1 Introduction
UWB (Ultra-Wideband, ultra-wideband) pulse wireless transmission technology is a revolutionary wireless communication technology that has emerged internationally in the past two or three years. It is very different from other wireless communication technologies: it does not require the use of carrier waves. Instead, it relies on a continuous, very short time baseband pulse signal (usually) to transmit data, so the occupied frequency band is very wide, usually on the order of a few GHz.
UWB technology is synonymous with the following terms: very short pulse, no carrier, time domain, non-sinusoidal, orthogonal function, and large relative bandwidth wireless / radar signal. Due to its excellent and unique technical characteristics, UWB pulse communication is more and more valued by the communication academia and industry, and is also concerned by all sectors of society. It will be in a small range and indoor large-capacity high-speed wireless multimedia communication, radar, precision Positioning, penetration through the wall, imaging, and measurement are gaining increasing popularity.
2. Overview of UWB
The UWB currently studied is essentially a carrier-free spread-spectrum technique that uses a shock pulse with a very low duty cycle (as low as 0.5%) as an information carrier. It is directly modulated by shock pulses with very steep rise and fall times. A typical UWB directly transmits an impulse burst, which no longer has the traditional concepts of intermediate frequency and radio frequency. At this time, the transmitted signal can be regarded as a baseband signal (in terms of conventional radio) or a radio frequency signal (from the spectrum of the transmitted signal Weight considerations). Shock pulses usually use single-cycle Gaussian pulses, and one information bit can be mapped into hundreds of such pulses. The width of a single-cycle pulse is in the ns level and has a very wide spectrum. UWB has developed a new wireless channel with GHz capacity and maximum space capacity.
The basic composition of CDMA-based UWB pulse wireless transceiver is shown in Figure 1. At the sending end, the clock generator generates a pulse sequence with a certain repetitive period. The information to be transmitted by the user and the pseudo-random code indicating the user's address separately or combined to modulate the above-mentioned periodic pulse sequence in a certain manner. The modulated pulse sequence drives the pulse. Generate a circuit to form a certain pulse shape and regular pulse sequence, and then amplify to the required power, and then coupled to the UWB antenna for transmission.
At the receiving end, the signal received by the UWB antenna is amplified by a low-noise amplifier and sent to one input of the correlator. The other input of the correlator is added with a locally generated pulse modulated by the user's pseudo-random code synchronized with the transmitting end. Sequence, the signal at the receiving end and the pulse sequence modulated by the local synchronous pseudo-random code are subjected to multiplication, integration, and sample-and-hold operations in the correlator to produce a signal that separates user address information, which contains only user transmission information and other interference. Then, the signal is demodulated, that is, each pulse is judged according to the modulation mode of the transmitting end, and the transmitted information is recovered. The synchronization circuit includes a capture and tracking circuit, whose function is to accurately extract the information of the position and repetition period of the clock pulse, and apply it to the local timing circuit to generate various clocks and timing signals required by the receiver.
2.1 UWB main indicators
Frequency range: 3.1-10.6GHz;
System power consumption: 1-4mW;
Pulse width: 0.2-1.5ns, repetition period: 25ns-1ms;
Transmitting power: <-41.3dBm / MHz;
Data rate: tens to hundreds of Mbit / s;
Decomposition multipath delay: ≤1ns;
Multipath fading: ≤5dB;
System capacity: much higher than 3G system;
Space capacity: 1000kB / m?
3. The key technology of UWB
3.1 Generation of pulse signals
Essentially, generating a signal source with a pulse width of nanoseconds (10-9s) is a prerequisite for UWB technology. A single carrier-free narrow pulse signal has two characteristics: One is that the waveform of the excitation signal has steep front and back edges A single short pulse, the second is that the excitation signal includes a very wide spectrum from DC to microwave. At present, the two types of methods for generating pulse sources are: (1) photoelectric method, the basic principle is to use the steep rising / falling edge of the photoconductive switch to obtain the pulse signal. The pulse width obtained by laser pulse signal excitation can reach the order of picoseconds (10-12s), which is the most promising method. (2) Electronic method, the basic principle is to use the transistor PN junction to reverse power, to obtain a steep rising edge at the instant of the avalanche state, and to obtain an extremely short pulse after shaping, which is the most widely used solution. Limited by the withstand voltage characteristics of transistors, this method can generally only generate pulses of tens of volts to hundreds of volts, and the pulse width can reach less than 1ns. In practice, a long series of ultra-short pulses are used.
3.2 UWB modulation and multiple access methods
3.2.1 Modulation method
The transmission power of UWB is limited by the power spectral density of the transmitted signal, so it affects the choice of modulation in two ways: one is to provide the best error performance for each bit of energy modulation; the other is that the choice of modulation scheme affects the signal power The structure of the spectral density, it is possible to put some additional restrictions on the transmission power.
In UWB, information is modulated and transmitted on pulses. A single pulse can be used to transmit different information, or multiple pulses can be used to transmit the same information.
(1) Single pulse modulation
For a single pulse, pulse amplitude, position and polarity changes can be used to convey information. The main single pulse modulation technologies suitable for UWB include: pulse amplitude modulation (PAM), pulse position modulation (PPM), on-off keying (OOK), two-phase modulation (BPM), and time-hopping / direct spread binary phase shift keying Modulate TH / DS-BPSK, etc.
PAM is a pulse modulation technique that transfers information by changing the magnitude of the pulse amplitude. PAM can change the polarity of the pulse amplitude or only the absolute value of the pulse amplitude. Generally speaking, PAM only changes the absolute value of the pulse amplitude. BPM and OOK are two simplified forms of PAM. BPM modulates binary information by changing the positive and negative polarities of pulses, and the absolute value of all pulse amplitudes is the same. OOK transmits information through the presence or absence of pulses. In PAM, BPM and OOK modulation, the time interval for transmitting pulses is fixed. In fact, we can also transfer information by changing the time interval of the transmitted pulse or the position of the transmitted pulse relative to the reference time, which is the basic principle of PPM. In PPM, the polarity and amplitude of the pulse do not change.
The common advantage of PAM, OOK and PPM is that information can be recovered through non-coherent detection. PAM and PPM can also increase the information transmission rate through multiple amplitude modulation or multiple position modulation. However, PAM, OOK and PPM all have a common disadvantage: the pulse signal modulated by these methods will have a line spectrum. The line spectrum not only makes it difficult for the signals of the UWB pulse system to meet certain spectrum requirements (for example, FCC regulations on the spectrum of UWB signals), but also reduces the power utilization.
As far as the above five modulation methods are concerned, considering the factors of reliability, effectiveness and multiple access performance, what are the two modulation methods that are widely concerned at present? ? TH-PPM and TH / DS-BPSK. The difference between the two is that when single-user detection using matched filters, the performance of TH / DS-BPSK is better than that of TH-PPM. For TH / DS-BPSK, when the rate is high, the DS-BPSK mode should be selected first; when the rate is low, the TH-BPSK mode should be selected because the TH-BPSK is less affected by the near-far effect. In the case of multi-user receivers using the minimum mean square error (MMSE) detection method, the difference between the two is not significant; but at higher rates, the performance of TH / DS-BPSK is still better than that of TH-PPM systems. But BPM can avoid the line spectrum phenomenon, and is the most efficient pulse modulation technology. For UWB pulse wireless systems with restricted power spectral density and limited power, in order to obtain better communication quality or higher communication capacity, BPM is an ideal pulse modulation technology.
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