Abstract: Introduce the design of 2.4GHz fast frequency hopping system based on high performance and low cost ML2724 and DSP, discuss the allocation of frequency hopping channels, the design of frequency hopping patterns, and the problem of frequency hopping synchronization, and give some software implementations flow chart.
2. 4GHz is the most widely used public frequency band for wireless product development. At present, a very popular technical topic-the 802.11 standard of wireless local area network is to use the 2.4 GHz frequency band. For wireless LAN, the biggest argument is its security and stability. Many domestic and foreign literatures pointed out: In addition to using a better key mechanism in wireless LAN, technologies such as spread spectrum and frequency hopping should be widely used to increase its wireless network. Stability and security on the channel. Comparing the performance of using direct sequence spread spectrum and frequency hopping in a wireless local area network, it can be concluded that it is better to use frequency hopping in a wireless local area network. At present, the design of the frequency hopping system usually uses CPLD + FPGA + DSP cooperative frequency synthesizer to achieve, which not only increases the size of the system, but also leads to a high cost of the system. This paper introduces the design of 2.4GHz fast frequency hopping system based on ML2724 and DSP with high performance and low cost. Because the ML2724 integrates a programmable frequency synthesizer, quadrature modulator and various filters, and has a convenient control interface, this can reduce the size and cost; it introduces the channel allocation and PN code design in detail , And the problem of frequency hopping synchronization, and gives a flowchart of some software implementation.
figure 1
1 Introduction to ML2724
ML2724 is a high-performance single-chip integrated transceiver chip widely used in 2.4 GHz fast frequency-hopping communication system by Micro Linear Company. It integrates local oscillator, anti-image â…£ filter, baseband low-pass filter and limiter , Data arbiter, and comes with a programmable frequency synthesizer, with synchronization instructions and various ports connected to baseband processing. It has the following main features:
(1) An integrated single chip capable of completing the transceiver function of the 2.4 GHz communication system;
(2) The channel interval is 2.048MHz, with 80 channels;
(3) All â…¢ filters and data filters are fully integrated;
(4) The sensitivity is -90dBm;
(5) A complete 1.6GHz frequency synthesizer is integrated inside;
(6) As FHSS launch, it can reach 1600 hops / second;
(7) A 3-wire interface can be used to programmatically control the PLL;
(8) With analog received signal strength indicator (RSS1);
(9) At 2.4GHz, the transmission distance is 10m ~ 1000m, and the transmission speed can reach 1.5Mbps;
(10) It can be applied to TDD and TDMA communication systems.
Its internal block diagram is shown in Figure 1.
figure 2
2 System composition principle and design
2.1 The composition principle of the system
The composition principle of the 2.4GHz fast frequency hopping system designed by the author is shown in Figure 2. The system is mainly completed by ML2724 and DSP. The system can reach 1600 hops / second. If the frequency hopping pattern is orthogonal arrangement, the maximum number of frequency hopping points of this system can be 80, and the non-orthogonal arrangement is 40. Within the frequency hopping bandwidth range, the frequency hopping interval is 1.024MHz and the center frequency is 2. At 4GHz, the frequency hopping processing gains are 16dB (orthogonal) and 16dB (non-orthogonal) respectively; DSP completes the encoding and decoding processing of baseband signals, and frequency hopping is the same. Step control, transceiver control, and extraction of frequency hopping synchronization information and control of voice codec. Voice coding can be implemented using AMBE2000, or MC3518 can be used to implement CVSD voice coding. The following will discuss the allocation of frequency hopping channels, the design of frequency hopping patterns, and the problem of frequency hopping synchronization.
2.2 Setting of frequency hopping channel
According to the internal frequency division rule of ML2724, the adjacent frequency intervals do not overlap (non-orthogonal arrangement), the number of frequency hopping points of the system can only be up to 40 frequency points, because the number of frequency hopping points is too small to meet the system anti-interference index, so The orthogonal arrangement between adjacent frequency points is adopted in this system, as shown in Figure 3. That is, the interval of the transmission frequency is allowed to overlap, and the interval is 1.024MHz. Although the orthogonal arrangement is the same as the system bandwidth of the conventional arrangement, the number of frequency hopping points is doubled, and the frequency hopping processing gain is improved by 3dB.
2.3 Design of frequency hopping patterns and sequences
Due to the limitation of the system operating frequency point, the frequency point can only be selected pseudo-randomly among the 80 (orthogonal arrangement) frequency points. In terms of performance of frequency hopping systems, the system has the following requirements for the generation and performance of frequency hopping patterns:
(1) The periodicity of the frequency hopping pattern should be long enough, and the linear complexity should be large enough to achieve high-density performance and strong resistance to deciphering;
(2) The cross-correlation performance of frequency hopping patterns between users in the same network is better, and the auto-correlation performance of frequency hopping patterns is also good to reduce the loss of signal-to-noise ratio caused by collisions;
(3) It has good randomness, making it difficult for other parties to capture and decrypt;
(4) Each frequency point is evenly distributed within the frequency band to enhance anti-interference performance;
(5) The number of frequency-hopping instruction codes should be more and can be replaced to facilitate the use of multiple networks, which can further improve the system's resistance to interception. 
From the perspective of domestic and foreign data and research, the construction of frequency hopping patterns is usually generated using m-sequences, M-sequences, Gold codes, and clock codes. These codes have their own advantages and disadvantages. Among them, the clock code has the best performance and a large number; the nonlinear code has good correlation performance, high coding complexity, and is difficult to decipher, but the structure is complex, and it is difficult to select the code; the number of Gold codes is large, and the period is long , With three-value cross-correlation; RS code has good correlation but short period; M sequence is also a non-linear code, although its number is large, but the correlation is poor; m sequence has good correlation performance and simple implementation, but the number is small. Combining various factors, the author adopts the most complete and easy-to-produce m sequence in theoretical research, and increases the complexity of the sequence through the method of nonlinear transformation, and makes it nonlinear, with excellent autocorrelation and cross-correlation performance . A. Lempl and H. Greenberger proposed the construction model of the frequency-hopping sequence cluster with the best Hamming correlation performance in 1974, which is based on the n-level m-sequence generator on the finite field GF (P). After adding a number of adjacent stages (k≤n) of the generator to a certain k item-by-item mode P to control the frequency synthesizer, this model is called the LG model. There is a serious frequency retention problem in the LG model: whenever n times XX ... X, XεGF (P), X ≠0 appear in the shift register, the input to the frequency synthesizer is continuous n-k + 1 transition k weight XX ... X. In this way, the signal stays on a certain frequency for a long time, and it is easy to be detected by illegal receivers.
Therefore, the LG model is improved, k non-adjacent stages are used to alleviate the problem of frequency retention, and a smooth replacement algorithm [5] is used to process the wide interval of the frequency hopping pattern. Define frequency band F:
F = {f1 | 0≤i≤N-1}
As long as | fi + 1-fi | ≥d is satisfied, it is called a wide-interval frequency hopping point, otherwise it is called a narrow point. There is a correction relationship for narrow points:
PN (i + 1) = [PN (i) + d] modN
Among them, N is the frequency hopping frequency points; d is the frequency hopping interval; PN (i) is the frequency hopping code number.
In this way, after the correction process is performed on the narrow point, the frequency point determined in the frequency domain F constitutes the required wide interval frequency hopping pattern set. This method does not need to construct dual frequency points or dual frequency bands, which guarantees the randomness of the pseudocode sequence, and is equivalent to the second non-linear transformation, so that the degree of nonlinearity and anti-decoding ability of the pseudocode (m sequence) is greatly improved. Enhanced. Thus, a wide-interval frequency hopping pattern that satisfies the requirements of the wide interval and overcomes the shortcomings of the LG model is obtained. The construction model is shown in FIG. 4. This kind of frequency hopping sequence construction model is not difficult to realize with FPGA or CPLD, but from the system cost, using DSP with ARM core to achieve. Of course, the frequency hopping frequency synthesizer uses the frequency synthesizer integrated inside the ML2724. 
2.4 Consideration of system synchronization
The synchronization of frequency hopping systems is a prerequisite for successful communication. If there is no synchronization, the signal code cannot be demodulated, and the anti-interference of the frequency hopping system cannot be exerted. Due to the inconsistency of the sending and receiving clocks, the start-up time difference of the frequency hopping sequence, the propagation delay of the radio wave and other factors, the start of the frequency hopping sequence started by the receiving end and the received transmission frequency hopping sequence are always out of sync. Therefore, the receiving end must adopt certain technical measures to force the local frequency hopping sequence to synchronize with the transmitting frequency hopping sequence. This is the acquisition of the frequency hopping code; after synchronization is achieved, the interference of noise and some external factors will also force the synchronization that has been achieved There is a loss of lock phenomenon, for this reason, the technology of keeping synchronization should also be adopted, which is synchronous tracking.
Frequency hopping synchronization is an organic combination of system initial synchronization, late network entry service synchronization and fast synchronization. Initial synchronization means that users in the network can quickly achieve synchronization and enter normal communication by searching for initial synchronization information. 
It is the main means of communication between the two parties of communication; service synchronization is the user who is in the search state after the initial synchronization information is sent, and joins the network communication in time by searching the synchronization information inserted in the user data stream in the network, and the service information is also synchronized And the guarantee of fast synchronization. The establishment of synchronization is inseparable from the synchronization information. The synchronization information content designed by this system includes related codes, marks, bit synchronization, network number, frequency indication number, TOD and other information. At the beginning of the communication, the receiver continuously searches for synchronization information. When the synchronization information is detected twice from the four cyclic frequencies, it is considered that synchronization has been caught, and according to the relevant codes and flags, the start time of the communication is determined; and subsequently The frequency hopping point transmits the same bit synchronization, network number, and frequency number information. In order to increase the anti-interference ability, the majority decision is adopted. Finally, after receiving the TOD information, the corresponding frequency hopping pattern can be converted to start the transmission of data or digital voice information.
In order to enable the radio station to access the network later and maintain synchronization, it is necessary to carry certain synchronization information (duty synchronization) in the transmission data stream to meet the communication requirements. In late network synchronization, if the detection fails in this frame, the search continues in the next frame until the correct service synchronization information is grasped. In addition, using the service synchronization information in each frame can achieve synchronization maintenance. When the synchronization information of each frame appears, the receiver enlarges the search window in the frequency hopping area where the relevant code and mark appear, and uses the received relevant information to adjust the frequency hopping clock that has shifted to achieve the purpose of synchronization maintenance.
3 System software design
The software design of system synchronization mainly considers the initial synchronization acquisition and synchronization processing of the receiving part. The reception synchronization process is completed by the DSP and the ML2724 together. The DSP (with an ARM core) extracts synchronization information from the ML2724, and after related operations, determines whether the frequency hopping has reached synchronization. After obtaining the initial synchronization, the data is handed over to the DSP to complete the main synchronization process. The software flow of the system receiving synchronization process is shown in Figure 5.
In the field of communication technology, spreading and frequency hopping technologies show their unparalleled advantages in other transmission systems due to their low interception rate, good confidentiality, strong anti-interference and anti-fading capabilities, flexible multiple access connections, and low interference to narrowband signals. It is widely used in various fields such as radar, navigation, communication, remote control and telemetry, especially the successful application in military communications, which has been highly valued by the military of various countries. In civil use, frequency hopping technology is also widely used. In modern wireless communications where frequency resources are increasingly tight, frequency hopping communication systems achieve multiple access and increase system capacity through the excellent self- and cross-correlation characteristics of frequency hopping modulation pseudocode . The 2.4GHz fast frequency hopping system designed in this paper can be widely used in small local area wireless communication systems, such as the currently popular wireless local area network, especially the wireless local area network can be used to achieve wireless coverage in hot areas.
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