Introduction of Bluetooth Wireless access system

Personal communication is the highest goal of human communication, and it utilizes all kinds of possible network technologies to realize any time and place between people. Any kind of communication. In the near distance communication, Bluetooth wireless access technology makes the communication between wireless cells become very easy, the computer technology and communication technologies are more closely combined, people can exchange and transmit information anytime and anywhere. In addition, Bluetooth technology can provide a common interface for digital networks and peripherals to build a personal special connection device group away from the fixed network.
1 Wireless band selection and anti-jamming
Bluetooth technology uses the 2400~2483.5mhz ISM (industrial, scientific and medical) frequency band, this is because: (1) There is no other system in the band signal interference, at the same time the frequency band to the public, without concessions, (2) Frequency band in the global scope of effective. The relevant laws and regulations of different countries and regions in the world generally stipulate only the transmission range and maximum transmission power of the signal. For a system operating on a global scale, its chosen band must meet all the requirements at the same time so that any user can access it, so the required elements must be minimized. In the case of satisfying the rule, the wireless frequency band can be freely connected, at this time, the anti-interference problem becomes very important. Because the 2.45GHZ ISM band is open band, use any of these frequencies will encounter unpredictable sources of interference (such as some household appliances, cordless phones and car doors, etc.), in addition, external and other Bluetooth users of the source of interference should also be fully estimated.
Anti-jamming methods are divided into avoiding interference and restraining interference. Avoiding interference can be achieved by reducing the signal emission level of each communication unit, and suppressing interference is realized by encoding or direct sequence spread spectrum. However, in different wireless environments, the interference of the special-purpose system and the dynamic range of the useful signal change greatly. In the case of more than 50dB and different ambient power differences, it is not enough to achieve 1mb/s above rate, only by encoding and processing gain. Conversely, it is easier to avoid interference because the signal can be sent when the frequency (or time) is not disturbed (or when the interference is low). If the time-avoiding interference method is used, the signal sent will be aborted when the pulse interference is encountered in time domain. Most wireless systems are limited, and in the 2.45GHZ band, the system bandwidth is 80MHz, can find a period of no obvious interference spectrum, while using frequency domain filter to suppress the rest of the spectrum of wireless frequency band, to achieve the desired results. Therefore, it is more feasible to avoid interference method in frequency domain.
2 Multiple Access system and modulation mode
The choice of special system multiple access system is because there is no uniform regulation in the ISM band. The advantage of Frequency division multiple access (FDMA) is that the orthogonality of the channel depends only on the accuracy of the crystal oscillator, and the adaptive or dynamic channel allocation structure can avoid interference, but the single FDMA can't meet the spread spectrum requirement in the ISM band. The channel orthogonality of time division Multiple access (TDMA) requires strict clock synchronization, so it is very difficult to keep common timing reference in the connection of multi-user special system. Code Division Multiple Access (CDMA) can realize spread spectrum, apply to asymmetric system, can make the special system achieve the best performance. Direct sequence (DS) CDMA due to near-far effect, requires consistent power control or additional gain, same as TDMA, its channel orthogonality also need a common timing reference, with the increase in the number of uses, will require higher chip speed, wider bandwidth (anti-interference) and more circuit consumption. Frequency hopping (FH) CDMA combines the advantages of the special wireless system, the signal can spread to a very wide range, so the effect of narrowband interference becomes very small. Frequency hopping carrier is orthogonal, through filtering, adjacent frequency hopping interference can be effectively suppressed, and the communication interruption caused by narrowband and user interference can be solved by high level protocol. In the ISM band, the FH system has limited signal bandwidth within 1MHZ. In order to improve the robustness of the system, the binary modulation structure is selected. The data rate is lower than 1mb/s due to bandwidth constraints. In order to support burst data transmission, the best way is to adopt incoherent demodulation detection. The Bluetooth technology adopts Gaussian type frequency shift keying (GFSK) modulation, and the modulation coefficient is 0.3. The logic "1" sends the positive frequency deviation, the logic "0" sends the negative frequency deviation. Demodulation can be accomplished by a band-limited FM frequency discriminator.
3 Media access Control (MAC)
Bluetooth system can achieve a large number of asymmetric communication in the same area. With other special system to implement a certain range of units to share the same channel, Bluetooth system is designed to allow a large number of independent channels exist, each channel is limited User service. It can be seen from the modulation mode that, in the ISM band, the bit rate supported by a FH channel is 1wb/s. Theoretically, the 79 carrier spectrum support 79MB/S, because the frequency hopping sequence is not orthogonal, the theoretical capacity 79mb/s is impossible to achieve, but can be far more than 1mb/s.
A FH Bluetooth channel is connected to a tiny net. The Pico-network channel is defined by a main unit (which provides a frequency hopping sequence) and the system clock (which provides the frequency hopping phase) and the other is from the unit. Each Bluetooth wireless system has a local clock and does not have the usual timing reference. When a pico-net is established, the clock compensation from the unit is synchronized with the main unit, and the compensation is canceled after the Pico-Net is released, but it can be stored for reuse. Different channels have different main units, so there are different frequency hopping sequences and phases. The number of units in a common channel is 8 (1 main 7) to ensure effective addressing and bulk communication between cells. The Bluetooth system is based on the Peer-to-peer communication, the master-slave task is only valid in the Pico-net lifetime, when the Pico-Net is canceled, the master-slave task is canceled. Each unit can be a primary/from unit, which can be defined as the primary unit of a Pico net. In addition to the definition of a pico net, the main unit also controls the information flow of the Pico network and manages the access. Access is not free competition, the 625PS resident time allows only one packet to be sent. The access mode based on competition needs more overhead and less efficiency. In the Bluetooth system, the centralized control of the main unit is implemented, and the communication exists only between the main unit and one or more from the unit. When communication between master and slave units, the time slot is alternately used. In the main cell transmission, the main unit determines a communication from the cell address, in order to prevent the channel from sending conflicts from the unit, the use of rotation detection technology, that is, each from the main time slot, from the main unit to determine which to send from the unit. This decision is based on the information sent in the previous slot, and is only available for forwarding from the address selected from the previous primary. If the main unit sends information to a specific unit, it is detected from the unit and can send information. If the main cell sends a message at the end, it sends a detection packet to indicate the detection from the unit. The information flow system of the main unit includes uplink and downlink, and an intelligent system algorithm is considered from the element feature. Master unit control can effectively prevent cell collisions in the Pico network. Interference may occur when the same frequency hopping is used for each individual pico-net unit. The system utilizes Aloha technology, when the information transmits, does not detect whether the carrier is no-load (does not listen), if the information receives is not correct, will carry on the postback (only has the data). Because of the short dwell period, the FH system is not suitable to avoid the conflict structure, for each frequency hopping, will encounter different competition units, backward (Backoff) mechanism efficiency is not high.
4 Packet-based communications
Bluetooth system is based on packet transmission: The information Flow Fragmentation (group) packaging, in each time slot only send a packet. All packet formats are the same: start with an access code, then the header, and finally the load.
Access codes have pseudo random properties, and direct sequence encoding can be used in some pickup operations. The pickup code includes the primary cell logo of the Pico Net, on which the all packet exchanges are identified using the main unit logo, only access codes and access codes that are connected to the primary unit of the Pico network are matched, so that a pico-net packet is received just as it is loaded into another pico-net unit of the same frequency-hopping carrier. On the access side, the access code matches the code required in a sliding correlator, and the correlator provides direct sequence processing gain. Baotou contains: From the address of the control information 3bit to distinguish between the mesh from the unit, to indicate whether the need for automatic query mode (ARQ) Response/non-response 1bit, packet encoding type 4bit, define 16 different load types; head error detection code (HEC) 8bit, Cyclic redundancy detection Coding (CRC) is used to check header errors. In order to limit overhead, the packet header is only 18bit and header is further protected by a 1/3 rate forward error Correction code (FEC).
The Bluetooth system defines 4 kinds of control Packs: (1) ID control packs, contains only the access code, used for signaling, (2) null (NULL) packet, only access code and header, must be used to transmit connection information in Baotou, (3) detection (POLL) package, similar to empty package, for the main unit force to return a response from the unit (4) FHS packet, FH synchronous package, for exchanging real-time clock and sign information between cells (including all the information needed for two unit FH synchronization to define the synchronous or asynchronous business of the package).
In the time slot channel, synchronous and asynchronous connections are defined. At present, asynchronous connections are supported for 2/3-rate FEC coding, and can be used for single time slots, 3 time slots, and 5 o'clock-gap packets. The maximum user rate for an asynchronous connection is 723.2kb/s, at which point the reverse connection rate can reach 57.6kb/s. By exchanging packet length and FEC coding dependent on the connection condition, the adaptive connection can be used for the asynchronous chain, depending on the valid user data and the load length is variable. However, the maximum length is limited to the minimum exchange time between Rx and TX (200Ps). For synchronous connections, only a single time slot packet transmission is defined, with a fixed load length of 1/3, 2/3 or no FEC. The synchronous connection supports Full-duplex, and the user rate is 64kb/s in both directions.