IR UWB TOA Estimation Techniques and Comparison

International Journal of Engineering Science Invention
ISSN (Online): 2319 – 6734, ISSN (Print): 2319 – 6726
www.ijesi.org Volume 3 Issue 11 ǁ November 2014 ǁ PP.01-05
www.ijesi.org 1 | Page
IR UWB TOA Estimation Techniques and Comparison
1,
Sri Hareendra Bodduluri, 2,
Anil Solanki and Mani VV
Department of Electronics and Communication
National Institute Of Technology, Warangal, India
ABSTRACT: In the recent times Impulse Radio-Ultra wide bandwidth (IR-UWB) positioning techniques have
been gaining interest especially in agglomerated environments where signals from satellite navigation systems
are not reliable. Range measurements required by UWB positioning systems are usually obtained from the time-
of-arrival (TOA) estimation of the first path, which could be a challenging task in the presence of multipath and
interference. In this paper we discuss various Time of Arrival (TOA) estimation techniques which include De-
convolution, Maximum Likelihood (ML) estimation, Pseudo spectrum multiple signal classification (MUSIC)
and Two stage estimation algorithms, their performances against different signal to noise(Eb/No) ratios and
also their complexities. The performances of the proposed algorithms are investigated via simulations using
IEEE 802.15.4a UWB channel model in Line of Sight (LOS) environment.
KEYWORDS: UWB, TOA, ML estimation, MUSIC Pseudo spectrum, DE convolution, Two Stage Estimation.
I. INTRODUCTION:
In the recent times the need for accurate location has been gaining interest.Especially in cluttered
environments (inside buildings, in urban areas, tree canopies etc.), where Global Positioning system is
inaccessible.The ultra-wide band-width (UWB) technology offers potential for achieving high accuracy in such
cluttered environments [1]-[4] . Most positioning techniques are based on, the first path is often not the time-of-
arrival (ToA) estimation of the first path [2],[5]-[6]. TOA based position estimation systems measure the
distance between transmitter and receiver based on the estimation of propagation delay which is affected by
multi path channel and noise. In dense multipath channels, the first path is often not the strongest making the
TOA estimation challenging [7]-[8]. The Direction of Arrival is also used for the estimation position The
Direction of Arrival (DOA) based positioning technique involve the use of antenna arrays. Due to the large
bandwidth of the UWB signal, the number of paths may be very large, especially in indoor environments.
Therefore, accurate angle estimation becomes very challenging due to scattering from objects in the
environments. Moreover, time-based approaches can provide very precise location estimates, and therefore they
are better motivated for UWB.An UWB signal is defined as signal that possess an absolute bandwidth larger
than 500Mhz or fractional energy bandwidth about 0.2-0.25 .The short pulse duration of UWB offers a lucrative
application which is positon estimation especially indoor positioning. In the following estimation techniques
LOS scenario is assumed. In general different measures are used to evaluate a TOA estimation algorithm in
wireless channel like noise sensitivity, complexity of the algorithm and priori information.
Many high resolution time delay estimating algorithms have been proposed in order to accurately
measure TOA by determining the delay of the first incoming signal path. These techniques can either be
performed in time domain or in frequency domain. Energy detection(ED) based estimations are growing popular
due to their low complexity implementation at sub-NY Quist sampling rates.The main purpose of our work is to
investigate the effects of multipath propagation on ToA estimation using real measurement data by considering
different algorithms with different levels of complexity. The trade-off between estimation accuracy, complexity
and sensitivity to parameter choice for different propagation conditions is discussed.The outline of this paper is
as follows. Section-II introduces the IR UWB signal model. Different estimation techniques like DE
convolution, ML correlation, pseudo spectrum MUSIC and Two step coarse-fine estimation techniques are
discussed in section III. Numerical analysis and conclusion are discussed in section IV and section V
respectively.
II. SYSTEM MODEL:
We consider an IR UWB system where transmission of symbol happens in repeated pulses Nf . The
mathematical model for transmitted signal is given as
(1)

p(t) refers to a Gaussian pulse or one of its derivatives with a pulse duration of Tp. Symbol duration Tsym=Nf :Tf
which is much greater than the pulse duration Tp ,where Nf is number of frames and Tf is frame duration. Tδ is
Pulse position Modulation (PPM) interval. Nc is the number of chips per frame cj which is the time hopping
sequence which takes values from {0, Nc-1}. An ideal received wideband signal template with Tp = 0.77 ns is
shown in Fig.1.
Fig. 1. An ideal UWB pulse template
Mathematical model for the general considered multicluster, multipath fading channel [9] is given by
(2)
In the above equation, L represents the number of multipath components, αk and τk are the complex
attenuation and propagation delays of the kth
path respectively and δ(t) is the unit impulse response. In general,
due to the mobility of the transmitter, the receiver, and objects in the environment, the parameters αk and τk are
time varying random variables. A realization of the Amplitude response of IEEE 802.15.4a is shown in Fig.2
Fig. 2.Amplitude response of the channel.
The multipath affected received signal r(t) consists of superimposition of several attenuated delayed
and eventually distorted replicas of a transmitted wave from s(t).When propagation fluctuation with in an
observation time T>>Tsym and path dependent distortions can be neglected.r(t) can be expressed as follows
(3)
Where s(t) is the transmitted signal. L is is the number of multipath components. αk and τk are the
complex attenuation and propagation delays of the kth
path respectively. n(t) is the Gaussian noise present
during the transmission.
III. TOA ESTIMATION TECHNIQUES:
A. Deconcolution : Deconvolution of the time domain waveforms can be used to determine impulse response of
Linear Time Invariant (LTI) systems. Deconvolution methods are essentially inverse filters [10]. The received
signal r(t) can be represented as a convolution of the transmitted signal and channel impulse response (CIR).
r(t) = s(t) * h(t) + n(t) (4)
where n(t) is the additive white Gaussian noise. In the frequency domain convolution transforms into
multiplication. So the above equation (4) becomes
R(f) = S(f) H(f) + N(f) (5)

In the above equation R(f),S(f), H(f) and N(f) are Fourier transforms of r(t), s(t), h(t), n(t) respectively. In ideal
conditions we divide R(f) by S(f) to obtain the channel response and estimate the TOA of the first path. The
vector form representation of the above equation (5) is R = SH + N
Where R = [R(1) ……. R(N)] ∈
S = dia[S(1) ……. S(N)] ∈
N = [N(1) ……. N(N)] ∈
H = [H(1) ……. H(N)] ∈
Let Y = then we have Y = H + W, where W = . One of the adverse effects of inverse filtering is noise
enhancement. The following constrained inverse filter provides a better performance against noise
(6)
whereλ is taken as 50 [11]
B. ML Estimation: If Gaussian noise is added then ML criterion is equivalent to the minimum mean squared
error (MMSE) criterion. Given an observation vector r of the received signal
r = S(τ)α + n (7)
Where n ∈ with elements for i = 1, 2, … , N
r∈ with elements for i = 1, 2, … , N
S(τ) =
for l = 1, 2, ………
where p ∈ with elements , for i = 1, 2, ….. , Z.
p(t) is the transmitted pulse, Z is the number of sample of the pulse shaper, is the discretized version of time
delay , such that , TS is the sampling time and Nis the number of samples of the received signal.
The ML estimate of the delay vector τ is the value that minimize the following mean squared error
where
The ML estimate of τ in the continuous-time domain, reformulated in the discrete-time domain, is given by [12]
(8)
Where is the correlation between the received signal and different delayed versions of S(τ) and
is the auto correlation matrix of S(τ). Hence Eq. (8) can be written as
(9)
C. MUSIC Pseudo Spectrum Algorithm: The MUSIC algorithm is a typical sub spaced based method. The
sub spaced based methods are better than parametric methods like ML algorithm. The MUSIC algorithm is
based on Eigen vector decomposition (EVD) of the covariance matrix of received signal. The vector form of the
signal in frequency domain is written as
R = SH + N (10)
Where R, S, H, N are vector representation of frequency domain of received signal, transmitted signal, channel
and noise respectively. From vector form (10) of received signal, if we assume that the auto correlation matrix
of the received signal can be obtained as
(11)

= SE[H = SP , where P = E[H and P is symmetric and positive definite with
rank L. Let the number of samples N be larger than the number of paths L, and rank of SP will be L. Then
from the formulation of received signals and additive noise, we obtain
SP = (12)
and = SP (13)
whereei is the eigenvector and associated with corresponding Eigen value . The Eigen decomposition of auto
correlation matrix gives two orthogonal sub spaces known as signal and noise subspace. The principle largest L
Eigen values span the signal subspace and the rest span the noise subspace. The Eigen values of the noise sub
space { } are equal to the variance of the noise . Let and be the noise and signal sub
spaces respectively, then , . The MUSIC pseuedospectrum is given by [13]
(14)
Where , .
The Toa of the first arriving path can be estimated by finding the delay at which pseuedospectrum MUSIC
achieve the maximum value.
D. Two Stage Estimation: In two stage estimation first stage is a simple coarse estimation that provides the
time reference for the symbol synchronization and estimates the threshold used in Toa estimation algorithm
[14]. The two stages estimation process is performed in frequency domain. It is assumed that an all zero training
sequence is used for the estimation. The coarse estimation consists of an energy estimator and a simple search
algorithm that identifies the beginning of the symbol by applying a minimum distance criterion. In coarse
estimation an acquisition time of length equal to the duration of is considered. Minimum acquisition
window shall be equivalent to two symbol duration in order to perform the fine estimation over a single symbol
. The fine TOA estimation and a high resolution time delay estimates of the first arriving path with
respect to the time reference obtained in the coarse estimation stage. The TOA estimation resulting from the fine
estimation is given by
(15)
The fine TOA estimator consists of finding the first delay that exceeds the given threshold Pth in the power
delay profile summed overall direction [15]. Pth is assumed based on coarse estimation
(16)
Where P(τ) = (17)
The signature vector is represented as , where .
For more robust estimation is averaged over symbols.
E. Numerical Results: For numerical evaluation of the above algorithms we consider the channel models
developed within the framework of the IEEE 802.15.4a. In particular it is used the CM1 Residential LOS
channel model.The pulse p(t) is Gaussian second order derivative with a 3dB bandwidth of 1.3 GHz and a centre
frequency f0 = 4:5GHz according to the European ECC. The pulse repetition period is Tf = Tsym/Nf = 128ns.All
simulations are given for 100 independent channel realizations.
Simulation Parameters
Parameter Value
Pulse duration P(t), 0.77 ns
Band Width BW 1.3GHz
Number of frames per symbol, 31
Number of symbols in the acquisition interval, 47 symbols
Symbol duration, 3974.4 ns
Sampling rate, 1/ 3GHz
The remaining parameters are chosen according to the algorithm in consideration.

Fig.3. depicts the normalized root mean square error (RMSE) of the estimated TOA in nanoseconds (ns) of
various algorithms against varying snr
Fig. 3. Error performance of various algorithms as a function of Eb/No in LOS environment.
F. Conclusion: Various TOA estimation techniques for UWB system complaint with 802.15.4a have been presented. It is
observed that music psuedospectrum has higher performance due to separating into signal and noise subspaces, the effect of
noise is reduced but also this method has high complexity due to matrix Eigen decompositionmaking this algorithm quite
difficult to be used in practical applications. Another drawback of this method is that the number of paths L should be known
as priori information of channel. ML has the second best performance against noise; no prior information is needed forthe
implementation of ML estimation method, but it has high complexity due high sampling rate. Two Stage Estimation has a
good performance and also low complexity but is asymptotic at higher SNR in nature due blind resolution. De-convolution is
poor against lower SNR another drawback associated with the De-convolution methods is their increased complexity and
memory requirements (mainly due to matrix inversions), which increase the implementation cost of this method in a real
system. So we can observe a trade-off between complexitiesandaccuracy so far two stage estimation has the best trade-off
between accuracy and complexity.
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