer D... max The maximum Doppler frequency shift of the user in the system after frequency hopping interval normalization and rounding up is given by d, where the positive integer d is the minimum Doppler distance between groups, and d and D are... max It must satisfy equation (10):

[0024] 0≤D max <d (10)

[0025] The minimum Doppler distance d is determined according to equation (10); where F b For frequency hopping, f d This represents the maximum Doppler frequency shift in the actual system. v is the maximum speed of an object in the communication environment, and λ is the signal wavelength.

[0026] Furthermore, in step 3, let the non-negative integer τ be... max The maximum multipath delay within the cell after frequency hopping time slot normalization and rounding up is given. The positive integer d1 represents the minimum delay distance between optimal frequency hopping maps within the same optimal frequency hopping map family. d1 and τ... max Satisfying equation (11):

[0027] 0≤τ max <d1 (11)

[0028] The minimum time delay distance d1 is determined according to equation (10); where t d For the maximum multipath delay, R is the radius of the cell, c is the speed of light, and T is the speed of light. b This refers to the frequency hopping time slot.

[0029] Furthermore, in step 4, the minimum Doppler distance d and the optimal frequency hopping pattern family number F need to satisfy equation (3):

[0030] d·F≤p (3)

[0031] The minimum delay distance d1 and the optimal number of frequency hopping patterns F1 need to satisfy equation (8):

[0032] d1·F1≤p-1 (8)

[0033] The value of p is determined by equations (3) and (8), and then a Welch Costas sequence is constructed based on the finite field GF(p). Assume that α is a primitive element of the finite field GF(p), p is a prime number, and the necessary and sufficient condition for an n = p-1 order permutation sequence C to be a Welch Costas sequence is that its placement function is:

[0034] y(k)≡α k (mod p), 1≤k≤p-1 (1)

[0035] The symbol “≡(mod p)” indicates congruence modulo p.

[0036] Furthermore, in step 5, the number of columns of the Costas sequence cyclically shifted in the horizontal direction is defined as the time delay distance between the original sequence and the new sequence formed after the cyclic shift, and the number of rows of the Costas sequence cyclically shifted in the vertical direction is defined as the Doppler distance between the original sequence and the new sequence formed after the cyclic shift.

[0037] Furthermore, in step 5, a blank line is added to the n = p-1 order Welch-Costas sequence C obtained in step 4, that is, the y(k) = n+1th line, so that sequence C becomes a p × (p-1) sequence C1. The placement function of sequence C1 is:

[0038] y1(k)=α k ,1≤k≤p-1 (2)

[0039] Furthermore, in step 5, sequence C1 is cyclically shifted vertically by d rows, 2d rows, ..., (F-1)d rows respectively, resulting in F-1 sequences C2, C3, ..., C..., each containing one gap row. F Their placement functions are as follows:

[0040] y2(k)=α k +d, 1≤k≤p-1 (4)

[0041] y3(k)=α k +2d, 1≤k≤p-1 (5)

[0042]

[0043] y F (k)=α k+(F-1)d,1≤k≤p-1 (6)

[0044] Sequence C2, C3, ..., C F This refers to the frequency hopping patterns of the F group heads, and the minimum Doppler distance between these frequency hopping patterns is d.

[0045] Furthermore, in step 6, the frequency hopping map C of the j-th optimal frequency hopping map family head obtained in step 5 is... j Shifting the data horizontally to the left by columns 0, d1, 2d1, ..., (F1-1)d1 respectively yields F1 sequences containing a single gap row, where the nth... j The placement function for each sequence is:

[0046]

[0047] Where j = 1, 2, ..., F; n j =1,2,…,F1; That is, the nth element in the j-th optimal frequency hopping family. j The optimal frequency hopping diagram, that is, the first optimal frequency hopping diagram of the j-th family is the frequency hopping diagram of the head of the j-th family.

[0048] The beneficial effects achieved by this invention are as follows:

[0049] (1) In the OFDM communication method based on the optimal frequency hopping diagram proposed in this invention, the frequency of the subcarrier is no longer constant, but changes in different time slots. The hopping pattern is controlled by the pre-designed optimal frequency hopping diagram. (2) By using the optimal frequency hopping diagram to control the carrier frequency hopping, since the optimal frequency hopping diagram has ideal autocorrelation characteristics, it can effectively reduce or eliminate the inter-carrier interference caused by time delay and Doppler frequency shift. (3) At the same time, since the optimal frequency hopping diagram has ideal cross-correlation, it can effectively eliminate the interference between users. Attached Figure Description

[0050] Figure 1 This refers to the frequency hopping diagram of the head of each optimal frequency hopping diagram family in the embodiments of the present invention.

[0051] Figure 2 This is the frequency hopping diagram of 6 users in the first cell in this embodiment of the invention.

[0052] Figure 3 This is a block diagram of the transmitter system in an embodiment of the present invention.

[0053] Figure 4 This is a block diagram of the receiving end system in an embodiment of the present invention. Detailed Implementation

[0054] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings.

[0055] A frequency hopping sequence with ideal autocorrelation characteristics is called a Costas sequence. Although the Costas sequence has ideal autocorrelation, it does not have ideal cross-correlation. Therefore, in scenarios that require multiple frequency hopping patterns with ideal autocorrelation and cross-correlation characteristics, especially in mobile communication systems that require a large number of frequency hopping patterns with ideal autocorrelation and cross-correlation characteristics, the Costas sequence cannot meet the requirements.

[0056] This invention proposes the concept of optimal frequency hopping patterns (OFHPs) with ideal two-dimensional autocorrelation and cross-correlation, and a method for constructing optimal frequency hopping patterns using the two-dimensional cyclic shift method. It is the first to propose applying optimal frequency hopping patterns to OFDM communication systems, and designs frequency hopping patterns for users in cellular mobile communication systems based on the system architecture, so that users in OFDM systems can avoid interference between OFDM subcarriers and between users caused by multipath propagation and Doppler frequency shift.

[0057] The optimal frequency hopping pattern is based on the algebraically constructed Costas sequence. Algebraic construction methods for Costas sequences include the Welch method and the Golomb method. This section focuses on the design method of the optimal frequency hopping pattern based on the Welch Costas sequence, then designs the user frequency hopping pattern in an OFDM system for the cellular system architecture, and finally provides an example.

[0058] Determine the order of the Welch Costas sequence and construct the Welch Costas sequence.

[0059] First, construct the Welch-Costas sequence based on the finite field GF(p). Assume α is a primitive element of the finite field GF(p), p is a prime number, and the necessary and sufficient condition for an n = p-1 order permutation sequence C to be a Welch-Costas sequence is that its placement function is:

[0060] y(k)≡α k (mod p), 1≤k≤p-1 (1)

[0061] The symbol “≡(mod p)” indicates congruence modulo p. In subsequent parts, “≡(mod p)” is represented by the symbol “=” to keep the expression concise, without causing misunderstanding.

[0062] First, design the frequency hopping diagram of the optimal frequency hopping diagram family head.

[0063] First, let's introduce the concepts of Doppler distance and time delay distance: Two new sequences C1 and C2 are formed by cyclically shifting two Costas sequences in the horizontal and vertical directions. If sequence C1 is first cyclically shifted by w1 columns in the horizontal direction and then cyclically shifted by w2 rows in the vertical direction to obtain sequence C2, then w1 is called the time delay distance between the two sequences C1 and C2, and w2 is called the Doppler distance between the two sequences.

[0064] Add a blank line (the y(k) = n+1th row) to the designed n = p-1 order Welch-Costas sequence C, making sequence C a p × (p-1) sequence C1. The placement function of sequence C1 is:

[0065] y1(k)=α k ,1≤k≤p-1 (2)

[0066] Suppose there are F families of optimal frequency hopping patterns, and the minimum Doppler distance between families is d (d is a positive integer). d and F need to satisfy equation (3):

[0067] d·F≤p (3)

[0068] Sequence C1 is cyclically shifted vertically by d rows, 2d rows, ..., (F-1)d rows respectively, resulting in F-1 sequences C2, C3, ..., C..., each containing a gap row. F Their placement functions are as follows:

[0069] y2(k)=α k +d, 1≤k≤p-1 (4)

[0070] y3(k)=α k +2d, 1≤k≤p-1 (5)

[0071]

[0072] y F (k)=α k +(F-1)d,1≤k≤p-1 (6)

[0073] Sequence C2, C3, ..., C F This refers to the frequency hopping patterns of the F group heads, and the minimum Doppler distance between these frequency hopping patterns is d.

[0074] Then, design the frequency hopping diagram within each family.

[0075] The frequency hopping diagram C of the obtained j-th (j=1,2,…,F) optimal frequency hopping diagram family head. j Shifting the data horizontally to the left by columns 0, d1, 2d1, ..., (F1-1)d1 respectively yields F1 sequences containing a single gap row, where the nth...j The placement function for (n = 1, 2, ..., F1) sequences is:

[0076]

[0077] Where j = 1, 2, ..., F; n j =1,2,…,F1. That is, the nth element in the j-th optimal frequency hopping family. j There are several optimal frequency hopping patterns. It can be seen that the first optimal frequency hopping pattern in the j-th group is the frequency hopping pattern at the head of the j-th group. d1 (d1 is a positive integer) is the minimum time delay distance between the frequency hopping patterns in each group, and the number of frequency hopping patterns in each group is F1. d1 and F1 need to satisfy equation (8).

[0078] d1·F1≤p-1 (8)

[0079] The above describes the method and steps for constructing the optimal frequency hopping map based on the Welch-Costas sequence using the two-bit cyclic shift method. The optimal frequency hopping map possesses ideal autocorrelation characteristics. Furthermore, if the Doppler distance between two frequency hopping maps is greater than the maximum Doppler shift in the system (Doppler shift normalized using the frequency hopping interval), then these two frequency hopping maps exhibit ideal cross-correlation regardless of the multipath delay. Similarly, if the delay distance between two frequency hopping maps is greater than the maximum multipath delay (multipath delay normalized using the frequency hopping slot), then these two frequency hopping maps exhibit ideal cross-correlation regardless of the Doppler shift. Based on this characteristic of the optimal frequency hopping map, the following section designs the user frequency hopping map in an OFDM system for the architecture of a cellular mobile communication system.

[0080] For cellular cell structures, cellular mobile communication systems employing optimal frequency hopping diagrams utilize the same frequency hopping diagram reuse technique. This means that if the distance between two users exceeds the protection distance of the same optimal frequency hopping diagram, they can use the same optimal frequency hopping diagram. Therefore, designing the optimal frequency hopping diagram for OFDM cellular mobile communication systems only requires designing the optimal frequency hopping diagram for all users within a single cell. The reuse distance D of the optimal frequency hopping diagram can be expressed as:

[0081]

[0082] In equation (9), R is the cell radius, N is the number of cells in a cell cluster, and N = i 2 +im+m 2 , where i and m are non-negative integers that are not both 0.

[0083] Regarding the number of cells and the number of optimal frequency hopping map families in a cluster, in principle, the number of optimal frequency hopping map families should not be less than the number of cells in the cluster. Each cell should be assigned at least one optimal frequency hopping map family, and any extra families can be reserved. If multiple cells are small, they can share one optimal frequency hopping map family; or if a cell is too large, it can be assigned multiple optimal frequency hopping map families. In this invention, it is assumed that one cell uses one optimal frequency hopping map family. Within a cell, each user should be assigned at least one optimal frequency hopping map. If a cell uses one optimal frequency hopping map family, the number of optimal frequency hopping maps within that family should not be less than the number of users.

[0084] Let D be a non-negative integer. max Let d be the maximum Doppler frequency shift of the user in the system after normalization and rounding up of the frequency hopping interval. Assume that the minimum Doppler distance between the optimal frequency hopping families is d, and d and D are... max It must satisfy equation (10):

[0085] 0≤D max <d (10)

[0086] Assume a non-negative integer τ max The maximum multipath delay within the cell after frequency hopping time slot normalization and rounding up is given by d1, where d1 and τ are the minimum delay distances between optimal frequency hopping maps within the same optimal frequency hopping map family. max Satisfying equation (11):

[0087] 0≤τ max <d1 (11)

[0088] According to the optimal frequency hopping diagram design method introduced above, equations (4), (5), and (6) are the placement functions of the first frequency hopping diagram of the F optimal frequency hopping diagram families, and equation (7) is the nth frequency hopping diagram in the j-th cell. j The placement function of the optimal frequency hopping diagram.

[0089] The following example illustrates a method for designing the optimal frequency hopping diagram for users in an OFDM system based on the Welch-Costas sequence.

[0090] (1) Design of frequency hopping diagrams for different cells in an OFDM system:

[0091] Mobile communication systems are all Doppler shift-limited systems because the user's movement speed is finite. The maximum Doppler shift of the OFDM system can be determined based on the movement speed. For example, if the maximum moving speed of an object in the communication environment is v = 20 m / s and the signal wavelength λ is 10 cm, then the maximum Doppler shift is... Use frequency hopping slot F b For the maximum Doppler frequency shift f in the actual system dNormalize, and the maximum Doppler shift after normalization and rounding up is Then, according to the formula 0 ≤ D max <d, d can be determined, and d is the minimum Doppler distance between the optimal frequency-hopping map families allocated to different cells.

[0092] The maximum multipath delay of a cell can be calculated based on the radius of the cell. The speed of light c = 3×10 8 m / s. Assuming the radius of the cell is R, the maximum multipath delay of the cell Use the frequency-hopping time slot T b Normalize the maximum multipath delay t in the actual system d Normalize, and the maximum multipath delay after normalization and rounding up Then, according to the formula 0 ≤ τ max <d1, d1 can be determined, and d1 is the minimum delay distance between the optimal frequency-hopping maps allocated to different users within the cell. Now, substitute numerical values for calculation.

[0093] Suppose there are 4 cells in a cell cluster, take F = 4, that is, there are 4 optimal frequency-hopping map families. Suppose the maximum number of users in each cell is 6, take F1 = 6, that is, there are 6 optimal frequency-hopping maps in each family. Assume that the calculated normalized maximum Doppler shift of the system is 2, that is, D max = 2. To satisfy equation (10), select d = 3. Assume that the calculated normalized maximum multipath delay τ within the cell max = 1. To satisfy equation (11), select d1 = 2. According to equations (3) and (8), it can be calculated that the prime number p is not less than 13. Here, take p = 13, and a primitive element of the finite field GF(13) is α = 2. Construct a 12th-order Welch Costas sequence C, and the placement function is y(k) = 2 k , 1 ≤ k ≤ 12.

[0094] First, add a blank line at the top of the 12th-order sequence C as a gap line to obtain a 13×12 sequence C 1,1 , as Figure 1 (a). Then, shift the sequence C 1,1 upward cyclically 3 times in the vertical direction, with each shift distance being 3 rows, to obtain sequences C 2,1 , C 3,1 and C 4,1 , and the placement functions are C 2,1 : y 2,1 (k) = 2 k + 3, 1 ≤ k ≤ 12, C 3,1 : y 3,1 (k) = 2 k + 6, 1 ≤ k ≤ 12, C 4,1 : y 4,1 (k) = 2k +9, where 1 ≤ k ≤ 12. As Figure 1 (b), Figure 1 (c) and Figure 1 (d) shown. These 4 sequences are respectively the first-hop frequency hopping diagrams of the best frequency hopping diagram families of 4 cells for the first users of the 4 cells. By cyclically shifting the frequency hopping diagrams of these 4 family heads horizontally, the best frequency hopping diagrams of 4 families can be obtained. The value of the cross-correlation function between any two different frequency hopping diagrams in these 4 frequency hopping diagram families does not exceed 1 within the range where the Doppler frequency shift v satisfies 0 ≤ v < d. This ensures that the signals of all users in different cells within the cell group do not interfere with each other.

[0095] (2) Design of the best frequency hopping diagram in the same cell of the OFDM system:

[0096] Cyclically shift the 4 family head sequences C 1,1 , C 2,1 , C 3,1 and C 4,1 horizontally to the left by 5 times respectively, with the moving distance of 2 columns each time. The 5 sequences obtained after the shift can be used as the frequency hopping diagrams for the other 5 users in each cell. For example, the 5 in-family sequences C 1,1 obtained by cyclically shifting the first-family head sequence C 1,2 , C 1,3 , C 1,4 , C 1,5 , and C 1,6 are respectively assigned to the 5 users in the first cell as frequency hopping diagrams. Note: The frequency hopping diagram of the first user in cell 1 uses the frequency hopping diagram of the first-family head. The 6 user frequency hopping diagrams are as Figure 2 (a), Figure 2 (b), Figure 2 (c), Figure 2 (d), Figure 2 (e) and Figure 2 (f) shown, and the time delay distance between them is 2. When the time delay τ satisfies 0 ≤ τ < d1, the value of the cross-correlation function of this in-family sequence does not exceed 1. This can ensure that the signals between all users in the same cell do not interfere with each other.

[0097] Figure 3 and Figure 4 show the principle block diagram of the OFDM communication system applying the best frequency hopping diagram. Compared with the traditional OFDM system, the main difference is that the frequency of the subcarriers is no longer constant, but jumps and changes in different time slots, and the jumping rule is controlled by the pre-designed best frequency hopping diagram.

[0098] Figure 3The diagram shows the transmitter block of an OFDM communication system employing an optimal frequency hopping diagram. The transmitted signal length is N. At the transmitter, N optimal frequency hopping diagram generators control N frequency synthesizers to output N subcarriers. These subcarriers are multiplied by N baseband signals, and then summed to obtain a single signal. This process is similar to IFFT. After completion, up-conversion is performed to generate the final transmitted signal. A i (l) represents the real part of the baseband signal of the l-th path in the i-th time slot, B i (l) represents the imaginary part of the baseband signal in the i-th time slot. cos(Qnk) i,l ) represents the carrier wave for the frequency hopping transition at the output frequency of the frequency synthesizer, where the optimal frequency hopping diagram is used. i,l ) is cos(Qnk) i,l The carrier wave obtained by phase shifting by -90° using a phase shifter, wherein...

[0099] like Figure 4 As shown, a i (l) represents the real part of the received signal of the l-th channel in the i-th time slot, b i (l) represents the imaginary part of the l-th received signal in the i-th time slot. At the receiver, each signal has an optimal frequency hopping diagram generator to generate the optimal frequency hopping diagram, which then controls the frequency synthesizer to output the carrier cos(Qn) for frequency hopping. i,l k), cos(Qn) i,l k) After a -90° phase shift using a phase shifter, sin(Qn) is obtained. i,l k). The OFDM communication system first down-converts the received signal, then performs serial-to-parallel conversion, dividing the signal into N channels. Then, the real and imaginary parts of the received signal in each time slot of the N channels are respectively compared with cos(Qn). i,l k) and sin(Qn) i,l k) Perform multiplication operations and sum them to obtain a total signal. This process is similar to FFT. Finally, perform serial-to-parallel conversion to recover the original baseband signal.

[0100] The above description is only a preferred embodiment of the present invention. The scope of protection of the present invention is not limited to the above embodiments. Any equivalent modifications or changes made by those skilled in the art based on the content disclosed in the present invention should be included within the scope of protection set forth in the claims.

Claims

1. An OFDM communication method based on an optimal frequency hopping diagram, characterized in that: The method includes the following steps: Step 1: Determine the number of optimal frequency hopping diagram families F based on the number of cells in the cellular system; determine the number of optimal frequency hopping diagrams F1 in each cell based on the maximum number of users in the cell; for the structure of the cellular cell system, cellular mobile communication systems using optimal frequency hopping diagrams use the same frequency hopping diagram reuse, that is, if the distance between two users is greater than the protection distance of the same optimal frequency hopping diagram, the same optimal frequency hopping diagram is used. Step 2: Based on the maximum Doppler frequency shift and frequency hopping gap in the system, obtain the normalized and rounded maximum Doppler frequency shift D. max This allows us to determine the minimum Doppler distance d between groups; Step 3: Based on the maximum multipath delay and frequency hopping time slot in the system, obtain the normalized and rounded maximum multipath delay τ. max This allows us to determine the minimum time delay distance d1 between the optimal frequency hopping maps within the same optimal frequency hopping map family; Step 4: Based on the obtained minimum Doppler distance d and minimum time delay distance d1, combined with the optimal number of frequency hopping pattern families F and the optimal number of frequency hopping patterns F1, determine the order n of the Welch Costas sequence and construct the Welch Costas sequence C. Step 5: Design the frequency hopping diagram of the head of the optimal frequency hopping diagram family; add a blank line to the (n+1)th row of the sequence C obtained in Step 4 to obtain the sequence C1; perform a vertical cyclic shift on the sequence C1 according to the minimum Doppler distance d to obtain the placement function of the corresponding sequence. The new sequence is the head frequency hopping diagram of each optimal frequency hopping diagram family. Step 6: Design the frequency hopping diagram within each family; based on the minimum delay distance d1, perform a horizontal cyclic shift on the frequency hopping diagram of the family head obtained in Step 5 to obtain the placement function of the corresponding sequence; Step 7: Following the optimal frequency hopping map design method described above, obtain the placement function of the sequence corresponding to the first frequency hopping map of the F optimal frequency hopping map families, and the nth sequence in the j-th cell. j The placement function of the sequence corresponding to the optimal frequency hopping diagram; Step 8: Apply the optimal frequency hopping diagram to the transmitting end of the OFDM communication system; assuming the transmitted signal length is N, N optimal frequency hopping diagram generators at the transmitting end control N frequency synthesizers to output N subcarriers, and multiply these subcarriers with N baseband signals, then sum them to obtain one signal; at the receiving end, each signal has an optimal frequency hopping diagram generator to generate the optimal frequency hopping diagram, which then controls the frequency synthesizer to output a frequency-hopping carrier, and performs a -90° phase shift via a phase shifter to obtain the shifted carrier; The OFDM communication system first performs down-conversion on the received signal, then performs serial-to-parallel conversion on the signal, dividing it into N signals. Then, it multiplies the real and imaginary parts of the received signal in each time slot of the N signals with the frequency-jumped carrier and the shifted carrier, respectively, and sums them to obtain a total signal, thus completing OFDM communication.

2. The OFDM communication method based on the optimal frequency hopping diagram according to claim 1, characterized in that: In step 1, when designing the optimal frequency hopping diagram for use in an OFDM cellular mobile communication system, it is only necessary to design the optimal frequency hopping diagram for all users within a single cell group. The reuse distance D of the optimal frequency hopping diagram is expressed as: In equation (9), R is the cell radius, N is the number of cells in a cell cluster, and N = i 2 +im+m 2 , where i and m are non-negative integers that are not both 0.

3. The OFDM communication method based on the optimal frequency hopping diagram according to claim 2, characterized in that: In step 1, for a cell cluster, the number of optimal frequency hopping map families should not be less than the number of cells in the cluster; each cell should be assigned at least one optimal frequency hopping map family, and any extra optimal frequency hopping map families should be reserved. If multiple cells are small in size, they can share a single optimal frequency hopping pattern family; or if a cell is too large in size, it can be assigned multiple optimal frequency hopping pattern families. In a cell, each user is assigned at least one optimal frequency hopping map; if a cell uses a family of optimal frequency hopping maps, the number of optimal frequency hopping maps in the family should not be less than the number of users.

4. The OFDM communication method based on the optimal frequency hopping diagram according to claim 1, characterized in that: In step 2, let the non-negative integer D be... max The maximum Doppler frequency shift of the user in the system after frequency hopping interval normalization and rounding up is given by d, where the positive integer d is the minimum Doppler distance between groups, and d and D are... max It must satisfy equation (10): 0≤D max <d (10) The minimum Doppler distance d is determined according to equation (10); where F b For frequency hopping, f d This represents the maximum Doppler frequency shift in the actual system. v is the maximum speed of an object in the communication environment, and λ is the signal wavelength.

5. The OFDM communication method based on the optimal frequency hopping diagram according to claim 1, characterized in that: In step 3, let the non-negative integer τ be... max The maximum multipath delay within the cell after frequency hopping time slot normalization and rounding up is given. The positive integer d1 represents the minimum delay distance between optimal frequency hopping maps within the same optimal frequency hopping map family. d1 and τ... max Satisfying equation (11): 0≤τ max <d1 (11) The minimum time delay distance d1 is determined according to equation (11); where t d For the maximum multipath delay, R is the radius of the cell, c is the speed of light, and T is the speed of light. b This refers to the frequency hopping time slot.

6. The OFDM communication method based on the optimal frequency hopping diagram according to claim 1, characterized in that: In step 4, the minimum Doppler distance d and the optimal frequency hopping pattern family number F need to satisfy equation (3): d·F≤p (3) The minimum delay distance d1 and the optimal number of frequency hopping patterns F1 need to satisfy equation (8): d1·F1≤p-1 (8) The value of p is determined by equations (3) and (8), and then a Welch Costas sequence is constructed based on the finite field GF(p). Assume that α is a primitive element of the finite field GF(p), p is a prime number, and the necessary and sufficient condition for an n = p-1 order permutation sequence C to be a Welch Costas sequence is that its placement function is: y(k)≡α k (mod p),1≤k≤p-1 (1) The symbol "≡(mod p)" indicates congruence modulo p.

7. The OFDM communication method based on the optimal frequency hopping diagram according to claim 6, characterized in that: In step 5, the number of columns cyclically shifted in the horizontal direction of the Costas sequence is defined as the time delay distance between the original sequence and the new sequence formed after the cyclic shift, and the number of rows cyclically shifted in the vertical direction of the Costas sequence is defined as the Doppler distance between the original sequence and the new sequence formed after the cyclic shift.

8. The OFDM communication method based on the optimal frequency hopping diagram according to claim 7, characterized in that: In step 5, a blank line is added to the n = p-1 order Welch-Costas sequence C obtained in step 4, that is, the y(k) = n+1th line, so that sequence C becomes a p × (p-1) sequence C1. The placement function of sequence C1 is: y1(k)=α k ,1≤k≤p-1 (2).

9. The OFDM communication method based on the optimal frequency hopping diagram according to claim 8, characterized in that: In step 5, sequence C1 is cyclically shifted vertically by d rows, 2d rows, ..., (F-1)d rows respectively, resulting in F-1 sequences C2, C3, ..., C..., each containing one gap row. F Their placement functions are as follows: Sequence C2, C3, ..., C F This refers to the frequency hopping patterns of the F group heads, and the minimum Doppler distance between these frequency hopping patterns is d.

10. The OFDM communication method based on the optimal frequency hopping diagram according to claim 9, characterized in that: In step 6, the frequency hopping diagram C of the j-th optimal frequency hopping diagram family head obtained in step 5 is... j Shifting the data horizontally to the left by columns 0, d1, 2d1, ..., (F1-1)d1 respectively yields F1 sequences containing a single gap row, where the nth... j The placement function for each sequence is: Where j = 1, 2, ..., F; n j =1,2,…,F1; That is, the nth element in the j-th optimal frequency hopping family. j The optimal frequency hopping diagram, that is, the first optimal frequency hopping diagram of the j-th family is the frequency hopping diagram of the head of the j-th family.

Images