Ultra-wideband communication technology and its application
Abstract: Ultra-Wide Bandwidth (Ultra-Wide Bandwidth) impulse communication (Impulse Radio) technology is very different from other communication technologies. It has the advantages of low signal power spectral density, difficult detection, and low system complexity. It is especially suitable for indoor use. High-speed wireless access and military communications in dense multi-path locations. The signal representation of UWB system is introduced, its characteristics are analyzed, and the current research and application of UWB communication are introduced.
UWB technology is a new type of wireless communication technology. It directly modulates impulse pulses with very steep rise and fall times, so that the signal has a bandwidth on the order of GHz. Ultra-wideband technology solves the major propagation problems that have plagued traditional wireless technology for many years. It has insensitivity to channel fading, low transmit signal power spectral density, low interception capability, low system complexity, and can provide positioning accuracy of several centimeters. advantage.
1 UWB signal and its characteristics
The Federal Communications Commission (FCC) provides:
Partial bandwidth 
The number is called UWB signal. Among them, part of the bandwidth is the value measured at -10dB of the signal power spectral density. Figure 1 compares the power spectral density of UWB signals and narrow-width signals; the UWB signal format is shown in Figure 2. 
A typical pulse position modulation (PPM) UWB signal form [1], [2] is:
Str (k) (t) represents the transmitted signal of the k-th user, which is the sum of a large number of single-cycle pulses with different time shifts. w (t) represents the transmitted single-cycle pulse waveform, which can be a single-cycle Gaussian pulse or its first- and second-order differential pulses, starting from the zero time of the transmitter clock (t (k) = 0). The start time of the jth pulse is 
. Carefully analyze each time-shift component:
(1) Pulse sequence with the same time shift: 
The pulse in the form represents a single-cycle pulse with a time step of Tf, and its duty cycle is extremely low. The typical value of the frame length or pulse repetition time Tf (Frame TIme) is one hundred to one thousand times the width of the single-cycle pulse. Similar to the ALOHA system, such a pulse sequence can easily cause random collisions.
(2) Pseudo-random time hopping: In order to reduce conflicts during multiple access, assign a specific pseudo-random sequence to each user 
, Called time-hopping code, whose period is Np. Each symbol of the 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 single-cycle pulse is 
second.
Since it takes a certain time to read the output of the single-cycle pulse correlator, NhTc / Tf should be strictly less than 1. However, if NhTc is too small, then the probability of collisions when multiple users access is still high. On the contrary, if the NhTc is large enough and the time-hopping code design is reasonable, the multi-user interference can be approximated as an additive white Gaussian noise (AWGN) signal.
Since the time-hopping code is a periodic sequence with a period of Np, then 
It is also a Np periodic sequence, whose period is Tp = NpTf. Another role of time-hopping code is to make the power spectral density of UWB signal more flat.
(3) Data modulation: The data sequence {di (k)} sent by the k-th user is a binary data stream. Each symbol transmits Ns single-cycle pulses, which increases the processing gain of the signal.
In this modulation method, the duration of one symbol (or symbol) is Ts = NsTf. For a fixed pulse repetition time Tf, the binary symbol rate Rs is:
Obviously, the ultra-wideband pulse communication system using the above signal has the following characteristics: the signal duration is extremely short, which is nanosecond or sub-nanosecond pulse, and the signal duty cycle is extremely low (1% ~ 0.1%) Good multipath immunity; the spectrum is quite wide, up to the order of GHz, and the power spectral density is low, so the UWB signal has little interference with other systems and strong anti-interception ability; the UWB system has a high processing gain, and its total processing gain PC is
For example, when a binary UWB communication system has Tf = 1μs, Tc = 1ns, Ns = 100, and bit rate Rs = 10kbps, the processing gain of the system UWB signal is 50dB. Compared with other communication systems, its processing gain is very high.
In addition, the UWB signal is a very narrow pulse sequence, so it has a very strong penetration ability, can identify hidden objects or objects moving behind the wall, can realize the combination of three functions of radar, positioning, and communication, suitable for military use Tactical communication.
image 3
2 Basic structure of UWB signal transmitter and receiver
2.1 Transmitter and related receiver models
Compared with the traditional wireless transceiver structure, the structure of the UWB transceiver is relatively simple. As shown in Fig. 3, at the transmitting end, the data directly modulates the RF pulse, and then the pulse is further controlled by a 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 after integration, it is sent to the baseband signal processing circuit through the sample and hold circuit. The programmable delay is controlled by the capture tracking part, clock oscillator and (time-hopping) code generator The generator generates a local template waveform according to the corresponding delay and multiplies it with the received signal. The entire transceiver is almost entirely composed of digital circuits, which is convenient for cost reduction and miniaturization.
2. 2 Rake receiver model
Because the UWB signal needs to be analyzed in the time domain method, it is mostly used under the condition of indoor dense multipath (multipath can reach 30), and the signal energy of each path is very small, making it difficult to estimate each channel. Therefore, Rake reception of UWB signals is possible. The Rake receiver makes the original multi-path signal with very small energy to improve the performance of the signal-to-noise ratio after energy combination. Suppose there are Nu users in a UWB communication system, and their transmitted signals are 
The signal received by a receiver is r (t). If you want to get the data sent by the first user, then the implementation block diagram of its Rake receiver is shown in Figure 4.
Figure 4
3 Comparison of UWB and several other wireless personal area network technologies
Due to various advantages of UWB technology, it has become one of the main technologies of WPAN (Wireless Personal Area Network). The goal of WPAN is to replace traditional wired cables with radio or infrared, realize the intelligent interconnection of personal information terminals at a low price and low power consumption within 10m, and establish a personalized information network. Its most common application is to connect devices such as computers, printers, cordless phones, PDAs, and information appliances. At present, the main technologies to realize WPAN are: IEEE802.11b (Win), Home RF, IrDA, Bluetooth (Bluetooth) and ultra-wideband.
It can be seen from Figure 5 that the advantages of UWB technology are more obvious. The main disadvantage is that the transmission power is too small, which limits its transmission distance (as shown in Figure 6). In other words, within 10m, UWB can exert a transmission performance of up to hundreds of Mbps, and the performance of IEEE802.11b or Home RF wireless PAN for long-distance applications will be stronger than UWB. UWB will not compete directly with the popular IEEE802.11b and Home RF, because UWB is more used indoors at a distance of about 10m. In fact, it may be more suitable to think of UWB as a substitute for Bluetooth technology, because the latter has a transmission rate far less than the former, and the protocol of Bluetooth technology is also more complicated. 
4 Research and development at home and abroad
4.1 Current status of foreign research
Military aspect: As early as 1965, the United States established the technical foundation of UWB. In the next two decades, UWB technology was mainly used for military applications in the United States, and its research institutions were limited to military-related enterprises and research institutions and groups. At present, the U.S. Department of Defense is developing dozens of UWB systems, including battlefield eavesdropping prevention networks.
Civilian aspect: Due to the advantages of ultra-wideband technology, it has great potential in wireless communication. In recent years, foreign research on UWB signal applications has been more popular, mainly used in communications (such as home and personal networks, highway information service systems And wireless audio, data and video distribution, etc.), radar (such as vehicle and aircraft collision / fault avoidance, intrusion detection and ground penetrating radar, etc.) and precise positioning (such as asset tracking, personnel positioning, etc.). High-tech companies such as Sony, Time Domain, Motorola, Intel, Daimler-Chrysler, etc. have all been involved in the development of UWB technology, connecting various consumer electronic devices with a high data transmission rate to meet consumers' short-range wireless Communication miniaturization, low cost, low power, high speed data transmission and other requirements. 
The international academic community has also conducted in-depth research on ultra-wideband wireless communications. From May 20 to 23, 2002, IEEE held a conference dedicated to UWB technology and its applications. On February 14, 2002, the Federal Communications Commission (FCC) formally adopted the proposal to apply UWB technology to civilian use, defining three UWB systems: imaging system, communication and measurement system, vehicle-mounted radar system, and the three systems The EIRP (omnidirectional effective radiated power) is specified separately. However, the agreement and standards of UWB technology have not yet been determined. At present, only the United States allows the use of civilian UWB devices; while Europe is discussing the further use of UWB and watching the UWB standard in the United States.
4.2 Status of domestic research
In the "Tenth Five-Year Plan" 863 Program Communication Technology Research Project released in early September 2001, the key technologies of ultra-broadband wireless communication and their coexistence and compatibility were regarded as the research content of wireless communication common technology and innovative technology. R & D work. However, the current 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.
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