Keywords: UWB pulse communication signal application
Ultra-wideband technology is a new wireless communication technology. By directly modulating pulses with steep rise and fall times, the signal has a bandwidth of GHz. Ultra-wideband (UWB) technology has solved the main propagation problems that have plagued traditional wireless technology for many years. It has the advantages of insensitivity to channel fading, low power spectral density of transmitted signal, low interception ability, low system complexity and positioning accuracy of several centimeters.
1 UWB signal and its characteristics
The Federal Communications Commission (FCC) stipulates that:
Part of the bandwidth is called UWB signal. Among them, part of the bandwidth is the value measured at the signal power spectral density of-10dB. Figure 1 shows the comparison of power spectral density between UWB signal and narrow-width signal; Fig. 2 shows the UWB signal format.
Typical pulse position modulation (PPM) UWB signal form [1], [2] is:
Str(k)(t) represents the transmission signal of the kth user, which is the sum of a large number of monocycle pulses with different time shifts. W(t) represents the transmitted monocycle waveform, which can be monocycle Gaussian pulse or its first-order and second-order differential pulse, starting from the zero time of the transmitter clock (t(k)=0). The start time of the j th pulse is. Carefully analyze each time-shift component:
(1) pulse sequence with the same time shift: the formal pulse represents a single-cycle pulse with a time step of Tf, and the duty cycle is extremely low. Typical values of the frame length or pulse repetition time Tf (frame time) are 100 to 1000 times of the single-cycle pulse width. Similar to the ALOHA system, such a pulse sequence can easily lead to random collisions.
(2) Pseudo-random time hopping: In order to reduce the collision in multiple access, each user is assigned a specific pseudo-random sequence, which is called time hopping code, and its period is Np. Every symbol of time-hopping code is an integer and satisfies. In this way, the time-hopping code adds a time shift to each pulse, and the additional time shift of the j-th monocycle is seconds.
Because it takes some time to read out the output of the one-cycle pulse correlator, NhTc/Tf should be strictly less than 1. However, if the NhTc is too small, the probability of collision when multiple users access will still be great. On the contrary, if the NhTc is large enough and the time-hopping code is designed reasonably, the multi-user interference can be approximated as AWGN (Additive White Gaussian Noise) signal.
Because the time-hopping code is a periodic sequence with a period Np, it is also a Np periodic sequence with a period Tp=NpTf. Another function of time-hopping code is to make the power spectral density of UWB signal flatter.
(3) Data modulation: The data sequence {di (k)} sent by the kth user is a binary data stream. Each symbol transmits ns monocycle pulses, which increases the processing gain of the signal.
In this modulation mode, the duration of a symbol (or symbols) is Ts=NsTf. For a fixed pulse repetition time Tf, the binary symbol rate Rs is:
Obviously, the UWB pulse communication system using the above signals has the following characteristics: the signal duration is very short, which is nanosecond and sub-nanosecond pulse, and the signal duty ratio is extremely low (1% ~ 0. 1%), so it has good multipath immunity; The frequency spectrum is quite wide, reaching the order of GHz, and the power spectral density is low, so UWB signals have little interference to other systems and strong anti-interception ability; The processing gain of UWB system is very high, and its total processing gain PC is:
For example, when a binary UWB communication system Tf= 1μs, Tc= 1ns, Ns= 100, and the bit rate Rs= 10kbps, the processing gain of UWB signals in this system is 50dB. Compared with other communication systems, its processing gain is very high.
In addition, UWB signal is a very narrow pulse sequence, so it has a very strong penetration ability, can distinguish hidden objects or moving objects behind the wall, can realize the combination of radar, positioning and communication, and is suitable for military tactical communication.
Basic structure of ultra-wideband signal transmitter and receiver
2. 1 transmitter and related receiver models
Compared with the traditional wireless transceiver structure, the structure of UWB transceiver is relatively simple. As shown in Figure 3, at the transmitting end, the data directly modulates the RF pulse, and then the pulse is further delayed by the programmable delay device, and finally transmitted through the ultra-wideband antenna. At the receiving end, the signal is multiplied by the local template waveform through the correlator, and then integrated and sent to the baseband signal processing circuit through the sample-and-hold circuit. The programmable delay is controlled by the capture and tracking part, the clock oscillator and the (time-hopping) code generator. According to the corresponding delay, the local template waveform is generated and multiplied by the received signal. The whole transceiver is almost entirely composed of digital circuits, which is convenient for cost reduction and miniaturization.
2.2 rake receiver model
Because UWB signals need time domain analysis, they are mostly used in indoor dense multipath (multipath can reach 30), and the energy of each path is very small, so it is difficult to estimate each channel, so it is possible to receive UWB signals by Rake. Rake receiver can improve the signal-to-noise ratio and system performance after combining the multipath signals with less energy.
Comparison between UWB and other wireless personal area network technologies
Due to its various advantages, ultra-wideband technology has become one of the main technologies of wireless personal area network (WPAN). The goal of WPAN is to replace the traditional wired cable with radio or infrared ray, realize the intelligent interconnection of personal information terminals within the range of 10m with low price and low power consumption, and form a personalized information network. Its most common application is to connect computers, printers, cordless phones, PDA and information equipment. At present, there are five main technologies to realize WPAN: IEEE 802. 1 1b (win), HORRF, IrDA, Bluetooth and UWB. It can be seen that the advantages of UWB technology are obvious, and the main disadvantage is that the transmission distance is limited by the low transmission power. That is to say, in the range of 10m m, UWB can exert the transmission performance of several hundred Mbps, and the performance of IEEE802. 1 1b+0B or household RF wireless PAN will be stronger than UWB for long-distance applications. UWB will not directly compete with IEEE802. 1 1b+0b and household radio frequency, because UWB is more used indoors, and the distance is about 10m. In fact, it may be more appropriate to regard UWB as a substitute for Bluetooth technology, because the transmission rate of the latter is far less than that of the former, and the protocol of Bluetooth technology is more complicated.
4 Research and development at home and abroad
4. 1 research status abroad
Military: As early as 1965, the United States established the technical foundation of UWB. In the next two decades, UWB technology will be mainly used in military applications in the United States, and its research institutions are limited to military-related enterprises, research institutions and groups. At present, the U.S. Department of Defense is developing dozens of ultra-wideband systems, including battlefield eavesdropping prevention networks.
Civil use: Because of the advantages of UWB technology, it has great potential in wireless communication. In recent years, the research on the application of UWB signals abroad is a hot spot, which is mainly used in communication (such as home and personal networks, highway information service systems and wireless audio, data and video distribution, etc.). ), radar (such as collision/fault avoidance between vehicles and aircraft, intrusion detection and ground penetrating radar, etc.). ) and accurate positioning (such as asset tracking and personnel positioning, etc. Sony, Time Domain, Motorola, Intel, Daimler-Chrysler and other high-tech companies have all participated in the development of UWB technology, connecting various consumer electronic devices with high data transmission rate to meet consumers' requirements for miniaturization, low cost, low power consumption and high-speed data transmission of short-distance wireless communication.
The research on ultra-wideband wireless communication is getting deeper and deeper in international academic circles. From May 20 to 23, 2002, IEEE held a conference, which was devoted to ultra-wideband technology and its application. On February 14, 2002, the Federal Communications Commission (FCC) of the United States formally adopted the proposal of applying UWB technology to civil use, defined three UWB systems: imaging system, communication and measurement system and vehicle-mounted radar system, and specified the EIRP (Omni-directional Effective Radiation Power) of the three systems respectively. However, the protocol and standard of UWB technology have not yet been determined. At present, only the United States allows the use of civil UWB equipment. Europe is discussing the further use of UWB, and look at the UWB standard in the United States.
4.2 Domestic research status
In the "Tenth Five-Year Plan" 863 communication technology research project released in early September, 2000/KLOC-0, the key technologies of ultra-wideband wireless communication and its * * * storage and compatibility technologies were taken as the research contents of wireless communication * * * technologies and innovative technologies, and domestic scholars were encouraged to strengthen research and development in this field. However, the in-depth research on UWB technology in China is limited to radar, and the research on UWB communication system has not yet formed a scale.
References: Wang, Kan Chunrong, Xu.