employing the very same electromagnetic wave ...ofdm: well established technology (lte, wlan, dvb,...
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KIT – University of the State of Baden-Wuerttemberg and National Research Center of the Helmholtz Association
Communications Engineering LabUniv.-Prof. Dr.rer.nat. Friedrich K. Jondral
www.kit.edu
Employing the very same electromagnetic wave simultaneously for communications and radar: It works!
Martin Braun, Friedrich Jondral
LS Summit, Lichtenau, July 2, 2014
2 03.06.2014 Communications Engineering LabUniv.-Prof. Dr.rer.nat. Friedrich K. Jondral
Frequency as a resource is scarce an should be used economicallySimilar techniques are employed by radio communications and radarCommunalities in hardware and software
Motivation
11001011 . . . communicationscommunications
radar radar
radar + communications
radar & communications
3 03.06.2014 Communications Engineering LabUniv.-Prof. Dr.rer.nat. Friedrich K. Jondral
Radar Networks
Benefits: Frequencies are used economically.A combination of radar and radio communications may lead to enhanced road safety.
Challenge:Networked radar and radio communi-cations systems are to be coordinated (differing requirements w.r.t. mediumaccess).
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Topics
OFDM Radar
Estimation Theory
OFDM Radar Networks
Further Investigations
Summary
5 03.06.2014 Communications Engineering LabUniv.-Prof. Dr.rer.nat. Friedrich K. Jondral
OFDM Radar: Review
OFDM: Well established technology (LTE, WLAN, DVB, …), convenient for wireless communications
2000: First application of OFDM for radar
2008 - 2010: Research Project RadCom (JO 258/15-1, ZW 1801/1-1),simultaneous usage of an OFDM signal in radar and radiocommunications, partially sponsored by the German Research Foundation (DFG).
6 03.06.2014 Communications Engineering LabUniv.-Prof. Dr.rer.nat. Friedrich K. Jondral
OFDM: Orthogonal Frequency Division Multiplex
OFDM signals are matrices carrying information:sub carriers, OFDM symbols, modulation alphabet
OFDM Basics
cyclic prefix
∩ C
7 03.06.2014 Communications Engineering LabUniv.-Prof. Dr.rer.nat. Friedrich K. Jondral
Relative velocity (Doppler)Distance (delay)Complex attenuationNoise (AWGN assumption)
The elements of the received matrix are divided by the elements of the transmission matrix:
Signal Processing by the Radar Receiver(Initially Single Target)
8 03.06.2014 Communications Engineering LabUniv.-Prof. Dr.rer.nat. Friedrich K. Jondral
Maximum Likelihood Estimator
Derivation of the ML estimator for ODFM radar:
Parameter vector:
Estimation:
Function to be maximized (noise is assumed to be AWGN):
9 03.06.2014 Communications Engineering LabUniv.-Prof. Dr.rer.nat. Friedrich K. Jondral
Einfache Umrechnung von Indizes in Distanz und Doppler:
The log likelihood function is maximized by finding the maximum of a two-dimensional peridogram.
Discretization narrows down the search domain.
ML Estimation at Discrete Arguments
The indices m and n are easily conver-ted into distance and relative velocity:
dist
ance
/ m
relative velocity / ms-1
pow
er /
dBm
10 03.06.2014 Communications Engineering LabUniv.-Prof. Dr.rer.nat. Friedrich K. Jondral
Outage Probability of a Radar Network
The outage probability is complementary to the reference object‘s detection probability PRx,ref = 1 – Pout .
The outage probability is a function of the subscriber density .
Definition:The outage probability Pout of a radar network is given by the probability by which a reference object cannot be detected caused to network internal interferences.
G(Φ)
dref
Φ
11 03.06.2014 Communications Engineering LabUniv.-Prof. Dr.rer.nat. Friedrich K. Jondral
Quantization Error
Discretization allows for a numerical search for a ML estimator but causes errors (quantization loss, quantization noise).
The maximum of the discretized peridogram is always found within half a quantizing step around the true maximum.
frequency index
12 03.06.2014 Communications Engineering LabUniv.-Prof. Dr.rer.nat. Friedrich K. Jondral
Estimator Performance
Numerical evaluation (by simulation)
Signal parameters:center frequency 24 GHzbandwidth 93,1 MHzradar cross section 10 m²noise figure 10 dBtemperature 290 K
An object with fixed radar cross section is moved away from the radar.If the distance from the radar becomes larger than a certain threshold1), the probability for estimating the true index pair decreases rapidly.The probability for estimating the true index pair is an alternative of using bias or variance as a performance metric.
1) This threshold may be interpreted as the radar’s maximum operation distance.
125 m
distance / m
13 03.06.2014 Communications Engineering LabUniv.-Prof. Dr.rer.nat. Friedrich K. Jondral
The quantization error is dominant below the thresholdThere is no measurable difference between optimization and quadraticinterpolation (that is suboptimal)
Simulation: Mean Estimation Error
distance / m
mea
n es
timat
ion
erro
r / m
quantizedquadratic Interpolationoptimizationhalf quantization step
quantizedquadratic Interpolationoptimizationhalf quantization step
14 03.06.2014 Communications Engineering LabUniv.-Prof. Dr.rer.nat. Friedrich K. Jondral
Simulation: Variance
Velocity and distance are uncorrelated.Accurate predictions are based on quadratic interpolation or on theaveraged Cramér-Rao Bound (CRB).
distance / m
varia
nce
/ m2 s
-2
quantizedquadratic interpolationoptimizationaveraged CRBquantization noise
15 03.06.2014 Communications Engineering LabUniv.-Prof. Dr.rer.nat. Friedrich K. Jondral
Multiple targets superimpose linearly (in ) :
Discrimination between targets and noise employs a threshold
depends of the false alarm rate
should be chosen such thatthe estimator works in a domainof small variances
Multi Target Detection
−24
−32
−40
−48
−50
−56
−72
−80
pow
er /
dBm
−20
−25
−30
−35
−40
−45
−50
−55
−60
−65
relative velocity / ms-1
dist
ance
/ m
−50 −25 0 25 50
80
60
40
20
0
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OFDM-Radar-Networks
Radar systems are transmitting periodicallyHow to deal with collisions?
Solution:Clocked access
different sub-carrier sets
For high subscriber densities collisions (interferences within the network) cannot be avoided!Wanted: Theoretical background for evaluating the performance networked radars.
-su
bcar
rier
inde
x
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Detection in a Networked Radar System
Medium access is globally clocked accessprobability
Power received from the th interferer:
Interferers are distributed over the two-dimensional plane according to a Poisson point process
At the receiver the sum of all interferences form an AWGN (Central Limit Theorem)Successful detection, if a threshold is exceeded:
18 03.06.2014 Communications Engineering LabUniv.-Prof. Dr.rer.nat. Friedrich K. Jondral
A Lower Bound for the Outage Probability
By which probability is the presence of one interferer sufficient in order to provoke radar outage?
Dominant Interferer PhenomenonBy which probability is no interferertoo close to the reference system?
The bound depends on the choice of the radar system parameters.All radar system parameters are integrated in .
φ
19 03.06.2014 Communications Engineering LabUniv.-Prof. Dr.rer.nat. Friedrich K. Jondral
Empirical Verification
Directional radiation pattern: Cone
Approximation of the outage probability:
The approximation works well for small subscriber densities.
Simulations are costly andtime-consuming.The approximation facilitatesthe verification of parameters.
subscriber density / m-2
upper boundsimulationlower bound
20 03.06.2014 Communications Engineering LabUniv.-Prof. Dr.rer.nat. Friedrich K. Jondral
Further Investigations
Signal ParametersComprehensive discussion of all signal parameters with respect to the communication as well as to the radar subsystem.Investigation of WLAN as a radar waveform.
Alternative Estimators
Multi Target DetectionProfound analysis of
radar signals.
DemonstratorModular, configurableBased on software defined radioConsideration of implementation aspects
relative velocity / ms-1relative velocity / ms-1
dist
ance
/ m
dist
ance
/ m
х target position+ estimation
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Demonstrator
Signal parameters: Accordng to IEEE 802.11pFrame repetition rate: 5 Hz
dist
ance
/ m
relative velocity / ms-1
left lane right lane
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Summary
OFDM is suitable for communications and radar systems using simultaneously the very same electromagnetic wave
Periodogram based method provides statistically optimal (maximum likelihood) estimators
Networking AspectsDefinition of outage probability in radar networksAnalytic approximation of outage probability in radar networksTheoretical limits as benchmarks for implementationsFirst elements for a radar networking theory
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Detailed Discussion
24 03.06.2014 Communications Engineering LabUniv.-Prof. Dr.rer.nat. Friedrich K. Jondral
Thanks for your attention!