Co-channel interference limitations of OFDM communication-radar networks
© Braun et al.; licensee Springer. 2013
Received: 25 March 2013
Accepted: 22 July 2013
Published: 14 August 2013
Orthogonal frequency-division multiplexing (OFDM) communication-radar networks are systems where individual nodes use OFDM signals to both communicate and perform radar simultaneously. As a research subject, such networks are fairly new and lack some research covering fundamental limits. In particular, it is unclear how the reliability of the radar component is affected by a network operation, as several nodes might attempt to access the medium at the same time, thereby increasing interference and reducing the radar capabilities of individual nodes. In this paper, we apply the notion of outage (which was originally introduced for communication networks) to radar networks and introduce the outage probability as a performance metric. Using stochastic geometry, we are able to give tight bounds on the outage probability and demonstrate how this is useful for testing OFDM radar parametrizations and algorithms. It is possible to show that the outage probability is smaller than 1% for previously suggested OFDM radar parametrizations without having to resort to empirical methods.
In 2009, Sturm et al.  presented a novel concept for radar systems based on orthogonal frequency-division multiplexing (OFDM) signals, the major innovation being that the radar signal can be used to simultaneously transmit information to other nodes in the range of the radio signal. Unlike previous attempts to combine radar and communication (e.g., ), the result of the radar processing is independent from the transmitted data. Also, the radar processing algorithms are not very complex and can easily be implemented on digital signal processing platforms. A possible application of such technology is for automotive systems.
Transceivers which do both radar and communication can thus cooperate and create a communication-radar networka, thereby enhancing the radar as well as the communication features .
This also brings disadvantages. Specifically, the problem of interference by simultaneous access to the physical medium by different nodes (i.e., collisions) is exacerbated when compared to networks without radar components: On one hand, such a collision does not only disturb communication but also affects the radar and can thus be potentially relevant for traffic safety. On the other hand, automotive radar systems must access the medium on a regular basis to obtain a permanent measurement of the environment, thereby preventing usage of typical carrier sensing schemes. In the domain of pure radar systems for vehicular applications, intra-system interference has been identified as a performance-limiting factor. The European Union (EU) has even launched a project to investigate these matters .
From pure communication networks, we know they only work reliably for a limited density of nodes. In radar networks, both communication and radar can fail. In this paper, we aim to provide a new tool for analyzing the performance of OFDM radar networks with respect to the signal parametrization and the network geometry. When the interference level rises, the ability to detect specific objects decreases. We therefore chose to introduce radar network outage as the relevant metric, which describes the situation when a radar network is no longer able to detect a specific object due to interference by others. Choosing successful detection as the main performance metric makes sense for several reasons: It is the first and most basic step in any radar system, and all subsequent operations (range estimation, etc.) depend on it. Also, unlike the detection, the range and Doppler accuracy do not change significantly when operating in a network (cf. [3, 5] for resources on OFDM radar accuracy).
A common characteristic of vehicular-based networks is high node mobility, which causes the geometrical configuration of nodes to change rapidly - and sometimes also unpredictably. Since an analytical description of these complex spatial fluctuations is difficult, an exact modeling and analysis are not appealing. A remedy to this problem is given by the stochastic geometry framework [6–10], which models the node locations as a realization of a point process, thereby essentially accounting for the spatial dynamics of the network. In this work, we will apply stochastic geometry tools to model the interferer locations. The chosen approach will be instrumental for determining the outage probability of such an OFDM radar network given the physical parameters and providing a definitive answer on how many participants may partake in such a network before the radar system becomes unreliable. Most importantly, we can provide analytical bounds for the probability of a radar outage. At low outage probabilities, which is typically the targeted region of operation in car-safety applications, these bounds coincide with the exact outage probability. This is a significant advantage over current evaluation methods, which can include time-consuming simulations, raytracing setups, or costly measurements.
Using these bounds, we now have an objective metric to analyze various aspects of OFDM radar networks. We give two application examples of our bounds: the quality of the target detection in a multi-target environment and an answer on how the sub-carrier spacing affects the outage probability.
This paper is structured as follows: Section 2 briefly describes the OFDM radar processing used here. The system model for OFDM radar networks and how interference is handled are discussed in Section 3.
In the next two sections, we recapitulate all the basics of OFDM radar networks and clarify which assumptions are made. The new results are derived in Section 4, where we obtain analytical bounds on the outage probability of radar networks, which are further verified by simulations. With these results, we will give a discussion on how this affects the OFDM radar system in Section 5. Section 6 concludes.
1.1 Previous research
The topic of mutual interference between radar systems has recently become a focus of research due to the popularization of vehicular radar systems. The previously cited EU research project MOSARIM  is an example of how relevant this topic has become.
Radar interference has been researched both analytically and empirically. Brooker  provides a very thorough analysis of interference in automotive radar systems at 77 and 94 GHz; his metric of choice is the probability of interference. Brooker argues that in case of interference, radar systems cease to work and interference-avoidance techniques must be introduced. This is not a suitable metric for OFDM radar systems (and possibly for any radar system where the interfering radar signals appear as additional white noise).
Goppelt et al. chose the probability of ghost target detection as a figure of merit . However, their results cannot be generalized to OFDM radar, as the derivations are specific to FMCW radar (despite being a commonly applied waveform, OFDM is rarely considered for radar networks). They also lack a random modeling of the interfering signal’s attenuation. Similarly, the analysis of Oprisan et al.  is also very specific to certain waveforms.
In general, current research focuses on interference-avoidance and mitigation techniques, which is exemplified by the results from the MOSARIM project, e.g., . Here, OFDM is in fact considered as a method to cope with mutual interference, but further analysis is not given. One suggestion to handle interference instead of avoiding it is given in , which is also the only publication which directly researches interference in OFDM radar networks. The paper suggests an interference mitigation technique but only for the very specific scenario of one single interferer.
The difficulty of limiting the scope to a single interferer is also identified by Hischke . He introduces a very useful quantity: The distribution of signal to interference plus noise ratio (SINR) as cause of mutual interference, as a function of the spacing between vehicles, from a given geometry. The results are derived from simulations, though, emphasizing the need for an analytical solution.
The importance of simulations is underlined by the work of Zwick et al. , who have done considerable work in the research of mutual radar interference. Their approach is empirical in nature and consists of elaborate software tools packaged under the name Virtual Drive. The results generated are highly useful but emphasize the fact that highly sophisticated simulations are the only means to research radar networks, motivating the derivation of analytical solutions.
In general, there has been little effort to create a ‘fundamental radar network theory’, analogous to what information theory is for communication networks. Hischke’s approach seems the most promising in this respect: If a probability distribution of the SINR could be derived for a given radar interferer density, this would allow a stochastic analysis of the interferer problem. More importantly, it would ground the research on radar networks with theoretical results and provide benchmarks for the empirical results - at this point, there is no theoretical bound for the performance of radar networks. This paper aims to be the first step toward a theoretical understanding of radars operating under mutual interference.
Stochastic geometry as a tool for research on vehicular networks has previously been suggested in , which suggests its suitability in this context.
2 OFDM radar signal processing principles
In the following, we present a very brief introduction to OFDM radarb.
For every radar measurement, one OFDM frame is transmitted. It consists of M consecutive OFDM symbols, with N active sub-carriers. Such a frame is represented by the transmit matrix, using the notation introduced in . Every column of this matrix represents an OFDM symbol, every row a sub-carrier. The elements (FTx)k,l () are symbols from a complex modulation alphabet (e.g., QPSK).
Relevant parameters of an OFDM transmitter
OFDM symbol duration
11 μ s
Cyclic prefix duration
1.375 μ s
Total OFDM symbol duration
12.375 μ s
Number of active sub-carriers
1,024 (for U=1)
OFDM symbols per frame
Leaving aside synchronization and equalization, receiving OFDM signals is the exact same procedure as the transmission, only in inverse order. The samples corresponding to the cyclic prefix are discarded, and the samples are processed with a fast Fourier transform (FFT). Every FFT output is then assigned to a column of a matrix FRx which represents the received signal.
2.2 OFDM radar fundamentals
To extend an OFDM transceiver into a radar system, it must be made sure that during the transmission of every OFDM frame, the receiver is active synchronously, i.e., transmitter and receiver use the same local oscillator and, derived from this, an identical clock (this setup describes monostatic radar systems). This ensures that the transmitted signal is delayed at the receiver by the round-trip propagation time of the electromagnetic wave whenever it is scattered back to the original node from a radar target. If the target is moving relative to the radar system, this causes a Doppler shift which appears as a frequency deviation of the received signal relative to the transmitted signal.
b h is the attenuation of the signal reflected from the h-th target; this includes both free space path loss and different reflectivity characteristics (i.e., radar cross sections).
f h is a random phase shift.
The Doppler shift causes a frequency offset (vrel,h being the relative speed of the h-th target, c0 the speed of light). We assume the bandwidth of the signal to be much smaller than the center frequency B«f c , so we can approximate an identical Doppler shift on all sub-carriers. A frequency-shift of one line of FTx is equivalent to multiplying it with a complex sinusoid .
The round-trip delay (with r h being the range of the h-th target) causes a phase rotation of the received symbols depending on the sub-carrier frequency, .
Additionally, there is white Gaussian noise (WGN), which is represented by the matrix .
For phase-shift keying (PSK) modulation schemes, the noise retains its statistical properties after the division , as the phase of circular complex Gaussian random variables is uniformly distributed and thus has the same probability distribution after the division. The elements (Z)k,l of Z are thus still realizations of a WGN process with zero mean and variance .
Now, the radar estimation problem can be expressed as a problem of spectral estimation. An OFDM radar algorithm based on Eq. (3) must consist of at least two steps: (1) estimating the number H of sinusoid-pairs and (2) identifying their frequencies, which then translate into distances and relative velocities.
The dimensions of this 2D periodogram can be chosen larger than those of F, NPer > N, MPer > M, which can be achieved by zero-padding F. This interpolates the periodogram, resulting in a smaller quantization error. The matrix W is a real tapering window matrix, the choice of which is outside the scope of this workd; unless stated otherwise, we choose a boxcar window ((W)k,l =1).
Because the Doppler (unlike the range) can be both negative and positive, we thus constrain the index ranges of Per(n,m) to 0≤n≤Nmax−1 and −Mmax≤m≤Mmax−1, thereby cropping the periodogram to a smaller size (2Mmax+1)×Nmax.
The choice of Nmax and Mmax is relevant to the estimation process: Of course, they should not be chosen too small, as valid targets might not be detected, but choosing them too large leads to an increased false alarm rate.
which we take as a maximum likelihood estimate for the noise power given a single row of the periodogram.
2.3 Target detection and false alarm rate
Having calculated the periodogram, the next step is to identify peaks within the periodogram and to return a list of targets, including their ranges and relative velocities. We call this process target detection.
Here, G is the combined antenna gain of transmit and receive paths.
where z is complex, Gaussian distributed with the variance .
We can justify the approximation by noting that the factor NM is a very large value for typical setups and therefore when the interference is low. Only when the noise power becomes too large (e.g., when there is a very high number of interferers, see the following section), this approximation becomes inaccurate. In this region, a radar system becomes highly unusable anyway.
3 Radar network setup
The previous section discussed the operation of a single OFDM radar unit. The next step is to describe how these systems work in a multi-user environment. Furthermore, we introduce the concept of outage in OFDM radar networks, which will become the basis of further analyses. When designing an OFDM radar network, we must make sure to satisfy the assumption that all interference can be modeled as AWGN (this becomes relevant for the derivations in Section 4).
3.1 Time-slotted multi-user access
When two OFDM signals interfere, we must distinguish two cases: synchronous and asynchronous interference. In the former case, all nodes start transmitting simultaneously, whereas in the latter case, both transmitters may transmit at any given time. This second case is far worse, as the interfering OFDM signal would appear to have a different OFDM symbol duration, and its energy would randomly leak across sub-carriers. This causes additional problems, which are avoided by enforcing simultaneous medium access.
Access to the medium can only happen at the beginning of a time slot, which is known to all nodes.
Medium access is allowed on one of U logical channels (as demonstrated in Figure?2). Every channel may only utilise a subset of the OFDM sub-carriers which consist of every U-th sub-carrier, starting at sub-carrier u, where u denotes the channel number. Note that this changes the OFDM processing only in a way such that the sub-carrier spacing is increased by U and the number of rows in F is reduced by 1/U. Consequently, the power per active sub-carrier is increased by U, in order to maintain the same transmit power.
Every node may randomly access any channel starting at the beginning of any time slot with probability pTx. While this allows no sophisticated MAC (e.g., through back-off mechanisms or similar collision avoidance techniques), it allows a radar system to access the medium at regular intervals, which is a requirement for safety-ensuring radar systems.
Having multiple channels in the frequency domain allows for an OFDMA-like multiple access scheme; if two or more nodes access the medium in the same time slot, they only collide if they use the same channel.
From a practical point of view, stipulating synchronous access by time slots requires a global clock. This could be provided by global positioning system (GPS)f. The guard interval T G must then be chosen large enough to allow for clock inaccuracies and different signal arrival times due to different distances to the other radar transmitters.
If clocks differ too much between nodes, minor interference can also be caused by nodes which have chosen a different channel u. For this work, we accept a certain inaccuracy of the model and assume perfect orthogonality between channels, as we focus on the definition of radar network outage and how to derive it.
3.2 Radar network outage
In communication networks, outage is a common concept to describe the case where an ongoing transmission fails to achieve a given rate. Goldsmith  defines outage as the event where SINR (the ratio of received signal power to the sum of noise and interference power) drops below a certain value due to slowly varying, random channel conditions. We can directly transfer this notion to a radar network by defining outage as the case when the reflected power at the receiver drops below a certain threshold due to the interference created by the randomly located nodes.
The outage definition is therefore the same for any object with the same backscattered power PRx,Ref. The performance of an OFDM radar network is completely defined by the detection of this target.
By applying the approximation (13), outage probability is complementary to the detection probability p D .
This also implies that multi-path propagation of the radar signal does not affect the outage probability and is thus not considered here. A multi-path backscattering might produce additional peaks in the periodogram which do not correspond to true targets, but this is a common problem of all radar systems and must be treated downstream in the processing chain.
3.3 Interference model
The reason we choose to model the geometry by a PPP is because not only does this provide us with the mathematical tools to analyze such a scenario but also because the main application we have in mind for radar networks is for vehicular technology, where mobility causes a high amount of ‘spatial randomness.’ Such a spatial model has been shown to properly capture these random spatial dynamics affecting the interference .
We emphasize that all nodes represented by this PPP are OFDM transmitters of the same type as the reference node. Because of the homogeneous setup, the results for the reference node are representative for all other nodes as well.
As this is a radar system, we need to be able to determine the azimuth ϕ of the targets as well as the range and Doppler. How the radar system implementation solves this problem is irrelevant for this work, what matters is that the angular resolution results in a receiver directivity which can be expressed as azimuth-dependant gain G(ϕ). For the transmitters, we assume that they emit omnidirectional so they can communicate with all other nodes.
At this point, we have all the knowledge required to calculate the outage probability for an OFDM radar network with a given network density λ. To recapitulate, the reason for outage is that whenever the reference node tries to obtain a radar image, a number of interferers might be also transmitting. This increases the interference level and thus the threshold θ.
Remember that the (FIx)k,l are zero for interferers which use a different channel than the reference node (assuming perfect orthogonality).
in correspondence with free space path loss. g i is an optional random small-scale power attenuation parameter (caused by fading) with distribution function F g(g)h; we discuss the cases where g i =1 (i.e., no fading), or i.i.d. exponentially distributed with unit mean (Rayleigh fading). This fading parameter covers multi-path propagation of the interference signals. A very similar model is also described in .
Any fading type can be inserted, as long as its distribution function can be given. Here, Rayleigh fading was chosen as an example for a fading model, as it is a popular choice for the modeling of wireless fading channels and its probability density function (pdf) is mathematically easy to describe.
In the special case where the modulation has constant amplitude (e.g., as in PSK) and the amplitude is Rayleigh-distributed, Ztotal is a sum of normally distributed random variables and therefore is normally distributed by the central limit theorem. For the more general case where the b i follow any distribution (in Eq. (18), we state that they depend on the distance of the interferers to the reference node and are thus not identically distributed), we may still refer to the central limit theorem, which states that a small number of summands (10 to 12) suffice for Ztotal to be approximately Gaussian (, Chap. 2).
For the rest of this work, we will omit the index ‘total’ and use Z to describe the compound noise with total two-sided noise power , where denotes the random variable representing the total interference power.
We have deliberately omitted a modeling of clutter, for two reasons: First, there are many different clutter models, and incorporating one specific model would make our results less versatile. Second, the outage probability is a theoretical boundary depending on the specifications of the reference target, and as such is unaffected by clutter.
This approach is not unusual in vehicular radar applications, where clutter is commonly not treated differently than other targets at the detector; an example would be the detection of a road sign when intended targets are other vehicles. Both these objects register as a backscattering object at the receiver (and therefore as a peak in the periodogram).
In the case where we need to specify the parameters (e.g., for simulations), we use the values given in Table 1, which coincide with the parameters chosen in previous publications [1, 27]. These parameters are also discussed in , and research on the radar accuracy for a single node (outside of a network) has been done through simulations in  and by measurements in . They provide a range resolution of approximately 3.2 m, and a Doppler resolution of approximately 2.5 m, although some modifications of the parametrization (e.g., other window matrices W) can degrade this value. For a detailed discussion on the choice of these parameters, we refer to .
Radar network setup
Thermal noise power
The target parameters are chosen to work within a vehicular scenario. For the receiver, the noise figure is one compound value encompassing all non-idealities and distortions caused by the amplifiers and the digital signal acquisition. The receiver noise power is then calculated using the given bandwidth (B=N Δ f=93 MHz) and at a noise temperature of 290 K.
As for the network parameters, if we specify a time slot duration of 5 ms, the average medium access rate is 20 Hz, a typical value for automotive radar systems. The false alarm rate is the targeted false alarm rate at the detector; in practical systems, tracking devices or similar systems will further decrease the false alarm rate for practical purposes.
These are only crude approximations of true antenna directivity functions but suffice for the following derivations. The cone-shaped antenna gain function has the additional advantage of making the derivations in the following sections entirely analytical.
It must be emphasized that the choice of parameters is only relevant for the simulations; none of the derivations made in this paper are specific to a certain set of parameters, as long as the medium access occurs as described in Section 3.1 and the interferer positions can be modeled as a PPP. The signal parameter most relevant for this is the sub-carrier spacing, as it affects the Doppler tolerance of the OFDM signal (and thus guarantees orthogonality as postulated in Section 3.1).
4 Analytical results
To analytically calculate the outage probability, we first re-iterate on the assumptions and approximations made.
The total, additive interference Z is modeled by WGN.
The medium access can be described by a transmit probability pTx, and a transmitting node will randomly choose one of U logical channels, each with equal probability (cf. Section 3.1).
The attenuation between interfering transmitters and the reference system is modeled by path loss and a (random) fading coefficient, see Eq. (18).
Nodes are distributed uniformly and independently.
This allows us to use the tools described in Section 4.3. It is motivated by the fact that due to the mobility of the nodes, their relative position changes all the time and is thus different between timeslots.
Sub-carriers are always orthogonal, interference on a different channel thus does not affect the radar system.
An ideal radar system can always detect the reference target if , i.e., Eq. (13) is an equality.
These are the only simplifications in this model which are designed to facilitate the derivation of analytical bounds. Their influence is discussed in Section 5.2.
4.3 Stochastic model
Given the synchronized slotted medium access of the nodes, we consider the network in an arbitrarily chosen slot (snapshot). In this snapshot, the locations of the potentially interfering nodes are given by the PPP with density λ, as already explained in Section 3.3. From Slyvniak’s theorem [6, 29], it follows that the law of the PPP is not changed by adding a node. Due to the stationarity, this node can be placed in the origin without loss of generality. We will refer to this node as the reference node as it will allow us to measure the typical performance in such a network.
Since the medium access is uncoordinated among the nodes, i.e., each node accesses the medium independently of each other with probability pTx, we can obtain the set of interfering nodes by independent thinning of the original PPP . The resulting point process is again Poisson with density pTxλ.
We treat the sub-carriers chosen by the i-th node as mark attached to this node. Formally, if node i chooses the u-th sub-carrier for transmission, we assign the mark u i to this node. The small-scale channel fading experienced between the i-th interferer and our reference node is denoted by another mark, g i .
of (spatial) density pTxλ, where the x i denote the random interferer locations, and u i and g i are the marks associated with interferer i. Note that , where .
where ∥x i ∥−α describes the large-scale path loss between interferer location and the origin, and ∠ x i the interferer’s azimuth.
At the receiver, the sum interference is superimposed by thermal noise of power . As shown previously, the interference noise can be assumed to be conditionally AWGN. Consequently, the total noise is conditionally AWGN as well with a (random) power equal to .
The factor β plays the same role as in Eq. (17). U is necessary because the power on the individual sub-carriers is scaled by the same factor to retain a constant transmit power.
4.4 Outage probability analysis
Hence, computing pout requires the evaluation of the tail probability of Y (note that Eq. (27) assumes that c−1 is positive, but unless p F is close to 1, this is always the case). Unfortunately, solving Eq. (27) directly is an intractable problem; therefore, we present bounds in the following:
4.4.1 Lower bound
At this point, the bound only depends on the probability distribution of the fading. For the fading cases discussed in Section 3.3, we can give solutions for μ:
188.8.131.52 Pure path loss (g≡1)
184.108.40.206 Rayleigh fading (g∼Exp(1))
The only difference between the Rayleigh fading and the path-loss-only scenario is the function, which is not present for no fading.
220.127.116.11 Approximating the directivity
4.4.2 Upper bound
where the second equation stems from the fact that dominant interferers always cause outage when present.
18.104.22.168 Pure path loss (g≡1)
4.5 Empirical verification
4.5.1 Verification of bounds
For every density λ, a radius R was chosen such that the average number of nodes satisfies , but the radius is not less than R≥200 m. For all simulations, the value λ is interpreted as average number of nodes per square meter.
A number I of interferers was randomly chosen from a Poisson distribution with mean parameter p Tx λ π R 2. Each of these interferers was assigned a random position x uniformly on the disc with radius R.
- 3.The total interference power for this realization is thus(57)
using β from Eq. (19).
If is true, this scenario counts as a detection. For every set of parameters, 10,000 realizations were run to estimate the outage probability as the fraction of scenarios where the target was not detected.
4.5.2 Model robustness for real estimates
The previous simulation assumes perfect detection; i.e., the reference target is guaranteed to be detected when Ppeak>θ. More realistic simulations are required to verify if the model works with the signal processing presented in Section 2 and the approximations shown in Section 4.1.
However, it is important to separate the detection from the multi-target identification, which will be discussed in Section 5.2. A target identification system has to distinguish sidelobes from targets, handle the case where targets are very close and have overlapping main lobes, etc., and therefore is an additional source of errors. To simulate an OFDM radar system without introducing multi-target-related errors, we create a scenario where everything except the reference target has zero RCS, and the reference target is therefore the only scatterer. The interferers only produce interference noise.
The simulation works as follows: For every iteration, a number of interferers (and their positions) is modeled the same way as in Section 4.5.1. The reference node, as well as every active interferer node, transmit an OFDM frame. The signal of the reference node is attenuated according to Eq. (14) and delayed by . The interference signals are attenuated according to their azimuth and antenna function as well as the path loss (with α=4). To reduce the amount of random effects, no small-scale fading is applied here.
All signals (reflected reference signal and all interferer signals) and the thermal noise are added up and passed to an estimator that works as described in Section 2.2. We confirm a detection by checking that the periodogram bin corresponding to the reference target’s distance and Doppler (rounded to the nearest integer) is larger than the threshold, P e r(n0,0)>θ. At every simulation point, 1,000 iterations were run.
Because this simulation omits all multi-target-related errors, it is called an ideal detector in the following. It is clear that such a detector cannot be implemented in reality, where there are many targets with non-zero RCS.
5 Consequences for the OFDM radar network parametrization
As an approximation for the outage probability, the bounds can be an extremely useful tool. The following section gives some practical examples of their utility.
5.1 Network feasibility study
The most obvious use for these new metrics is a feasibility check of a given radar network to determine whether or not a network would fulfill certain QoS requirements. In this case, we can take a set of system parameters and use the bounds for the detection probability to see if the radar system is expected to work reliably.
As an example, consider the results from Figure 4. Say that we require a detection probability of 99% for safety reasons, the node density cannot increase beyond λ=102.2, which corresponds to one node per 158.5 m2 on average. In a vehicular scenario, this corresponds to an average of one vehicle equipped with an OFDM radar system every 53 m for a lane width of 3 m, which seems realistic given that most likely, not all vehicles would be equipped with such a radar system. For other node densities, the corresponding detection probability can be read from Figure 4. If the radar system must work with a specific detection probability with a higher node density, the parametrization can be changed in several ways: Either the number of channels U is increased, or ϕ0 is reduced.
It is worth pointing out the analogy to the outage capacity of communication networks, which is a data rate that can be achieved with a given probability. Here, a target with peak amplitude Ppeak can be detected with probability p D .
This demonstrates how these simple bounds provide a powerful tool for benchmarking the network performance of an OFDM radar system.
5.2 Evaluation of the detection performance
Section 4.5 discussed how well an ideal detector would perform. This can be used as a benchmark for practical implementations.
To demonstrate this, the simulation in Section 4.5.2 was modified to a more realistic scenario: First, every interferer is assigned a random RCS which obeys an exponential distribution with pdf . The mean RCS was chosen as .
Next, the radar signal processing uses a modified CLEAN algorithm  to estimate Doppler and range of all interferences and the reference target. This algorithm detects the largest peak in the periodogram, estimates range and Doppler, and then subtracts the components from the periodogram caused by the estimated values. This process is repeated until the largest remaining peak is smaller than θ. To make sure peaks are not caused by residuals during the subtraction process, peaks detected within the main lobe of a previously detected target are discarded during the process.
For large λ, the probability of a large target being in close proximity to the reference target increases, in which case the reference target might be overshadowed and therefore not detected.
For the values used to create Figure 9, we obtain a received power of −127 dBm, or an 18-dB detector loss. Other multi-target detection algorithms might achieve better values, but we now have a practical limit for how good such an algorithm can get.
We emphasize that this benchmark is only valid for a fixed scenario.
5.3 Choice of sub-carrier spacing
In Section 3.1, we introduce the possibility of using a carrier spacing method (U>1) to allow multiple user access to the medium. This has two effects: first, it decreases the chance of a collision, as different users may access the medium on different logical channels. On the other hand, whenever a collision occurs, the influence of the interfering signal is in fact worse than it were for U=1, because the signal power per active carrier is increased by a factor U. This requires a closer analysis on how the choice of U affects the multi-user performance.
These results provide a simple answer for this: consider Eq. (46), where U appears twice: once in the PPP density (pTxλ/U), and once in . The argument of the exponential function therefore depends on . Because , the outage probability will always decrease for higher U, and this can thus be chosen as large as external constraints permit.
In this paper, we have suggested a novel figure of merit for radar systems: the radar outage probability. This is a scalar value which describes the capability to detect a specific target. It depends on the density of nodes and the waveform parameters.
More importantly, we could derive an asymptotically tight bound, which provides an analytical expression of radar systems without simulations or measurement campaigns (although empirical methods are used to verify the results). As an example, we can calculate that the OFDM radar system suggested in  would have an outage probability of less than 1% for traffic densities below one node per 158.5 m2.
Despite being a theoretical model, only few assumptions and approximations are made (see the introduction of Section 4). This makes the result applicable in practice, e.g., as a quick, computationally inexpensive tests to verify if a given waveform achieves a certain outage probability.
It can also be used to benchmark implementations of radar systems. The complementary probability of pout=1−p D is an ideal detection rate. If the measured detection rate lies below that, the loss due to the implementation can be quantified.
a Multiple radar sensors working cooperatively are sometimes also referred to as radar networks. Here, the network aspect describes the distributed nature of nodes which can also communicate among each others.
b We assume that the reader is familiar with OFDM; for a more detailed introduction, we refer to standard textbooks, e.g., [32, 33]. OFDM radar, in particular the signal processing components, is explained in .
c Here, anything backscattering energy is considered, be it a regular target or clutter.
d In , the authors discuss the usage of a Hamming window.
e This is further discussed in Section 5.2.
f In the context of vehicular technology, we can safely assume the existence of a GPS receiver.
h The fading of the nodes is identically distributed, the distribution itself does not depend on i.
i sinc(x)= sin(π x)/π x
j This does not state that the individual analog transmit signals are pure white Gaussian noise, rather that the superposition of such signals in the discrete-time domain are uncorrelated.
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