Ultra-wideband impulse radio ranging (or UWB-IR ranging) is a wireless positioning technology based on IEEE 802.15.4z standard, which is a wireless communication protocol introduced by IEEE, for systems operating in unlicensed spectrum, equipped with extremely large bandwidth transceivers. UWB enables very accurate ranging (in the order of centimeters) without introducing significant interference with narrowband systems. To achieve these stringent requirements, UWB-IR systems exploit the available bandwidth (which exceeds 500 MHz for systems compliant to IEEE 802.15.4z protocol) that they support, which guarantees very accurate timing (and thus ranging) and robustness against multipath, especially in indoor environments. The available bandwidth also enables UWB systems to spread the signal power over a large spectrum (this technology is thus called spread spectrum), avoiding narrowband interference.
Protocol UWB-IR relies on the low-power transmission of specific sequences of short-duration pulses. The transmit power is limited according to FCC regulations, in order to reduce interference and power consumption. The bands supported by the standard are the following ones:
The sub-gigahertz band, which contains only 1 channel and ranges from 249.6 MHz to 749.6 MHz. The low band, which contains 4 channels and ranges from 3.1 GHz to 4.8 GHz. The high band, which contains 11 channels and ranges from 6.0 GHz to 10.6 GHz. The primary time division in UWB systems is structured in frames. Each frame is composed by the concatenation of 2 sequences:
The first one is called preamble (also known as SHR or synchronization header) and consists of a header, known a priori both at transmitter and receiver side. It is employed for synchronization purposes. The second one is called physical layer protocol data unit (abbreviated to PPDU) and contains the data to communicate, which are known a priori only at transmitter side. The further time subdivisions of the preamble and the PPDU are organized in different ways. For localization purposes, only the preamble is employed (and described in detail later on), since it is specifically designed to perform accurate synchronization at receiver side. The SHR sequence is composed by the concatenation of 2 other subsequences:
The first one is called synchronization sequence (abbreviated to SYNC) and it is the longest one. Its purpose is to increase the effective SNR and simultaneously exhibit a highly peaked autocorrelation function, in order to enhance synchronization accuracy. The second one is called start of frame delimiter sequence (abbreviated to SFD) and, as the name suggests, it is employed to efficiently recognize the time delimitations of the various frames. Both the SYNC and the SFD sequences are furtherly time-divided into symbols. The SYNC sequence consists of N s y n c ∈ { 16 , 64 , 1024 , 4096 } {\displaystyle N_{\mathrm {sync} }\in \{16,64,1024,4096\}} identical symbols. The SFD sequence consists of N s f d ∈ { 8 , 64 } {\displaystyle N_{\mathrm {sfd} }\in \{8,64\}} different symbols, generated according to a specific code which enables to easily detect the time delimitations of the frame. Each symbol consists of a sequence of bursts, generated through a ternary code (the elements of the sequence can be 0, +1, -1) of length 31 or 127. These codes are specifically designed to have highly peaked autocorrelation and very-low cross-correlation, in order to simplify synchronization through peak-finding and simultaneously avoiding false alarms (i.e. detection of a code which was not transmitted). Each burst is furtherly divided in L {\displaystyle L} chips.
L {\displaystyle L} is called spreading factor and it is defined as the ratio between the PRF and the chip rate. L {\displaystyle L} can be equal to 16 or 64 for length-31 codes while it is set to 4 for length-127 codes; thus the number of chips per symbol N c p s ∈ { 496 , 508 , 1984 } {\displaystyle N_{\mathrm {cps} }\in \{496,508,1984\}} . The purpose of the spreading factor, as the name suggests, is to spread the signal in time domain, in order to make the chips very sparse in time, allowing to reduce interference with other systems operating in the same band. The chip rate is f c {\displaystyle f_{c}} = 500 MHz or, alternatively, the chip time is T c {\displaystyle T_{c}} = 2 ns. Each chip is modulated by the same pulse waveform. The pulse waveform is not specified by the protocol, thus it is left to user preference. However the spectrum of the pulses must be entirely contained in the allowed channels.
SHR waveform The transmitted SHR waveform (baseband equivalent) can be modeled as follows
x ( t ) = ∑ k , n c k n ⋅ p ( t − n L T c − k N c p s T c ) {\displaystyle x(t)=\sum _{k,n}c_{kn}\cdot p{\big (}t-nLT_{c}-kN_{\mathrm {cps} }T_{c}{\big )}}
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