Abstract: This paper introduces and analyzes the ultra-wideband UWB technology, and makes a preliminary theoretical discussion on its modulation method and the recently proposed new Pulsing Shape Modulation (PSM).
Ultra Wide Band is a new type of wireless communication technology that is quite different from traditional communication methods. Since it does not need to use a carrier circuit, but transmits data by transmitting nanosecond pulses, the technology has the advantages of simple transmitting and receiving circuits, low power consumption, low impact on existing communication systems, and high transmission rate. Multipath resolution, strong penetrability, good concealment, large system capacity, and high positioning accuracy. According to the FCC regulations, the 7.5 GHz bandwidth frequency from 3.1 GHz to 10.6 GHz will be used as a UWB communication device. However, due to the impact on existing wireless systems, UWB's transmit 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 to the interface card and access technology of wireless local area network LAN, personal area network PAN. It solves the major problems that have plagued traditional wireless technologies for many years, and has developed a transmission technology that has many advantages such as insensitivity to channel fading characteristics, low transmission power density, difficulty in interception, and low complexity. This technology is especially suitable for high-speed wireless access and military communication applications in dense multi-path locations such as indoors.
figure 1
1 basic concept
Ultra-wideband (UWB), also known as Impulse Radio, is specifically defined as a signal with a relative bandwidth (ratio of signal bandwidth to center frequency) greater than 25%, ie:
Bf=B/fc=(fh-fl)/[(fh+fl)/2]>25% (1)
Or the bandwidth is over 1.5GHz. In fact, the UWB signal is a short-time pulse with a very short duration and a wide bandwidth. Its main form is ultra-short baseband pulse, the width is generally 0.1-20ns, the pulse interval is 2~5000ns, the precision is controllable, the spectrum is 50MHz～10GHz, the frequency band is greater than 100% center frequency, and the typical point-to-space ratio is 0.1%.
Traditional UWB systems use a pulse called a "monocycle pulse." In general, it is generated by a pass diode or a mercury switch. Gaussian pulses are used in computer simulations to approximate it. Since the antenna has different effects on the pulse, it can be assumed that the transmitted pulse is:
The signal received by the receiver is:
Tc is the time shift of the pulse and 2tau is the width of the pulse. Figure 1 shows the time domain pulse of the transmitted and received pulses.
2 UWB performance characteristics
Ultra-wideband is different from other existing communication technologies. The most fundamental difference is that no carrier is needed, which greatly reduces the complexity of the transmitting and receiving devices, 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 transmitting and receiving devices are simple. Since the UWB signal is a very short time pulse with a high frequency, it does not need to be modulated to a certain transmission frequency to transmit in the channel like the conventional baseband signal. Therefore, the structure of the transmitter and the receiver is inevitably simplified.
figure 2
(2) Low power consumption. Since the UWB signal does not require a carrier and operates in the electronic noise band of the spectrum, it requires only a very low power supply. Generally, the UWB system only needs 50 to 70 mW of power, which is only one percent of the mobile phone and one tenth of the Bluetooth technology.
(3) The transmission rate is high. The extremely wide bandwidth allows UWB to have a very high transmission rate. In general, its maximum data transmission speed can reach several hundred Mbps to 1 Gbps. Intel Corporation demonstrated the technology in "IDF2002 Spring Japan" in April 2002, with a transmission rate of 100Mbps over a distance of several meters.
(4) Good concealment and high security. Since the bandwidth of the UWB signal is wide and the transmission power is low, this inevitably gives the communication technology the advantage of Low Probability of Detec (TID). In addition, UWB also adopts the TH (TIme Hopping) spread spectrum technology, and the receiving end must demodulate the transmitted data information only after knowing the spreading code of the transmitting end.
(5) Strong multipath resolution. From a time domain perspective, the UWB system uses a narrow signal with a pulse width of a few nanoseconds, so it has a high time resolution, and the corresponding multipath resolution is less than tens of centimeters; from the perspective of the frequency domain, due to the UWB signal The bandwidth is extremely wide, so the frequency selective fading of the signal during transmission is certain. However, it is because of the extremely wide bandwidth that multipath fading occurs only at certain frequencies. From the overall point of view, the fading energy is only a small part of the total energy of the signal, so the technology still has Lu in the anti-multipath. Great.
(6) The system capacity is large. Shannon formula gives
C=Blog2(1+S/N) (4)
It can be seen that the increase of the bandwidth makes the increase of the channel capacity far greater than the effect brought by the increase of the signal power, which is the theoretical mechanism of the proposed ultra-wideband technology.
(7) High-precision distance resolution. Since the time jitter of the ultra-wideband positioning device is less than 20 ps, ​​if the same working principle and algorithm of GPS are adopted, the corresponding distance uncertainty is less than 1 cm. In practical applications, ultra-wideband radar systems use ultra-narrow pulse signals with a range resolution of less than 30 cm.
(8) Strong penetrating power. In a wireless signal having the same bandwidth, the ultra-wideband has the lowest frequency, and therefore, it has a higher penetration capability with respect to a millimeter wave signal while having a large capacity and a high range resolution.
3 UWB signal modulation method
UWB has many modulation methods, and pulsed PPM (Pulse PosiTIon Modulation) is taken as an example for analysis.
First define a single-cycle pulse:
s(k) represents the signal kth, and w(t) is the transmitted single-cycle pulse.
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: 
Where d(k) is information data and δ is time shift. In order to meet the needs of multiple users, improve the security of communication and consider the power spectral density (PSD) of the system, a time hopping code is introduced. The problem is analyzed from the perspective of power spectral density.
Assume that the Gaussian single pulse given in Fig. 1(a) is used as the transmission signal, and it is only a series of periodic pulse sequences. Due to the periodicity of the time domain signal, a strong energy peak appears in the frequency domain. These peaks will be Interference with existing traditional wireless signals. So some measure is needed to smooth it. If the position of the pulse is adjusted by PPM modulation, it can be seen that the peak of the frequency domain is controlled to some extent due to the scrambling effect of the modulation, but it is still obvious at this time. In order to further reduce the amplitude of the peak, a time-hopping code is introduced, so that the power spectrum of the transmitted signal is further smoothed, which is almost similar to the background noise, which is one of the reasons why the UWB system can coexist with the existing wireless system. Figure 2 shows the PSD diagram for the different signals described above and the time domain waveform after the introduction of the time hopping code.
In addition to PPM, UWB signals can also use Pulse Amplitude Modulation (PAM), On-Off Key, and Bi-Phase Shift Key. At the receiving end, the single pulse signal can be reliably received by the related art. In practice, a correlator is often used, which multiplies the prepared RF signal by the prepared template waveform, and then integrates to obtain a DC output voltage. The correlator outputs the relative time position difference between the received single-cycle pulse and the template waveform. Finding the signal with the time difference of 0 from the output is the signal to be received.
In order to pursue more efficient information transmission, a new type of pulse modulation method, Pulse Shape Modulation (PSM), has recently been proposed. PSM is to modulate the shape of the pulse to achieve the information load, so the choice of pulse shape is very important. It is proposed thanks to the study of the hermite polynomial. Since the mathematical expression of the hermite polynomial is very close to the Gaussian single pulse, and the duration of the waveform does not change greatly with the order, it is thought that the variation of the hermite polynomial number is used to produce different shapes. Pulses to achieve diversified modulation. In order to seek orthogonal waveforms, the hermite polynomial needs to be corrected, namely:
After the modification, the order Hermite polynomials orthogonal to each other can be obtained. At this time, n different pulses of different shapes can be simultaneously transmitted at the transmitting end, and the orthogonality is such that they do not interfere with each other, and the receiving end can separate each signal by using the relevant receiving technology.
Figure 3 shows the improved hermite polynomial time domain waveform. At the same time, you can get the desired hermite polynomial pulses by constructing the simulink circuit. Figure 4 shows the setup circuit and simulation waveforms. In the simulink circuit, the order of the Hermite polynomial is controlled by the pulse order unit, and the oscilloscopes 1, 2 give the corresponding order and the hermite shape of the corresponding order minus 1 order.
The increase in transmission efficiency leads to a decline in system performance, which is not tolerated by many systems and therefore requires coding. First, the BCH (7, 4) is used to encode the signal in the shape field, so that the transmission rate is 4 times that of the single pulse, and the error performance is basically the same as that of the single pulse, and then the BCH is performed on the information frame in the time domain (31, 11) Encoding makes the performance further improved. Finally, it can be jointly coded in the time domain and the shape domain, and the bit error performance is 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 direction
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 allowed UWB technology to enter the civilian sector, provided that: "With a transmission power lower than the US emission noise specification of -41.3dBm/MHz (converted to a power of 1mW/MHz), 3.1G~ The 10.6 GHz band is used for imaging systems that scan underground and partition walls, automotive collision avoidance radars, and ranging and wireless data communication between home appliances terminals and portable terminals. Although the technology has so many limitations in its application, it is still favored by telecom developers. Companies such as Time Domain and Multispectral Solutions have proposed to the IEEE-802.15 committee the use of ultra-wideband technology, and many companies' research departments and even schools have brought the technology research to the agenda. Many 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 practical civilian products.
Figure 4
The author summarizes the application of ultra-wideband technology into three main aspects: short-range wireless communication, radar detection and precise positioning. Among them, in short-range wireless communication, it can be used in ciphertext transmission, audio/video stream transmission, radio frequency tag identification, and physical layer of Adhoc-free network; radar is mainly used for collision radar detection and precision altimetry. Learning, through-wall imaging and ground penetrating radar systems; precise positioning can be used for resource tracking and Global Positioning System (GPS). This shows that there are huge business opportunities behind UWB technology.
Of course, if UWB technology is to be used in people's daily life, there are still many challenging issues. This is the direction that UWB technology has recently been researched and developed for a long time.
(1) Establish a model of the ultra-wideband radio transmitter in the time domain, and design the transmission function of the antenna from a time domain perspective;
(2) Study the optimization of UWB signal generation and basic functions;
(3) Study the low-level broadband radio signal set 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 mobile Adhoc network protocol and routing protocol, apply UWB technology to distributed network structure, blind capture and self-configuration function; study the networking protocol applicable to UWB similar to "Bluetooth" system; 
(6) Research on wireless IP protocols based on ultra-wideband radio transmission technology;
(7) Study the testing technology of ultra-wideband radio, including transmission channel test, estimation, channel model and so on.
Nowadays, the scientific community is setting off a revolutionary wave of general UWB. UWB technology has become one of the top ten communication technologies with the most promising future. China also attaches great importance to the research of this revolutionary technology, and in the "10th Five-Year Plan" 863 Program communication technology theme research project released in early September 2001, the key technologies of ultra-wideband wireless communication and its coexistence and compatibility technology 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 opened up a new field in wireless communications and has a very broad market prospect. Perhaps in time, UWB will appear as the mainstream of wireless interconnection standards in front of people, let us wait and see.