Consumer Radar: Technology and Limitations R. J. Evans∗ , P.M. Farrell† , G. Felic∗ , Hoa Thai Duong† , Hoang Viet Le† , J. Li† , M. Li† , W. Moran‡ , M. Morelande† , E. Skafidas∗ ∗ NICTA
† Department
Victoria Research Laboratory of Electrical and Electronic Engineering, University of Melbourne ‡ Defence Science Research Institute
[email protected]
Abstract—Recent advances in micro-electronics have created the possibility of building very low cost, very small, high performance single chip Radar systems. Concurrent with this technological advance, a diverse range of new applications for such radar systems is emerging. The coming consumer radar revolution is on the verge of entering the market place in areas including automotive radar, bicycle radar, micro-UAV radar, and many other applications. This short paper briefly describes the current state of the technology covering RF, signal processing and antenna systems. We also introduce recent work on performance limitations of such systems including radar information theory and its connections with quantum mechanics. Index Terms—millimeter radar, single chip radar, CMOS, radar performance limitations
I. I NTRODUCTION The idea of radar is more than 100 years old. In 1900 Nikola Tesla foreshadowed the idea of remotely detecting and locating objects using radio waves but it was Christian Hulsmeyer who in Germany in 1904 first demonstrated the detection of ships using echoes of radio signals. While the essential idea of radar is very simple its significance in applications is such that the ongoing effort to improve the performance and capabilities has resulted in radar being a major technology driver since its inception. We believe this trend is set to continue at an accelerated pace given the emerging opportunities in singlechip millimeter-wave radar made possible by CMOS technology scaling (Moore’s law) and recent advances in adaptive waveform design. Interest in consumer applications of radar is of course not new. In 1993 the IEE held a symposium on this very topic [1] and there are already small low-cost hand held Doppler radar systems on the market (e.g. Pocket radar). This paper will briefly explore the coming consumer radar era where tiny single-chip radar systems will be available for just a few dollars. They will utilize highly sophisticated waveform diversity techniques and adaptive signal processing to extract optimal performance. Automotive radar is one of the current challenges driving innovation in small low cost millimeter radar. Another exciting application of small lightweight high performance radar technology concerns sensing systems for micro-UAV’s. The ready availability of tiny, cheap, radar systems is likely to open up a whole host of new applications mimicking the transformations arising from the availability of tiny and cheap GPS systems. Relentless electronic technology scaling
978-1-4673-5178-2/13/$31.00 © 2013 IEEE
Fig. 1. The single chip radar system developed at The University of Melbourne.
is providing the opportunity to move to higher and higher RF frequencies hence enabling complete radar systems (RF and DSP) to be integrated onto a single chip. Clever new waveform diversity techniques [2] together with innovations in small antenna technology together mean highly sophisticated consumer radar systems will become a reality over the next few years. The remainder of this paper presents certain aspects of millimeter wave consumer radar technology covering CMOS RF systems, array antenna systems, waveform and signal processing, and finally performance limitations considerations. II. S INGLE C HIP RF S YSTEMS Many groups around the world are developing various types of ‘single chip’ radar systems especially for automotive radar applications. For an overview of these activities see for example [3]–[6] and the references therein. In this brief paper we overview the radar-on-a-chip system developed in our Melbourne University Labs. The 5 mm by 5 mm CMOS chip shown in Figure 1 is built with 65 nm technology. The chip (Figure 2) contains a complete RF transceiver system operating at 76–77 GHz. As seen in Figure 3 a typical configuration consists of 2 transmit chains and 4 receive chains. All required components including passives, amplifiers,
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Fig. 2.
An image of the single chip radar.
Fig. 4.
Performance of the single chip radar.
synthesize a transmit waveform based on the desired ambiguity function. Significant research effort has been devoted to the synthesis problem including the deep work of Wilcox [8] in 1960 on the group theoretic foundations of ambiguity theory that enabled synthesis of a certain restricted class of waveforms. A few years later, Hilbert-Schmidtt operator approximation techniques were proposed by Sussman [9] and Vakman [10] to approximately synthesize waveforms with specified ambiguity properties. See also [11] for recent work in this direction. Progress on this difficult but deeply important problem has taken on a new flavour in recent years under names such as waveform diversity, adaptive waveforms, and waveform scheduling [2]. Digital waveform generation and fast adaptable digital matched filter implementation means that waveform diversity techniques can now be implemented even in low cost single chip radar systems. The CMOS radar-ona-chip developed at Melbourne University employs adaptive digital matched filtering and scheduling of advanced multifrequency coded waveforms, to reduce clutter, mitigate against interference, and also to reduce computational load. Typical performance is demonstrated on an automotive scenario consisting of 25 oncoming point scatterers centred on x = 3 and 5 targets centred around x = 0. All scatterers and targets have unity radar cross-section and the speed of each target/scatterer is chosen randomly within typical automobile speeds. The performance on the radar is shown in Figures 5 and 6. Figure 5 shows the range estimation error as a function of target range and Figure 6 shows Doppler estimation accuracy.
Fig. 3. A schematic showing the components and function of the single chip radar.
mixers, oscillators are contained on the chip. A transmit power of 10 dBm at 77 GHz has been achieved [5]. A receiver sensitivity of -100 dBm for an output SNR of 10 dB has been measured [6]. III. WAVEFORMS AND S IGNAL P ROCESSING Radar design was put on a sound theoretical footing in 1953 when Philip Woodward at TRE in England developed the radar ambiguity function [7]. This was based on the matched filter developed by Dwight North at RCA in USA in 1943. Woodward’s ambiguity function characterizes the performance of a matched-filter radar for any particular transmitted waveform. Unfortunately however, it is not possible to precisely
IV. A NTENNA S YSTEMS The antenna is a critical element for all radar systems. To satisfy the size and cost requirements of a consumer radar, our work has focused on innovative patch antenna structures
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Fig. 5.
Range estimation error of single chip radar as a function of range.
Fig. 7.
Serial patch antenna structure with a low cost plastic lens.
Fig. 6. Doppler estimation error of single chip radar as a function of range.
Fig. 8.
and low cost lens technology [12]. The antenna shown in Figure 7 consists of a serial patch structure with a low cost plastic lens. The measured performance at 77 GHz is shown in Figures 8, 9 and 10. Figure 8 is a plot of the antenna driving impedance while Figures 9 and 10 show the elevation and azimuth radiation patterns.
Antenna impedance as a function of frequency.
V. L IMITATIONS AND I NFORMATION T HEORY After Shannon had proved the possibility of constructing codes that allow the transmission of messages in the presence of noise without error up to limits set by the available bandwidth, the transmitted power and the signal to noise ratio, there was an explosion of interest in the connection of these mathematical ideas to the underlying physics of information flow. The information carrying capacity of the electromagnetic field subject to the limits set by quantum mechanics and statistical mechanics was found by constructing states of the field which maximized the physical entropy. By these means, Lebedev and Levitin [13] showed the channel capacity, Cp , of an electromagnetic field which was not constrained by
Fig. 9.
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Antenna elevation radiation pattern.
Fig. 10.
Antenna azimuth radiation pattern.
Fig. 11. The channel capacity of the electromagnetic field in units of bandwidth as a function of SN R = hfP∆f , illustrating the high power, P > hf ∆f , low power, P < hf ∆f , and the total.
bandwidth was given by r Cp = π
2P , 3h
and for the case of a bandwidth limited field by P Cp =∆f log2 1 + hf ∆f P hf ∆f + log2 1 + . hf P
(1)
(2)
Here, P is the average power received by the detector, f is the carrier frequency of the bandwidth-limited signal, ∆f is the bandwidth, and h is Planck’s constant. The optimum spectrum for the bandwidth unlimited case is that of a black body radiator with a temperature set by the average power. This gives an absolute upper limit to the channel capacity given an available power. The bandwidth-limited case gives more insight into the limits for circumstances that are routinely used in radar and communications. The limit contains two terms. The first dominates in the limit of high power, P > hf ∆f , and has the form of the classic version of Shannon’s channel capacity with the noise hf ∆f being the vacuum fluctuation noise in the bandwidth ∆f . The second term is scaled with the photon P arrival rate hf , and this term dominates when P < hf ∆f . Figure 11 illustrates the channel capacity contributions of the two terms. It is important that the only noise considered here is that set by the limits of quantum mechanics, the vacuum fluctuation noise. Any technical noise carried by the field is considered to be information about the source of that noise. In addition to this, the ability to transfer this information from the field into some other form is not guaranteed. The transfer of information from the field to an antenna and through an amplifier and filter to an analog to digital convertor will place further limits on the information which can be extracted from this field. These details of these limits are the subject of further study by the authors. The ultimate limits of the radar system presented above can be calculated subject to these provisos and are shown
Fig. 12. The channel capacity for an electromagnetic field versus received power with frequency 77 GHz and a bandwidth of 1 GHz. Also shown is the bandwidth unlimited case.
in Figure 12. Figure 12 shows the channel capacity for an electromagnetic field versus received power with frequency 77 GHz and a bandwidth of 1 GHz and the channel capacity for the same power with no bandwidth limit. The power shown is the average which is incident on the receive antenna. In order to achieve such a channel capacity the transmitted electromagnetic field would have to be modulated in a manner such that after the interaction with whatever is being observed by the radar system, the received field was in this maximum entropy state. This would require either near perfect knowledge of the state of the scene under observation, or a modulation scheme which adapts to the information which is obtained from continuing observations. A simple estimate of the limits imposed by the transfer of power from the field to the antenna may be found by considering the channel capacity of the antenna subject to the electromagnetic noise in the environment. If we take the noise as due to the temperature of the background radiation in the
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Fig. 13. The maximum background temperature observed by the antenna which allows complete matching of the field channel capacity to the antenna.
Fig. 14. The high SNR limit to the maximum allowed antenna temperature as a function of carrier frequency.
bandwidth of the antenna, and the bandwidth of the antenna to be the same as that of the incident signal we have a channel capacity which may be written as Pa , (3) Ca = ∆f log2 1 + kT ∆f where Ca is the channel capacity of the antenna, Pa is the power transferred from the field to the antenna, T is the temperature of the background radiation as seen by the antenna, and k is Boltzmann’s constant. We are interested in the circumstance such that Ca = Cp subject to Pa ≤ P . If all the radiation incident on the antenna is converted to electrical energy, i.e. Pa = P , we can find a condition for the maximum temperature experienced by the antenna to achieve equality of channel capacity, Tmax given by Tmax =
Fig. 15.
Figure where the RF chip described in Section 2 is coupled to a array antenna similar to that described in Section 4 sits alongside a DSP chip implementing the algorithms in Section 3 and all under a small plastic lens. The radar occupies less than 6 cubic centimeters. Early results towards a new performance theory for such radar systems are also presented in Section 5 showing the potential for a significant increase in radar performance is still available.
hf × k
SN R1+SN R , SN R (1 + SN R) 1 + SN1 R − SN RSN R
A complete radar including array antenna and DSP.
(4)
with SN R = hfP∆f as defined above. This maximum temperature is shown for a 77 GHz, 1 GHz bandwidth signal in Figure 13 with the cosmic microwave background radiation (CMBR) temperature [14]. Since the maximum allowed temperature reaches a limit with increasing SNR, for this example, no increase in received power will allow all information possibly carried by the electromagnetic field to be extracted by an antenna. The limiting maximum temperature is shown against wavelength in Figure 14 illustrating the fact that the limiting channel capacity changes from the antenna to the field at a frequency in the THz range.
ACKNOWLEDGEMENT The authors would like to thank the Australian Research Council, NICTA, the Victorian Government and the University of Melbourne for financial support. The authors would also like to thank Cadence, IBM, General Motors and DSTO for their generous assistance. R EFERENCES [1] IEEE Symposium on Consumer Applications of Radar and Sonar, 1993. [2] M. C. Wicks, E. L. Mokole, and S. D. Blunt, Principles of waveform diversity and design. SciTech, 2010. [3] M. Hartmann, C. Wagner, K. Seemann, J. Platz, H. Jager, and R. Weigel, “A low-power low-noise single-chip receiver front-end for automotive radar at 77 GHz in silicon-germanium bipolar technology,” in Radio Frequency Integrated Circuits (RFIC) Symposium, 2007 IEEE. IEEE, 2007, pp. 149–152.
VI. C ONCLUSIONS This paper provides a very brief glimpse of the coming consumer radar revolution and the current state of technological development for single chip CMOS millimeter wave radar systems. A complete radar can be constructed as shown in
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