The structure of the Bluetooth network piconet is the most basic way to achieve Bluetooth wireless communication. Each piconet has only one master device, and one master device can simultaneously communicate with up to seven slave devices at the same time. Multiple Bluetooth devices form a piconet as shown in FIG. 1.
The scatternet is a Bluetooth network formed by interconnecting multiple piconets with a larger coverage than the piconet. It is characterized by interconnected Bluetooth devices between different piconets, as shown in Figure 2.
Although each piconet has only one master device, the slave device can join different piconets based on the time division multiplexing mechanism, and the master device of one piconet can become the slave device of another piconet. Each piconet has its own independent frequency hopping sequence, and there is no frequency hopping synchronization between them, thereby avoiding co-frequency interference.
The Bluetooth physical layer link Bluetooth technical specifications stipulate that between the master device and the slave device, two different types of links can be established: synchronous link (Synchronous ConnecTIon-OrientaTIon, SCO) and asynchronous link (Asynchronous
ConnecTIon-Less, ACL).
The SCO link is a point-to-point link between the master device and the slave device. The master device runs the SCO link by using reserved slots at regular intervals. Its transmission rate is 64 kb / s, which is used to support voice communication. Each master device supports up to three SCO links simultaneously.
ACL link refers to the time slot reserved in non-SCO, the master device uses the time slot as a unit to exchange data packets with the slave device. Only one ACL link can exist between a master device and a slave device, and each master device supports up to seven slave devices simultaneously. For most ACL packets, packet retransmission is used to ensure data integrity and correctness. After the ACL link is established, the master and slave devices alternately transmit packets in a time-division multiplexing manner. The master device starts transmitting packets only in even-numbered time slots, and the slave device starts transmitting packets only in odd-numbered time slots. The transmission mode of 1 time slot, 3 time slots, and 5 time slots can be selected by the master device or the slave device for group transmission.
Bluetooth ACL packet transmission
The Bluetooth ACL packet data packet is divided into a medium data rate type (Data Medium, DM) and a high data rate type (Data High, DH). The DH packet uses less error correction to obtain a higher data transmission rate.
In a single-slot packet, the 72 b piconet access code and 54 b data packet header need to be transmitted, and a 220 μs guard band is required between the packets. Therefore, the data obtained on the actual transmitted wireless interface The transmission rate is much smaller than the maximum original data transmission rate of 1 Mb / s [5].
In different time slot transmission methods (DH1, DH2, DH3), the payload ratio of Bluetooth data packets is shown in Table 1 [5].
Table 1 Packet payload ratio than the number of slots per packet
1 slot
3 time slots
5 timeslot payload ratio
1/3
7/9
13/15
It can be seen that the use of large time slots as far as possible for data transmission can achieve the effect of improving the overall communication performance of the Bluetooth network through a higher payload.
2 The data transmission algorithm based on BER mode uses Bit Error Rate (BER) to describe the quality of transmission in the data transmission of Bluetooth applications. A larger BER value indicates a higher error rate during communication. The data transmission algorithm used in the current Bluetooth application, in order to simplify the connection management of the Bluetooth network, does not consider the change of the BER in the specific communication link, and uses a single link frame method throughout the transmission process, resulting in low transmission efficiency.
In the actual communication process, due to the influence of the noise of the surrounding environment, the effect of using the single slot mode for data communication is not optimal. If the BER parameter is used to describe when the external environment changes, and different types of data packets are dynamically selected according to the BER parameter, the situation that the data transmission is greatly reduced when the environment changes drastically can be improved.
In the Bluetooth protocol [5], the current link communication quality information can be obtained by calling the Get_Link_Quality function. The value range is between 0 and 255. The larger the value, the better the channel quality and allow the user to measure the link quality. The standard maps it to values ​​between 0 and 255.
Based on the data simulation results of the CSR chip in [6], the BER mode can be divided into four zone segments. During the data transmission process, the real-time detection of the link communication quality is based on the link communication over a period of time. The quality condition selects a better frame mode for data transmission.
According to the above analysis, in order to improve the data transmission efficiency of the Bluetooth network, an adaptive data transmission algorithm based on BER mode proposed in this paper is described as follows:
(1) Use the Get_Link_Quality () function to obtain the current BER status value.
(2) Calculate the average BER during the period.
(3) Analyze the statistical results and adjust the transmission method:
If BER <0.000 152 9, choose DH5 for data transmission;
If 0.000 152 9 ≤ BER <0.006 079 5, the DH3 mode is used for data transmission;
If 0.006 079 5 ≤ BER <0.015 781 3, the DH1 mode is used for data transmission;
If 0.015 781 3 ≤ BER, the DM1 mode is used for data transmission.
(4) Return to (1), continue the next timing statistics and adjust the transmission mode.
When the worst case occurs in the algorithm, the shortest time slot data packet with FEC error correction is used for data transmission in DM1 mode, and the remaining high-speed DH1, DH3, and DH5 methods without FEC error correction are used for data transmission.
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