Abstract: This paper introduces and analyzes UWB technology from ultra-wideband, and makes a preliminary theoretical discussion on its modulation method and the new high-efficiency pulse shape modulation PSM (Pulse Shape ModulaTIon) proposed recently.
Ultra wide band (Ultra Wide Band) as a new type of wireless communication technology is very different from traditional communication methods. Because it does not need to use a carrier circuit, but transmits data by sending nanosecond pulses, the technology has the advantages of simple transmission and reception circuits, low power consumption, little impact on existing communication systems, and high transmission rate. In addition, it has the advantages It has the advantages of strong multi-path resolution, strong penetrating power, good concealment, large system capacity and high positioning accuracy. According to FCC regulations, 7.5GHz bandwidth frequencies from 3.1GHz to 10.6GHz will be used as UWB communication equipment. However, in consideration of the impact on existing wireless systems, UWB transmission power is limited to less than 1mW / MHz.
UWB is a wireless communication technology that can bring low power consumption, high bandwidth, and relatively simple wireless interface technology to wireless local area network LAN, personal area network PAN interface cards and access technologies. It solves the major problems that have plagued traditional wireless technologies for many years, and developed a transmission technology that has many advantages such as insensitivity to channel fading characteristics, low general density of transmitted signal power, difficulty in interception, and low complexity. This technology is especially suitable for high-speed wireless access and military communications applications in dense multipath locations such as indoors.
figure 1
1 Basic concepts
Ultra-wideband (UWB) is also known as impulse radio (Impulse Radio), specifically defined as a signal with a relative bandwidth (the ratio of signal bandwidth to center frequency) greater than 25%, namely:
Bf = B / fc = (fh-fl) / [(fh + fl) / 2]> 25% (1)
Or the bandwidth exceeds 1.5GHz. In fact, UWB signal is a short-duration pulse with extremely short duration and wide bandwidth. Its main form is ultra-short baseband pulses, the width is generally 0.1 ~ 20ns, the pulse interval is 2 ~ 5000ns, the precision is controllable, the frequency spectrum is 50MHz ~ 10GHz, the frequency band is greater than 100% center frequency, the typical point-to-space ratio is 0.1%.
Traditional UWB systems use a pulse called "monocycle". In general, it is generated by a diode or a mercury switch. Gaussian pulses are used to approximate it in computer simulations. Since the antenna has different effects on the pulse, it can be assumed that the transmitted pulse is:
The signals received by the receiving end are:
tc is the time shift of the pulse, and 2tau is the width of the pulse. Figure 1 shows the time-domain pulse shape of the transmitted and received pulses.
2 Performance characteristics of UWB
UWB is different from other existing communication technologies. The most fundamental difference is that it does not require carrier waves, which greatly reduces the complexity of transmitting and receiving equipment and fundamentally reduces the cost of communication.
The advantages of UWB can be summarized into the following eight aspects:
(1) No carrier is required, and the sending and receiving equipment is simple. Because the UWB signal is some ultra-short pulses, the frequency is very high, so it does not need to modulate it to a certain transmission frequency like the traditional baseband signal to transmit in the channel. Therefore, the structure of the transmitter and receiver must be simplified.
figure 2
(2) Low power consumption. Because UWB signals do not require carrier waves and work in the electronic noise band of the spectrum, they only require very low power supply power. Generally UWB system only needs 50 ~ 70mW power supply, and this is only one percent of mobile phones, one tenth of Bluetooth technology.
(3) The transmission rate is high. The extremely wide bandwidth makes UWB have a very high transmission rate. Under normal circumstances, its maximum data transmission speed can reach several hundred Mbps ~ 1Gbps. Intel Corporation of America demonstrated the technology at "IDF2002 Spring Japan" in April 2002, with a transmission rate of up to 100 Mbps over a distance of several meters.
(4) Good concealment and high security. Because the bandwidth of UWB signals is very wide and the transmission power is very low, this communication technology must have the advantage of low interception capability LPD (Low Probability of DetecTIon). In addition, ultra-wideband also uses time-hopping TH (TIme Hopping) spread spectrum technology, the receiving end must be able to demodulate the data information sent under the condition of knowing the spreading code of the transmitting end.
(5) Strong multipath resolution capability. From the time domain perspective, ultra-wideband systems use narrow signals with a pulse width of a few nanoseconds, so they have high time resolution, and the corresponding multipath resolution is less than tens of centimeters; from a frequency domain perspective, due to UWB signals The bandwidth is extremely wide, so it is certain that the frequency selective fading appears during the transmission process. However, it is precisely because of the extremely wide bandwidth that multipath fading occurs only at certain frequency points. Overall, the energy faded out is only a small part of the total energy of the signal, so the technology still has a strong resistance to multipath. Greatness.
(6) Large system capacity. Shannon formula gives
C = Blog2 (1 + S / N) (4)
It can be seen that the increase in bandwidth makes the increase in channel capacity much greater than the effect brought by the increase in signal power. This is also the theoretical mechanism for the proposed ultra-wideband technology.
(7) High-resolution distance resolution. Because the time jitter of UWB positioning equipment is less than 20ps, if the same working principle and algorithm of GPS are used, the corresponding distance uncertainty is less than 1cm. In practical applications, the ultra-narrow pulse signals used by ultra-wideband radar systems have a range resolution of less than 30 cm.
(8) Strong penetration ability. Among the wireless signals with the same bandwidth, the ultra-wideband has the lowest frequency. Therefore, it has a large capacity and a high distance resolution and has a stronger penetration ability relative to the millimeter wave signal.
3 Modulation method of UWB signal
There are many modulation methods of UWB. Take pulse modulation PPM (Pulse PosiTIon Modulation) as an example for analysis.
First define a single-cycle pulse shape:
s (k) represents the signal kth, w (t) is a single-cycle pulse transmitted.
Move it to the beginning of each frame:
Tf represents the pulse repetition period, and j represents the jth single pulse.
Add pseudo-random time-hopping code:
Finally add modulation data: 
Among them, d (k) is information data, δ is time shift. In order to meet the needs of multiple users, improve the security of communications and consider the power spectral density PSD (Power Spectral Density) of the system, time-hopping codes are introduced, and the problem will be analyzed from the perspective of power spectral density.
Assuming that the Gaussian single pulse given in Figure 1 (a) is used as the transmitted signal and is only a series of periodic pulse sequences, due to the periodicity of the time-domain signal, strong energy-like peaks appear in the frequency domain. These peaks will be Interference with existing traditional wireless signals. Therefore, some measures need to be taken to smooth it. If you use PPM modulation to adjust the position of the pulse, you can see that due to the scrambling effect of the modulation, the peak in the frequency domain has been controlled to a certain extent, but it is still more obvious at this time. In order to further reduce the amplitude of peak-like, time-hopping codes are introduced, so that the power spectrum of the transmitted signal will be further smoothed, almost similar to background noise, which is one of the reasons why UWB systems can coexist with existing wireless systems. Figure 2 shows the PSD diagram of the above different signals and the time-domain waveform after the introduction of time-hopping codes.
In addition to PPM, UWB signals can also use pulse amplitude modulation PAM (Pulse Amplitude Modulation), on-off key OOK (On-Off Key) and two-phase shift keying BPSK (Bi-Phase Shift Key). At the receiving end, single-pulse signals can be reliably received by related technologies. Correlator is often used in practical applications. It uses the prepared template waveform to multiply the received RF signal, and then integrates to obtain a DC output voltage. The output of the correlator is the relative time position difference between the received single-cycle pulse and the template waveform, and the signal to be received is found from the output when the time position difference is 0.
In order to pursue higher-efficiency information transmission, recently a new type of pulse modulation-Pulse Shape Modulation (PSM) (Pulse Shape Modulation) has been proposed. PSM is to modulate the pulse shape to achieve the load of information, so the choice of pulse shape is very important. It was put forward because of people's research on hermite polynomials. Since the mathematical expression of the Hermite polynomial is very close to the Gaussian single pulse, and with the change of the order, the duration of the waveform will not change greatly, so people have thought of using the change of the Hermite polynomial number to produce different shapes Pulse to achieve multiple modulations. In order to seek orthogonal waveforms, the hermite polynomial needs to be modified, namely:
After modification, you can get hermite polynomials of each order orthogonal to each other. At this time, n single pulses of different shapes can be sent at the sending end at the same time, and the orthogonality makes them not interfere with each other. The receiving end can separate each signal using the relevant receiving technology.
Figure 3 shows the time domain waveform of the improved hermite polynomial. At the same time, you can get the desired hermite polynomial pulses of various orders by building a simulink circuit. Figure 4 shows the construction of the circuit and the simulation waveform. In the simulink circuit, the order of the Hermite polynomial is controlled by the pulse order unit, and the oscilloscopes 1 and 2 give the corresponding order and the corresponding order minus 1 order hermite pulse shape.
The increase in transmission efficiency leads to a decrease in system performance, which is not tolerated by many systems and therefore requires encoding. First, BCH (7, 4) is used to encode the signal in the shape domain. In this way, the transmission rate is 4 times that of the single pulse, and the error performance is basically the same as the single pulse. Then, the BCH (31, 11) Encoding to further improve performance. Finally, it can also be jointly encoded in the time domain and shape domain. The error performance will be greatly improved, and the transmission efficiency is still higher than that of the single pulse system. The performance curve is shown in Figure 5.
4 Application prospects and development directions
With its many advantages, UWB technology has broad application prospects. UWB first received substantial attention in the US military and government departments, and was quickly applied to the US military ’s radio network (Adhoc) and high-precision radar detection system. in. In February 2002, the FCC permitted UWB technology to enter the civilian field, under the condition that: "Under the condition that the transmission power is lower than the US radiated noise regulation value -41.3dBm / MHz (the conversion success rate is 1mW / MHz), the 3.1G ~ The 10.6 GHz frequency band is used for imaging systems that scan underground and partition walls, automotive collision avoidance radar, and ranging and wireless data communication between home appliance terminals and portable terminals. " Although this technology has so many restrictions in application, it is still favored by the majority of telecommunications developers. Companies such as Time Domain and Multispectral Solutions have proposed to the IEEE-802.15 committee to adopt ultra-wideband technology, and many companies' research departments and even schools have also put the technology research on the agenda. Many now mature technologies have been combined with UWB, such as UWB-OFDM, UWB-Ad hoc, UWB-Wavelet, UWB-Neural network, etc. Some companies have even used these technologies to produce actual civilian products.
Figure 4
The author summarizes the application of ultra-wideband technology into three main aspects of short-range wireless communication, radar detection and precise positioning. Among them, it can be used in short-range wireless communication for ciphertext transmission, audio / video streaming, radio frequency tag recognition, and the physical layer of a centerless self-textile network (Adhoc); radar is mainly used for collision detection radar detection and precision height measurement Learning, through-wall imaging and ground penetrating radar systems; precise positioning can be used for resource tracking and global positioning system GPS (Global Position System). This shows that there are huge business opportunities behind UWB technology.
Of course, if ultra-wideband technology is really used in people's daily life, there are still many challenging topics. This is also the direction of research and development of ultra-wideband technology for a long time now.
(1) Establish a model of the ultra-wideband radio transmitter in the time domain, and design the antenna transfer function from the time domain perspective;
(2) Research on UWB signal generation and optimization of basic functions;
(3) Study the low-level interference with broadband radio signals and tens of millions of interference, effectively balance the relationship between power and communication range;
(4) Research on ultra-wideband time-hopping codes;
(5) Study the mobile Adhoc network protocol and routing protocol, and apply ultra-wideband technology to distributed network structure, blind capture and self-configuration functions; study the application of ultra-wideband networking protocols similar to the "Bluetooth" system; 
(6) Study wireless IP protocol based on ultra-wideband radio transmission technology;
(7) Study the testing technology of ultra-wideband radio, including transmission channel testing, estimation, channel model, etc.
Now the scientific community is setting off a revolution wave of general UWB, and UWB technology has become one of the ten most promising communication technologies in the future. China also attaches great importance to the research on this revolutionary technology, and in the research project on the communication technology theme of the "Tenth Five-Year Plan" 863 program released in early September 2001, the key technology of UWB wireless communication and its coexistence and compatibility technology were regarded as wireless communication. The research content of common technology and innovative technology encourages domestic scholars to strengthen research and development in this area.
Ultra-wideband technology has created a brand-new field in wireless communication and has a very broad market prospect. Perhaps in time, UWB will appear in front of people as the mainstream of wireless interconnection standards, let us wait and see.
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