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Impulse Radio is a synonym for early ultra-wideband systems, specifically referring to non-sinusoidal carrier radio technology that uses impulse pulses (ultra-short pulses) as information carriers. This technology is different from the traditional narrowband wireless system that uses a sinusoidal carrier and belongs to the category of baseband and carrier-free communication.
(1) Commonly used UWB baseband narrow pulse waveforms
The baseband narrow pulse form is the earliest signal form adopted in UWB communication. Generally speaking, its working pulse width is in nanoseconds.
The shape of the power spectrum depends on the shape of the pulse signal. Therefore, the typical UWB pulse is a Gaussian Doublet pulse, which is often used because it is easy to generate.
The mathematical model of the time domain and frequency domain corresponding to a single-cycle Gaussian pulse can be expressed as:
The pulse modulation methods commonly used in UWB technology include pulse position modulation (PPM), pulse amplitude modulation (PAM) and binary phase modulation (BPSK).
(2) UWB pulse modulation method
Pulse Position Modulation (PPM
Information is transmitted by changing the time interval of the transmitted pulse or the position of the transmitted pulse relative to the reference time. Its advantage is simplicity, but it requires relatively precise time control.
Pulse Amplitude Modulation (PAM):
Information is transmitted by changing the pulse amplitude. It can change the polarity of the pulse amplitude or only change the absolute value of the pulse amplitude. The commonly mentioned PAM only changes the absolute value of the pulse amplitude, that is, OOK (On-off Keying).
Bi-Phase Modulation (BPSK):
BPSK modulates binary information by changing the positive and negative polarity of the pulse, and the absolute value of all pulse amplitudes is the same. One reason for using two-phase modulation is that it has a 3dB gain over PPM in terms of noise immunity.
Comparison of Several Modulation Methods
Common advantages of PAM, OOK and PPM:
The information can be recovered through incoherent detection.
PAM and PPM can also increase the information transmission rate through multiple amplitude modulation or multiple position modulation.
The common disadvantage of PAM, OOK and PPM is that the pulse signal modulated by these methods will have a line spectrum. The line spectrum not only makes it difficult for the signal of the ultra-wideband pulse system to meet certain spectrum requirements, but also reduces the power utilization.
BPM can avoid the line spectrum phenomenon and is the pulse modulation technology with the highest power efficiency. For ultra-wideband pulse wireless systems with constrained power spectrum density and limited power, BPM is an ideal pulse modulation technology.
Multiple access technology refers to the technology of connecting multiple users in different locations to a public transmission medium to achieve communication between users.
The actual multiple access technologies include: frequency hopping FH (Frequency Hopping) and direct spread DS (Direct Squence).
Direct spread spectrum uses pseudo-random codes for spectrum spread, and frequency hopping uses code sequences to form frequency hopping instructions to control the frequency of the frequency synthesizer output signal.
Combining multiple access with modulation methods, three typical ultra-wideband pulse radio systems can be obtained:
TH-PPM
TH-PAM
DS-BPSK.
UWB Technology Solution
Multi-Band OFDM (MB-OFDM) UWB Technology
After the FCC stipulated the spectrum usage range and power limit for UWB communications in 2002, major consumer electronics companies and their researchers around the world proposed the ultra-wideband scheme MB-OFDM (MultiBand OFDM), which is different from the baseband narrow pulse form, based on the perspective of traditional narrowband wireless communications.
In March 2005, the European Computer Manufacturers Association (ECMA) released the ECMA-368 and ECMA-369 standards based on the MB-OFDM scheme, which passed ISO certification in 2007 and became the first international standard for UWB.
OFDM (Orthogonal Frequency Division Multiplexing)
It is a MCM (Multi-Carrier Modulation) technology. Its main idea is to divide the channel into several orthogonal sub-channels, convert the high-speed data signal into parallel low-speed sub-data streams, and modulate them to transmit on each sub-channel. The orthogonal signals can be separated by using correlation technology at the receiving end, which can reduce the mutual interference (ICI) between the sub-channels.
MB-OFDM
The frequency band is divided into multiple 528MHz sub-bands, each of which uses OFDM and data is transmitted on each sub-band.
ECMA-368 Protocol
The ECMA-368 protocol specifies the characteristics of the physical layer and MAC layer of the UWB system used in high-speed short-range wireless networks. The frequency band used is 3.1 to 10.6 GHz, and the maximum rate can reach 480 Mbit/s.
(1) Frame structure
The frame structure in the ECMA-368 protocol is a PLCP protocol data unit (PPDU) consisting of three parts: the physical layer convergence protocol (PLCP) preamble, the PLCP header, and the physical layer service data unit (PSDU).
PLCP (Physical Layer Convergence Protocol: Physical Layer Convergence Protocol)
PSDU (PHYService Data Unit, physical layer service data unit)
PPDU (PLCP Protocol Data Unit, PLCP Protocol Data Unit)
There are two types of nPLCP preambles: standard and burst. The structure of nPLCP preamble is divided into two parts: synchronization sequence and channel estimation sequence.
Synchronization sequence length: 24 OFDM symbols for standard type and 12 OFDM symbols for burst type;
Channel estimation sequence length:
Both types have 6 OFDM symbols.
An OFDM symbol refers to the analog waveform that will be transmitted through radio frequency after the data bits at the transmitting end undergo a series of changes.
(2) Coding and modulation
The PPDU data is coded and modulated to form an OFDM time domain signal, which is then sent to the channel through the radio frequency. The PLCP preamble is a time domain signal, which directly forms an OFDM signal without coding and modulation. The process of the PLCP header and the PSDU part forming an OFDM symbol is shown in the figure:
Dual Carrier Modulation (DCM)
It maps the four interleaved bits to two hexadecimal (16-point) symbols simultaneously, and then maps the two hexadecimal symbols to two subcarriers separated by 50 subcarriers.
(3) Subband division
At the physical layer, the protocol divides the spectrum into 14 sub-bands with a bandwidth of 528 MHz. These 14 bands are divided into 5 groups, with the first 4 groups containing 3 sub-bands each and the fifth group containing 2 sub-bands.
In the actual scheme, only three sub-bands from 3.168 to 4.752 GHz are used in the early stage. In each sub-band, the information is modulated by OFDM with 128-point IFFT/FFT. IFFT converts the signal from the frequency domain to the time domain, and FFT converts the signal from the time domain to the frequency domain.
Time-Frequency Code (TFC)
MB-OFDM defines 7 sets of time-frequency codes, including 4 sets of frequency hopping modes and 3 sets of fixed frequency modes. The frequency hopping mode can reduce the interference between two uncoordinated piconets.
The figure lists 7 groups of time-frequency codes with a period of 6 symbol time slots. 1, 2, and 3 in each TFC represent the subband number.
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