Personal Area Network (PAN) is a new concept introduced in the field of short-distance (home and small office) wireless communication technology in recent years. The core idea of â€‹â€‹PAN is to replace the traditional wired transmission technology with new wireless transmission technology, realize the intelligent interconnection of personal information terminals, and set up a personalized office or home information network. The PAN technologies that have been implemented are: Ultra-Wideband (UWB), Bluetooth (Bluetoot h), IrDA (Inf rared Data Ass oc iaTI on), and HomeRF. Ultra-wideband has the advantages of high performance and low power consumption, making UWB one of the most competitive technologies in personal area network (PAN) research.
1 Ultra-wideband wireless transmission system
Ultra-wideband wireless technology can be abbreviated as "UWB". Ultra-wideband technology is a state-of-the-art wireless communication technology that uses bandwidths above 1 GHz. Since the frequency band is very wide, although it is wireless communication, the communication speed can still reach several hundred Mbps or more. Ultra-wideband is characterized by the use of an impulse (ultra-short pulse) instead of a carrier. The pulse wave of the UWB transmitter is transmitted directly according to 0 or 1. Since the pulse wave is transmitted only when needed, the power consumption is greatly reduced, and the power consumption of the circuit can be reduced to several tens of milliwatts. Therefore, compared with conventional radios, UWB has the advantages of frequency bandwidth, low average power, good anti-interception performance, strong penetrating power, low cost and excellent multipath resistance.
1. 1 Ultra-wideband (UW B) signal model
Ultra-wideband (UW B) technology refers to a radio technology that has a very high bandwidth ratio (the ratio of RF bandwidth to its center frequency). The fractional bandwidth (ratio of bandwidth to center frequency) of the UWB transmitted signal is greater than 25%. The primary task in achieving ultra-wideband communications is to generate ultra-wideband signals. Ultra-wideband is essentially a technique for transmitting and receiving high-frequency electromagnetic pulses. Different methods can be used to generate and receive these signals and encode the transmitted information. These pulses can be transmitted separately or in groups, and can be based on pulse amplitude. , phase and pulse position encode the information. Signal models are: Pulse Position Modulation (PPM), Pulse Amplitude Modulation (PAM), UWB-CDMA. In ultra-wideband communication, PPM is generally used to modulate the signal, and the spread spectrum method uses time-hopping (TH) spread spectrum. The PPM signal model is shown in equation (1):
Where W(t) is the single-cycle pulse sent, Satisfied to represent the kth user in the multi-user system, tf is the pulse repetition period, wd[ ]tf, d is the information sequence, Î´ represents the transmission pulse delay controlled by the information sequence, and transmits a binary symbol every Ns single-cycle pulse waveform , d denotes the binary "0" and "1" sequence transmitted, "[ ]" denotes the rounding operation, and tc is the transmission pulse delay controlled by the PN code Indicates the starting point of the jth pulse in the transmitted impulse burst, The j-th symbol of the k-th user PN code has a maximum value of NmaxC(k)j and a PN code period of Np.
1. 2 Ultra-wideband technology transmission and reception framework
From the above analysis of the signal model, it is known that the ultra-wideband technology is essentially a carrierless spread spectrum technique in which an impulse pulse with a very low duty cycle is used as an information carrier. A typical ultra-wideband radio directly transmits an impulse burst, which no longer has the traditional concept of intermediate frequency and radio frequency. The transmitted signal can be regarded as a baseband signal or as a radio frequency signal. Impulse pulses typically use a single-cycle Gaussian pulse, and one information bit can be mapped to hundreds of such pulses. Single-cycle pulses have a wide spectrum in the nanosecond range. PPM and PAM are the main modulation methods of UWB technology. The multiple access mode is Pulse Position Multiple Access (PPMA). The time-hopping PN code and information bits jointly control the transmission timing of the impulse. Ultra-wideband communication systems have their own unique system architecture. Figure 1 shows a block diagram of an ultra-wideband system using PPM modulation. The transmitting end transmits PPM modulation to the input binary data stream; the receiver adopts related reception, the input pulse passes through the pulse correlator, then performs pulse sequence integration, and finally detects the decision.
2 Ultra-wideband technology in personal area networks
2. 1 Future Wireless Personal Area Network
In recent years, the concept of a personal area network has been proposed in the field of short-distance (home and small office) wireless communication technologies. Wireless Personal Area Network (WPAN) refers to a network that can make short-range special connections (ad hocconnecTIvities) between portable appliances and communication devices. WPAN coverage is typically within 10m radius and requires support and services for high-rate wireless networks. The high-speed WPAN is mainly for future multimedia applications. It establishes a wireless connection between portable appliances and communication devices. It is a network with smaller WLAN coverage, higher data rate, certain QoS guarantee, and more flexible use. It will also have a place in the future fourth-generation mobile communications.
In order to set standards for high-speed WPANs, IEEE has set up an 802.15.3 high-rate WPAN task group. The 802.15.3 physical layer operates in the unlicensed band between 2.4 GHz and 2.4835 GHz. The data rate can reach 11Mbit/s to 55Mbit/s, which is suitable for the distribution of high-definition video and high-fidelity sound. The 802.15.3 Media Access Control (MAC) layer specification is designed to support special networks, provide multimedia QoS, and support power management. Compared to wireless LANs, the 802.15.3 high-rate WPAN technology is particularly well-suited for portable consumer appliances and communication devices and their applications. Table 1 shows the IEEE 802.15.3 high-rate WPAN standard.
2. 2 Features of UWB technology for personal area networks
Ultra-wideband technology can fully suppress the effects of multipath fading, achieve high-quality indoor short-range wireless communication, and transmit power is small, with little impact on other broadband transmissions. These make ultra-wideband technology a very popular among many technologies for implementing personal area networks. A technology that has great development advantages. Figure 2 shows an example of a personal area network based on ultra-wideband technology, in which the ultra-wideband system has the following characteristics:
2. 2. 1 Wide spectrum and low power consumption
Compared with narrowband transceivers and Bluetooth transceivers for short-range communication, UWB does not need to generate sinusoidal carrier signals, and can directly transmit impulse sequences controlled by hopping pseudo-random codes (PN codes) and information bits. Therefore, it has a wide spectrum and a low average power, which is conducive to coexistence with other systems and improve spectrum utilization.
2. 2. 2 Good concealment and high safety factor
Since the UWB signal adopts time-hopping spread spectrum, its RF bandwidth can reach more than 1 GHz, and the transmission power spectrum density is extremely low. The signal is concealed in the environmental noise and other information numbers. It cannot be distinguished and received by the traditional transceiver. A large number of noise-like transmission information can be demodulated by using a spreading code pulse sequence consistent with the transmitting end, thereby increasing the security of the ultra-wideband communication system.
2. 2. 3 Strong multipath resolution
The carrierless state of the ultra-wideband signal waveform results in little fading even when the pulses overlap. In order to achieve the same performance, only the signal-to-noise ratio required for the additive Gaussian noise channel is about 13.5d B, and the estimation is based on the measured channel data. A signal to noise ratio of about 15 dB is required.
2. 2. 4 The bandwidth of the ultra-wideband signal brings great system capacity.
2. 2. 5 high processing gain
From the above characteristics, UWB technology is most suitable for crowded indoor environment applications and is not suitable for outdoor applications. Ultra-wideband technology is ideal for next-generation physical layer (PHY) chips for high-speed, short/medium-range wireless home networks. It is predicted that UWB can be used as a physical layer for the pending Bluetooth ot h2.0 version and IEEE. A good candidate for the next generation of physical layers of 802. 15.3.
Figure 2 Schematic diagram of PAN based on ultra-wideband technology
2. 3 Research direction of ultra-wideband technology in personal area networks
2. 3. 1 A very practical problem affecting the use of ultra-wideband is interference.
Up to now, UWB uses a very wide bandwidth to transmit and receive radio signals. In fact, there is no such a wide free frequency band, and there is always a part overlapping with the frequency band used by existing wireless technologies, especially for designing to aviation. , military, security, astronomy and other fields. The use of ultra-wideband may interfere with other narrow-band wireless communication methods such as GPS. The tests conducted by the relevant departments on the interference of UW B operating from 400 MHz to 6000 MHz indicate that, under certain conditions, 3. 1 GH z ~ 5. 65 Certain bands of GH z can be used. There are still some problems to be solved for fixed satellite services (3.7 GHz to 4.2 GHz), microwave landing services (5.03 GHz to 5.091 GHz) and Doppler weather radars (5.6 GHz to 5.65 GHz). At present, UWB can only obtain limited applications, and the serious potential interference of UWB systems for narrowband systems is still under further study.
2.3.2 One of the key points of current research on the application of UWB in future WPAN is Ad hoc
Ad hoc is a special-purpose peer-to-peer network. It uses wireless communication technology. The nodes in the direct communication range of the network are routers of each other, and the internal hosts of the Adhoc and the internal hosts and external hosts are realized by the adjacent point forwarding. Communication between. The characteristics of ultra-wideband make it promising to be a flexible and particularly suitable transmission solution in a small space:
First, ultra-wideband provides high-speed data rates in dense multi-path environments, which is what is needed in future wireless systems;
Second, ultra-wideband has the flexibility to handle data rates and power consumption, matching the best data rates based on transmission parameters and environment. As far as the wireless terminal device is concerned, since the carrier is not required, the structure of the receiver will be very simple. Ultra-wideband devices will also be cheaper than traditional wireless technology terminal devices;
Third, although ultra-wideband requires synchronization between transmission and reception, it can work even if the links between the different links in the network are asynchronous. Therefore, for Ad hoc, there is no fixed network facility and the whole network terminals are connected. The synchronization is highly complex and ultra-wideband is also very suitable.
2 .3 .3 The rational design of the Media Access Control (MAC) layer is also the focus of research
High-speed data propagation in a wireless channel is prone to problems such as packet loss, burst loss, and packet delay. At the same time, since the wireless channel changes with time and place, it is difficult to maintain the same QoS service even when the user moves. To solve these problems, the MAC layer needs to be properly designed. The MAC layer design of the UWB system includes control channel access, QoS guarantee, and security.
The demand for capacity of personal area networks is constantly increasing and requires no impact on existing communication systems. Ultra-wideband pulse radio systems meet this requirement. UWB communication is a high-speed wireless access technology with great development potential. It is believed that with the deepening of UWB research, ultra-wideband radio technology will serve people's lives more perfect and more effectively.
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