Employing the very same electromagnetic wave ...OFDM: Well established technology (LTE, WLAN, DVB, …), convenient for wireless communications 2000: First application of OFDM for

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


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).


Topics

OFDM Radar

Estimation Theory

OFDM Radar Networks

Further Investigations

Summary


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).


OFDM: Orthogonal Frequency Division Multiplex

OFDM signals are matrices carrying information:sub carriers, OFDM symbols, modulation alphabet

OFDM Basics

cyclic prefix

∩ C


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)


Maximum Likelihood Estimator

Derivation of the ML estimator for ODFM radar:

Parameter vector:

Estimation:

Function to be maximized (noise is assumed to be AWGN):


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


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


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


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


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


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


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


dist

ance

/ m

−50 −25 0 25 50

80

60

40

20

0


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


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:


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


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


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


Demonstrator

Signal parameters: Accordng to IEEE 802.11pFrame repetition rate: 5 Hz

dist

ance

/ m


left lane right lane


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


Detailed Discussion


Thanks for your attention!

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Employing the very same electromagnetic wave ...OFDM: Well established technology (LTE, WLAN, DVB, …), convenient for wireless communications 2000: First application of OFDM for