Bluetooth Technology-frequency hopping technology (I)

Frequency-hopping spread spectrum; FHSS receives signals transmitted by narrow-frequency carriers of a specific type at both ends of synchronization and at the same time, for a non-specific receiver, the hop signal generated by FHSS is also called pulse noise. FHSS signals can be specially designed to avoid noise or non-repetitive channels of one-to-one channels, and these frequency hopping signals must comply with fcc requirements, the maximum interval between 75 or more frequency hopping signals and frequency hopping to the next frequency (dwell
Is 400 ms.

Purpose of Frequency Hopping Technology Introduction

With the rapid development of digital mobile communication networks and the rapid increase of mobile users, the traffic per unit area of the network is also increasing. In busy areas such as the city center of some big cities, even serious traffic congestion occurs. In the face of increasing traffic demands, the network needs to be expanded to meet the requirements of capacity and coverage.
　　
In the initial stage of network construction, due to the small number of users, the coverage problem is first considered in network planning. However, with the continuous expansion and coverage of the network, we found that the capacity problem has become a bottleneck restricting the further development of the network. Due to frequency band restrictions, the GSM network used in China is becoming more and more obvious after so many years of rapid resizing.

For Network resizing, we can usually use the following methods: cell splitting, adding new frequencies, and increasing frequency reusability to increase the configuration of each cell. Obviously, in the early stages of network construction, the Community is usually split, and new base stations (macro and micro cell Base Stations) are constantly added to achieve the purpose of expansion. However, as the distance is approaching, we found that network interference is constantly increasing. Therefore, when the macro cell base station distance reaches a certain level, it is difficult to add new base stations to the network. In this case, the GSM1800 network is introduced on the basis of the GSM900 network. This new frequency band is introduced to solve the problem of network bottle tightness, this is also the dual-band DCS network used by China Mobile and China Unicom in the current network. However, because the GSM900/GSM1800 frequency band is limited and the frequency resources allocated by various operators are different, and considering the high cost of introducing dual-band networks, therefore, you can increase the frequency reusability by using the existing 900 single-frequency network or the GSM1800 dual-band network, increase the capacity configuration per unit area to save network costs and increase the capacity. By introducing wireless link control technologies such as frequency hopping, power control, and discontinuous transmission, the expansion is achieved.

How the frequency hopping system works　　

Frequency Hopping is one of the most commonly used spread spectrum methods. its working principle is that the carrier frequency of the transmission signals between the receiving and receiving sides is discretely changed according to the predefined rules. That is to say, the carrier frequency used in communication is randomly changed due to pseudo-random variation codes.

Frequency-hopping communication principle

From the implementation method of communication technology, "frequency hopping" is a communication method that uses code sequence for multi-frequency Frequency Shift Keying. It is also a communication system that controls the frequency hopping. In the time domain, the frequency hopping signal is a multi-frequency Frequency Shift Keying Signal. in the frequency domain, the frequency spectrum of the frequency hopping signal is a random jump at an unequal interval in a very wide band. The frequency hopping controller is the core component, including the generation, synchronization, and adaptive control of the Frequency Hopping pattern. The frequency generator is used to synthesize the desired frequency under the control of the Frequency Hopping controller; data terminals include data error control. Compared with fixed-frequency communication, hop-frequency communication is relatively hidden and difficult to be intercepted. As long as the recipient does not know the frequency hopping rules, it is difficult to intercept our communication content. At the same time, frequency-hopping communication also has good anti-interference capabilities. Even if some frequency points are disturbed, it can still perform normal communication at other non-interference frequency points. Because the Frequency Hopping Communication System is an instantaneous narrow band system, it is easy to be compatible with other narrow band communication systems. That is to say, frequency hopping stations can communicate with conventional narrow band radio stations, which is conducive to device update.

We all know that the frequency hopping technology is a spread spectrum communication technology. Because the frequency hopping technology has communication secrets and anti-interference, it is first applied to military communication. However, with the development of mobile communication and digitalization, the frequency hopping technology has been applied in the digital cellular system. The GSM mobile communication system adopted in China adopts this technology.

Frequency Hopping means that the carrier frequency is changed according to a certain pattern (sequence) within a very wide band. Information Data D enters Carrier Modulation after it is modulated into a baseband signal with BD bandwidth. The carrier frequency is controlled by the pseudo-random code generator and is randomly changed in the bandwidth BSS (BSS> BD) band, the bandwidth of the baseband signal BD is extended to the bandwidth BSS used for transmitting signals. Variable Frequency Synthesizer is controlled by pseudo-random sequences (Frequency Hopping Sequences), so that the carrier frequency changes with the Sequence Value of the frequency hopping sequence. Therefore, carrier modulation is also called spread spectrum modulation.

Features of the Frequency Hopping System　　

The frequency hopping system has the following features:
　
* The frequency hopping system greatly improves the communication system's anti-interference and anti-fading capabilities;
* Multi-address operations without mutual interference;
* The near-far effect of Direct Spread Spectrum Communication systems does not exist, which can reduce the interference of strong signals from the near end to weak signals from the far end;
* The Anti-interference performance of the Frequency Hopping System is strictly "avoided". The frequency of external interference cannot keep up with the frequency change of the frequency of the Frequency Hopping System;
* The frequency hopping sequence has a low rate. Generally, the bitwise rate is less than or equal to the Information rate. In a TDMA system, the hop rate is usually equal to the number of frames transmitted per second. In the GSM system, each time slot is 576.9 μs long, and eight time slots are one frame. Therefore, the frame length is 4.615 Ms,Frequency Hopping is 217 times per second.

In the GSM digital cellular system, the frequency hopping technology can improve the anti-fading and anti-interference capabilities. The frequency hopping technology has a good anti-fading effect on static or slow-moving mobile stations, however, for the fast-moving Mobile Station, the burst pulse sequence of two connections in the same channel has a low gain because their location difference is sufficient to make them irrelevant to the change in the year over the year, this is the frequency diversity of frequency hopping. Because the frequency is constantly changing during frequency hopping, the frequency interference is instantaneous, so frequency hopping has interference diversity.

Regular frequency hopping

The frequency hopping pattern of the Communication Receiver and receiver is prepared in advance, and is synchronized Based on the frequency hopping pattern. This method is called normal FH ). With the increasing popularity of Electronic Confrontation in modern wars, adaptive frequency hopping is proposed based on conventional frequency hopping. It adds two aspects: frequency adaptive control and power adaptive control. In frequency-hopping communication, the frequency-hopping pattern reflects the signal carrier frequency of both parties, ensuring that the sending frequency of the Communication party can follow the pattern, but it is not easy to be discovered by the other party. The common frequency hopping code sequence is based on m
Pseudo-Random Sequences designed for the sequence, M sequence, and RS code. These pseudo-random code sequences are implemented through the shift register and feedback structure. The structure is simple, the performance is stable, and synchronization can be achieved quickly. They can achieve a long period of time, and the relevant characteristics of hamming are also good, but when there is human intentional interference (such as tracking interference after the prediction code sequence, these sequences have poor anti-interference capabilities.

Adaptive FM technology used by Bluetooth

   Working on Bluetooth 2.4 ~ 2.48 GHz
ISM frequency band, because the frequency band is very crowded (11b/g, microwave oven, cordless phone, etc.), and Bluetooth uses low power consumption (-6 ~ + 4dbm ). Therefore, in order to avoid frequency conflicts, Bluetooth uses AFH (Adaptive Frequency Hopping), LBT (Listen Before talk), power control and other anti-interference measures.
   AFH
The implementation process is device identification, channel classification, classification information exchange, and adaptive frequency hopping.
1. Device Identification: Before Bluetooth devices are interconnected, first exchange information between the two parties based on The Link Management Protocol (LMP: link manager Protocol) to determine whether both parties support the AFH mode, the LMP information contains the minimum number of channels used by both parties. This step is answered by the host.
2. Channel classification: firstly, each channel is evaluated according to the gate restrictions of plrs (Packet Loss ratios) and the CRC, Hec, and FEC Error Parameters of the payload. When the CRC value is measured from the device, the CRC value of this package is also automatically detected, and the correctness of this package has been determined. Then, the master and slave devices form a classification table according to the LMP format. Then, the frequency hopping of the master and slave devices is performed based on this classification table.
3. Channel Information exchange: the master/slave device uses the LMP command to notify all members of the network and exchange AFH information. The channel is divided into good channels, bad channels, and unused channels. Connect the master and slave devices to determine which channels are available and which ones are unavailable.
4. Execute AFH: Advanced FM editing to select the appropriate FM frequency. Due to sudden interference in the environment, the frequency modulation classification table needs to be updated periodically and communicate with each other in a timely manner.
AFH structure:

In the Bluetooth structure, a group Er (Adaptive Frequency selector) is added to the frequency synchronization and frequency modulation sequence generators ).

The Spread Spectrum Technology originally used in the military and intelligence fields is used for Bluetooth communication to ensure the anti-interference and confidentiality of communication. By modulation and Spread Spectrum of the narrow bandwidth information, the original signal is restored in the receiver in the agreed manner, because the spread spectrum will spread the interference signal at the same time, therefore, the power spectral density of the interference signal is greatly reduced, and the signal will be filtered out during decoding and expansion. Therefore, the interference intensity of the interference signal is greatly reduced, this ensures the anti-interference and eavesdropping of Bluetooth communication. The modulation and decoding adopt the general-purpose PSK method, while the Spread Spectrum and decoding adopt the pre-defined pseudo-random Spread Spectrum Code sequence.

Because the Spread Spectrum reduces the power spectral density of the signal, the listening may be greatly reduced, and the interference to other narrowband communication systems is also very small. Bluetooth uses the frequency hopping technology for spread spectrum. The above pseudo-random Spread Spectrum Code sequence is called the frequency hopping sequence in the bluetooth technology. The main bluetooth device determines the frequency hopping sequence. The device uses this sequence to perform frequency hopping at a frequency of 1600 hops per second. Both parties need to synchronize regularly to ensure that they can jump to the same frequency within the allowable error.