Joint wireless communication and radar sensing systems - IEEE Xplore

www.ietdl.org Published in IET Microwaves, Antennas & Propagation Received on 5th August 2012 Revised on 23rd May 2013 Accepted on 24th May 2013 doi: 10.1049/iet-map.2012.0450

Invited Paper

ISSN 1751-8725

Joint wireless communication and radar sensing systems – state of the art and future prospects Liang Han, Ke Wu Poly-Grames Research Center, Center for Radiofrequency Electronics Research of Quebec (CREER), Department of Electrical Engineering, Ecole Polytechnique, University of Montreal, Montreal, Quebec, Canada H3T 1J4 E-mail: [email protected]

Abstract: The historical development and current state-of-the-art of various joint wireless communication and radar sensing systems are reviewed and discussed in this study. Different kinds of systems are categorised according to their modulation waveforms and duplex schemes. Pros and cons of each category are highlighted. To showcase the current research advances, several demonstration systems are introduced with emphasis on proposed research contributions in this emerging area, and their performances are compared with respect to both communication and radar modes. Also, a number of challenges are identified for the near future system developments and applications.

1

Introduction

The invention of radio ushered the history of mankind into a new era. Among various types of radio applications known to date, the most ubiquitous and prominent two of them are wireless communication and radar sensing. Since the very beginning, these two sorts of systems have been designed and developed in a quite different manner, even though both make use of radiofrequency transceiver functional blocks. Of course, they have generally been independently and separately studied and developed from each other in most scenarios except for some primitive application examples of system fusion between them such as the so-called secondary surveillance radar system or the identification friend or foe (IFF) system [1]. Although information exchange is enabled in such kind of radar systems, the responder is in general not able to transmit data to the interrogator autonomously. Therefore data fusion schemes based on this kind of system operation, that is, radar-embedded communications [2] and UWB RFID [3], will not be further discussed in this paper because such systems do not present a ‘truly’ blended transceiver architecture that simultaneously possesses communication and radar functions. If successfully implemented, the system fusion of wireless communication and radar sensing would definitely bring up many benefits including architecture unification and simplification, functional reconfiguration and fusion and especially efficiency enhancement and cost reduction. Naturally, the fusion of two systems through one single transceiver allows the real-time interplay and ‘dialogue’ between two different functions. An early example of integrated radar and communication subsystem can be traced back to the NASA Space Shuttle Orbiter [4]. Recently, there has been a multitude of emerging needs for integrating wireless communication and radar sensing 876 © The Institution of Engineering and Technology 2013

systems. The intelligent transportation systems [5], for example, require intelligent vehicles to have the capability of autonomously sensing the driving environment and cooperatively exchanging information data such as velocity and braking between vehicles and also road, traffic and weather conditions as well as entertainment content among vehicles and beacons. Another example is the wireless sensor networks (WSN) [6] for positioning and monitoring purposes, in which each node detects targets and shares its information with other nodes through wireless communication links, which should be desirably part of the WSN system. Furthermore, it should be mentioned that multifunctional in-situ systems with active protection radar, wideband communications and combat identification are also highly demanded for modern military applications [7]. The unprecedented system fusion has generally presented many stringent design requirements such as low-cost, simple implementation, flexible functional reconfiguration and high-power efficiency as well as fast response. This is because the system fusion or integration should be much more advantageous than its pure mechanically assembled counterpart of two separate system architectures. In the past half-decade, a number of system concepts with respect to waveform design and transceiver development have been proposed to integrate both wireless communication and radar sensing functions within a single transceiver system. This paper briefly reviews and summarises the state-of-the-art systems, describes and discusses some demonstration systems, and points out several challenges for future system developments.

2

Waveform design

Different underlying operation principles of wireless communication and radar sensing systems suggest different IET Microw. Antennas Propag., 2013, Vol. 7, Iss. 11, pp. 876–885 doi: 10.1049/iet-map.2012.0450

www.ietdl.org requirements with regard to certain system and transceiver design considerations, such as duplex mode, modulation scheme and bandwidth as well as signal processing algorithms. For instance, frequency division duplex mode commonly used in wireless communication is not suitable for radar applications since the spectra of transmitted and received radar signals are almost co-located in the same frequency channel. In most cases, the design requirements for communication and radar functions may be in conflict, which pose a great challenge in the development of such system architecture fusions. With rapid progress of digital circuits, there has been a trend to simplify the design of radio-frequency (RF) front-end circuits by implementing most of the system functions based upon software-reconfigurable signal processing algorithms. These software-defined radio platforms provide us with a good opportunity to achieve joint communication and radar operations within the same RF front-end circuit.

Table 1 summarises the pros and cons of typical communication–radar fusion schemes that have recently been proposed and studied in [8–24]. The joint waveform scheme can be further categorised into single-carrier and multi-carrier systems. In the single-carrier systems, the frequency-domain scheme is found much less popular than its code-domain (spread spectrum) counterparts because of the mutual or cross-channel interference caused by the spectrum overlapping [8]. The spread spectrum technique was firstly implemented in a boomerang transmission system for vehicular communication and ranging applications [9]. Essentially, the proposed system is similar to an IFF system except that it adopts the direct-sequence spread spectrum (DSSS) technique to improve the system performance. Following that development, the spread spectrum technique has widely been exploited for fusing communication and radar functions, such as DSSS [9–13],

Table 1 Summary of communication–radar fusion schemes Domain

joint waveform

single-carrier system

multi-carrier system

time-domain duplex

—

frequency

Pros

1. simultaneous operation

code

1. simultaneous operation; 2. secure and robust communication; 3. strong resistance to interference and jamming; 4. low probability of intercept; 5. multiple access capability

—

1. simultaneous operation; 2. high spectral efficiency compared with spread spectrum systems; 3. no range-Doppler coupling; 4. strong resistance to inter-symbol and inter-channel interference; 5. flexible spectrum adaptation and subcarrier modulation

time

1. minimised mutual interference; 2. spectrum reutilisation; 3. possibility of functional reconfiguration and data fusion; 4. multiple access capability; 5. low cost

Cons

Ref.

1. spectrum overlapping introduces mutual interference 1. low spectral efficiency for communication; 2. low dynamic range in radar signal processing and not robust against Doppler shift; 3. require heavy computations for Doppler frequency estimation; 4. complex system design and implementation; 5. relatively high cost 1. complex system design and implementation; 2. high peak-to-average power ratio of OFDM signals; 3. sensitive to Doppler effect and frequency synchronisation; 4. degraded transmission efficiency because of guard interval; 5. high cost

[8]

pulse (DSSS)

ASK

[9] [10] [11] [12] [13] [14] [15]

Pulse Pulse (DSSS) pulse (DSSS) pulse (DSSS) pulse (DSSS) pulse (THSS) pulse (CSS)

DQPSK CCK BPSK PAM PPM PPM BPSK

[16]

pulse (matched filtering for range estimation) pulse (matched filtering for range estimation) CW (matched filtering for range and FFT for Doppler estimation) CW (modulation symbol domain processing for range and Doppler estimation) FMCW FSCW FMCW FMCW trapezoidal FMCW

1. no of simultaneous operation; 2. time synchronisation is required

[17]

[18]

[19]

[20] [21] [22] [23] [24]

Radar mode

Commun. mode

BPSK

OOK

M-PSK or M-QAM

QPSK

ASK FSK FSK BPSK BPSK

ACRONYMs: ASK − amplitude shift keying; BPSK − binary phase shift keying; CCK − complementary code keying; CSS − chirp spread spectrum; DQPSK − differential quadrature phase shift keying; DSSS − direct-sequence spread spectrum; FMCW − frequency-modulated continuous-wave; FSCW − frequency-stepped continuous-wave; FSK − frequency shift keying; PAM − pulse amplitude modulation; PPM − pulse position modulation; THSS − time-hopping spread spectrum IET Microw. Antennas Propag., 2013, Vol. 7, Iss. 11, pp. 876–885 doi: 10.1049/iet-map.2012.0450

877 © The Institution of Engineering and Technology 2013

www.ietdl.org THSS [14] and CSS [15]. The code-domain schemes allow for secure communication and high-resolution ranging at the expense of utilising excessive spectrum resources for data communication. Moreover, different users can share the same frequency band simultaneously by using different spreading codes, which is very beneficial for multiuser application scenarios. Regarding the radar ranging and Doppler estimation based on the spread spectrum technique, there are two main drawbacks. One is that the peak-to-sidelobe ratio is limited by the imperfect auto-correlation characteristics of the pseudorandom codes, and it is also impacted by the spreading factor and the Doppler shift. The other is that for Doppler processing, the spread spectrum technique requires a huge amount of computations. In general, the concern of using a spread spectrum technique is related to its high complexity and cost and low efficiency in terms of system implementation. Multi-carrier, especially the orthogonal frequency-division multiplexing (OFDM) technique, which has widely been used in wireless communications, was also introduced in the design of radar waveform [25]. The use of OFDM radar waveforms was demonstrated in multiple-input and multiple output (MIMO) radar systems [16] and radar networks [26, 27] as well as synthetic aperture radar (SAR) systems [17, 28–30]. One attractive feature of the OFDM radar signals is that there is no range–Doppler coupling issue and therefore independent range and Doppler processing becomes possible [31, 32]. In the past few years, different signal processing techniques have been proposed and implemented. In the beginning, matched filters were used to perform range and Doppler estimation in [18, 33–37]. However, correlation-based processing method still suffers from such problems as low dynamic range (or peak-to-sidelobe ratio) and ambiguities. In order to increase the dynamic range and preserve the transmitted communication data, a novel ‘modulation symbol domain’ OFDM processing algorithm was proposed [38–41] and validated through experimental system prototypes [19, 42]. Although preserving the resolution and the processing gain of the correlation-based processing method, this advanced joint range and Doppler estimation algorithm for the OFDM-based joint waveform has much higher dynamic range than the single-carrier spread spectrum approach by many orders of magnitude, especially for high SNR levels. Moreover, the Doppler frequency (or target relative velocity) can be easily estimated independently from the target range. A comprehensive review of the proposed signal processing technique and related system demonstration can be found in [43]. This technique has been recently extended to tackle a multipath and multiuser scenario in [44]. Although the OFDM techniques have presented a number of advantages listed in Table 1, the high implementation cost of OFDM systems because of the complex signal processing and high peak-to-average power ratio of OFDM signals still impede their widespread applications. Almost in parallel with the joint waveform scheme, the time-domain duplex scheme has also stimulated research interest because of its high spectral efficiency and easy system implementation as well as low cost [20–24]. In the time-domain platforms, communication and radar functions operate independently in order to minimise their mutual interference. Consequently, various kinds of radar waveforms and communication modulation techniques can be applied, respectively, in the radar cycle and the communication cycle, according to application scenarios. This allows the development of each system function to its 878 © The Institution of Engineering and Technology 2013

utmost possible. It should be noted that frequencymodulated continuous-wave (FMCW)/frequency-stepped continuous-wave (FSCW) radar waveforms are mostly used in the time-domain duplex scheme because of its low cost and easy implementation. However, the range and Doppler estimation based on the conventional FFT method suffers from the much higher peak-to-side ratio compared with the spread-spectrum and OFDM-based radar waveforms. For a conventional rectangular windowing, the peak-to-sidelobe ratio is about 13 dB. Therefore other window functions such as Hamming window are often used for improving the peak-to-sidelobe ratio to 43 dB with the sacrifice of 6 dB bandwidth. Among these time-domain schemes, a novel time-agile modulation scheme has recently been proposed and demonstrated by the authors [24] and it will be presented in detail in the next section.

3

Demonstration systems

For each category of modulation scheme in Section 2, we will present one system example that has been developed in the past few years with practical implementation and measured system performance of both communication and radar functions. Their reported performance will also be compared at the end of this section. 3.1

System based on the DSSS technique

In [13], a 60 GHz transceiver that is able to simultaneously perform communication and location functions was proposed. Fig. 1 shows the transceiver architecture. Generally, the proposed system is based on a pulse radar system and the radar pulse with a width of about 300 ps is modulated by the communication data using the pulse position modulation (PPM) technique. The receiver of the whole system is actually composed of four sub-receivers. In addition to a correlator for data demodulation in each sub-receiver, the four output baseband signals are combined

Fig. 1 Transceiver architecture of a joint communication and radar system based on the DSSS technique [13] IET Microw. Antennas Propag., 2013, Vol. 7, Iss. 11, pp. 876–885 doi: 10.1049/iet-map.2012.0450

www.ietdl.org to implement the time-reversal technique for the radar ranging. The system prototype has a communication range of 10 m with a data rate of 100 Mbps under a bit-error-rate (BER) of 1 × 10–6, and a radar range resolution of 12.4 cm within a range of 3 m. It should be noted that in a multi-user (multiple identical systems) environment, DSSS technique is often employed to enable code division multiple access (CDMA) [45]. 3.2

System based on the CSS technique

By making use of the quasi-orthogonality of the up-chirp and down-chirp, communication and radar functions can be also realised using the same transceiver platform [15]. In the detailed block diagram of Fig. 2, the pulse radar signal uses a down-chirp modulation whereas the binary phase shift keying (BPSK) communication signal uses an up-chirp modulation. The down-chirp modulation with a bandwidth of 500 MHz is achieved through a surface acoustic wave filter, and therefore the up-chirp signal is easily obtained by flipping the down-chirp signal with respect to the carrier frequency (750 MHz in this setup). Both communication and radar signals are combined and transmitted by a right-handed circularly polarised (RHCP) antenna. In the receiver, considering the reflection characteristic of the radar signal, a left-handed circularly polarised is used to receive the radar signal whereas an RHCP antenna is used for receiving the communication signal. Then, the radar and communication signals are pulse compressed using an up-chirp matched filter and a down-chirp matched filter, respectively, which finally generate the ranging output and communication data.

A number of experiments were conducted to evaluate both communication and radar performances. In the beginning, the radar range resolution was found to be 63 cm with two objects placed at 10 m away. Also, measured probability of false alarm was 7% when communication system was operating at 1 Mbps. Measured probability of detection was 99% for both communication system off and operating at 1 Mbps. On the other hand, communication performance evaluation was carried out by directly connecting the transmitter to the receiver with a 60 dB attenuator. Measured communication BER of 1 Mbps BPSK data drops from less than 1 × 10–5 to 2 × 10–3 with the radar operating at a pulse repetition frequency (PRF) of 150 kHz. Further, the BER almost increased exponentially as the increase of PRF from 100 to 1000 kHz. These experimental results assert that simultaneous operation may not be the most optimal if there exists residual mutual interference between communication and radar signals. Therefore the time-domain modulation scheme that will be showcased later becomes more attractive because of the sequential operations of the radar and communication modes. 3.3

System based on the OFDM technique

As described in Section 2, OFDM technique is another viable solution for simultaneous radar ranging and data communication. A multifunctional software-defined system based on the OFDM technique has been demonstrated in [17]. In the proposed system, radar function is achieved by directly transmitting the OFDM pulses whereas sub-bands of OFDM sequences are encoded with information data in the communication mode. As it can be seen from the block

Fig. 2 Block diagram of a multifunctional communication/radar system based on the CSS technique [15] IET Microw. Antennas Propag., 2013, Vol. 7, Iss. 11, pp. 876–885 doi: 10.1049/iet-map.2012.0450


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Fig. 3 Software-defined multifunctional platform [17] a Software-defined subsystem b Analogue front-end

diagram in Fig. 3, the entire system is highly replying on the digital signal processing and also presents high demands on the digital-to-analogue convertor (DAC) and the analogue-to-digital convertor (ADC). To generate baseband OFDM signals with a bandwidth of 500 MHz, the sampling rate of the DAC and correspondingly that of ADC are 1 GS/s. The resolution of the DAC and ADC are 12-bit and 8-bit, respectively. After the baseband OFDM signals are generated using soft code in MATLAB®, they are converted to analogue signals through DAC, up-converted to the carrier frequency of 7.5 GHz and then transmitted by the TX antenna. The received signals are processed in a reverse order. The system performance was evaluated based on two sets of experiments. First of all, the range resolution of the radar function was tested with three trihedral corner reflectors placed apart 1.6 m away from the system. Measured range resolution is about 34 cm, which is very close to the theoretically calculated range resolution of 30 cm. It should be noted that when calculating the theoretical range resolution, an effective bandwidth of 500 MHz is used in spite of the occupied bandwidth of 1 GHz. In addition, by moving the antenna platform over a straight path and taking a number of measurements at different positions, the proposed system can be used as a SAR. For testing the communication performance, a reflector was used to create a static flat-fading channel. Measured BER varied between approximately 0.625 × 10–2 and 3.125 × 10–2, and the degradation was attributed to the frequency offset-induced inter-carrier interference, that was estimated to be around 2 MHz from the experimental results. 3.4

duration of Ts. One beat frequency can be obtained in each period for a single target, which is then used to estimate the target’s range and velocity. Following the radar cycle, the radio (communication) cycle is another constant-frequency period, which can be modulated by communication data using various sorts of modulation techniques such as amplitude shift keying (ASK), frequency-shift keying (FSK) and phase-shift keying as well as high-order

System based on time-domain duplex

3.4.1 Modulation waveform: The authors have proposed a novel kind of time-agile modulation waveform for the first time, which is able to achieve both radar sensing and data communication functions using a single transceiver [24, 46]. Two cycles of the proposed periodic modulation waveform are illustrated in Fig. 4. In Fig. 4a, the black solid line is the transmitted signal, whereas the blue dashed line is the received signal. As shown in Fig. 4, each operation cycle of T consists of two different time slots. The first part is a radar cycle, which utilises a trapezoidal FMCW modulation consisting of an up-chirp, a constant-frequency period and a down-chirp with equal 880 © The Institution of Engineering and Technology 2013

Fig. 4 Time-frequency diagram of the proposed time-agile modulation waveform a Transmitted and received waveforms b Beat frequency IET Microw. Antennas Propag., 2013, Vol. 7, Iss. 11, pp. 876–885 doi: 10.1049/iet-map.2012.0450

www.ietdl.org modulations such as quadrature amplitude modulation (QAM). Our proposed modulation waveform is generated by making use of a direct-digital synthesiser (DDS), since it is able to realise frequency chirping and various kinds of modulations very easily and precisely through software programming. In such a software-defined platform, time-agile and sequential operation of communication and radar functions enables very flexible functional reconfiguration and potential data fusion between them. On one hand, this may be well pronounced in the case of a high-speed mobile broadband system for which the Doppler effect of a fast moving device may be compensated through the on-device radar sensing. This Doppler effect is a fundamental limiting factor for the moving speed of communication devices with respect to the device communication rate. On the other hand, the ranging information can be changed by communication links between different on-device radars, which effectively forms a radar network with increased detection range and accuracy. Moreover, the chirp rate can be varied in a number of consecutive operation cycles for multiple moving targets detection. 3.4.2 Transceiver architecture: To validate our proposed modulation waveform and system concept, a system demonstrator has been built up for 5.9 GHz US Federal Communications Commission (FCC)’s dedicated short range (DSRC) applications. Our transceiver architecture is sketched in Fig. 5. It can be seen from Fig. 5 that in order to adapt the conventional heterodyne transceiver architecture to our proposed modulation waveform, a power splitter is inserted in the transmitter. A pair of microstrip antennas is developed to increase the isolation between the transmitting and receiving channels. The system operation principle corresponding to the proposed modulation scheme is briefly described as follows. In the radar mode, the frequency-sweeping signals generated by the DDS are filtered by two low-pass filters and then up-converted to intermediate-frequency (IF). The IF signal is divided into two portions with one of them further translated to RF and sent by the transmitting antenna. The reflected wave from the target is captured by the receiving antenna and subsequently down-converted to IF after being amplified. The received IF signal is then mixed with the other portion of the transmitted IF signal.

As a result, the target range and velocity can be estimated from the beat signals in the radar cycle [46]. In the communication (radio) mode, the transceiver operates in a time division duplex mode, which means that the transmitter and the receiver cannot operate at the same time. For transmitting, data information sent by an advanced reduced instruction set computing machine (ARM) board modulates the constant-frequency output of the DDS, and then the modulated signal is transmitted in the same way as the radar mode. The reference signal is discarded in the receiving channel. On the other hand, when the system receives signal, the output of the DDS is kept unmodulated, which can be used for demodulating received signals from other onboard units. 3.4.3 Prototyping and experimental results: As shown in Fig. 6, the system is prototyped with commercial off-the-shelf components and subjected to a number of experiments for evaluating its performance in terms of BER in the communication mode and range and velocity detection accuracy in the radar mode. In our experiment, a multichannel emulator (EB Propsim C8) is used to generate different channels for system performance evaluation. In the communication mode, the channel emulator is set as an additive-white-Gaussian-noise (AWGN) channel and measured BER agrees well with our simulation results obtained from the Ptolemy simulator in a commercial circuit simulation package. On the other hand, in the radar mode, the channel emulator is configured to have a set of delays from 1500 to 5000 ns and a set of velocities from 10 to 80 m/s. The experimental results obtained from our system present a standard deviation of 3.34 ns for the delay measurement and 0.15 m/s for the velocity measurement, which fully demonstrates excellent ranging and velocity detection of our system. 3.4.4 Higher-frequency prototype: Both data communication and radar performance of our 5.9 GHz system demonstrator is limited by the available channel bandwidth of 20 MHz defined by the DSRC protocol. Nevertheless, there is a frequency band up to 250 MHz in the 24 GHz industrial, scientific and medical (ISM)-band, which has also been adopted for vehicular applications for a long time. Therefore an alternative higher-frequency system has been prototyped in the 24-GHz ISM band [47, 48] with the help of the emerging substrate integrated waveguide technology, which has presented a cost-effective solution

Fig. 5 Transceiver architecture for fused radar and radio functions [24] IET Microw. Antennas Propag., 2013, Vol. 7, Iss. 11, pp. 876–885 doi: 10.1049/iet-map.2012.0450


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Fig. 6 Photos of the fabricated 5.9 GHz system prototype [46] a Transmitter b Receiver

for microwave and millimetre-wave integrated systems [49]. In this higher-frequency system prototype, the sweeping bandwidth of the DDS is set as 100 MHz for our proof-of-concept study and the transceiver architecture is similar to that illustrated in Fig. 5. The pictures of the fabricated prototype are given in Fig. 7. Our measured constellations of demodulated I–Q signals at an input power level of − 60 dBm present good data communication capability of our system for both BPSK and quadrature phase shift keying (QPSK) modulations at a data rate of 50 Mbps. In the meanwhile, the ranging performance in the radar mode is evaluated by varying the distance of a target from our system and a very linear detection curve is obtained, which exhibits the high-accuracy range detection capability of our system. Fig. 7 Pictures of the fabricated 24 GHz prototype [47]

3.5 Performance comparison and remarks: Since the above-described architectures make use of different schemes that involve different hardware and software implementations with different design objectives, it is rather difficult to compare them for both communication and radar functions in a very quantitative manner. Table 2 compares the performance of radar and communication functions between different demonstration systems. In Table 2, the maximum operation range of both radar and communication modes for [47] is derived from the input signal level of the receiver by inversely deducting the free space path loss. A number of observations can be made from Table 2. First of all, the 60 GHz multifunction system in [13] targets at indoor applications since it supports the highest data rate of 200 Mbps but within a very limited range of 10 m because of a very high oxygen absorption. Second, similar data rate throughput is achieved between [17, 47]. However, it should be pointed out that the BER of [47] is estimated using the measured signal-to-noise ratio in the case of a

direct cable connection between the transmitter and the receiver, which can be modelled as an AWGN channel. However, when it comes to the over-the-air measurement with a reflector, the wireless channel will be fairly similar to a Rician channel with a dominant direct link and multiple reflected waves. Therefore the BER of the over-the-air measurement would be worse than that of the direct cable connection. For example, in the case of the same 10 dB Eb/N0, BER can degrade from 3.9 × 10–6 for an AWGN channel to 5e − 3 for a Rician channel with a power ratio of 6 dB between the dominant link and all other scattering. Thirdly, as described in Section 3.2, BER of 1 Mbps BPSK signal decreases from 1 × 10–5 to 2 × 10–3 in the case of a simultaneous radar operation [15], which


IET Microw. Antennas Propag., 2013, Vol. 7, Iss. 11, pp. 876–885 doi: 10.1049/iet-map.2012.0450

a Transmitter b Receiver

www.ietdl.org Table 2 Performance comparison of demonstration systems Ref.

[13] [15] [17] [23] [47]

Carrier frequency, GHz

60 0.75 7.5 5.875 24.125

Communication Maximum range, m

10 30 5 1000 200

Radar

Modulation

Data rate, Mbps

BER

Maximum range, m

Type

Occupied bandwidth, MHz

Range resolution, cm

PPM BPSK OOK BPSK BPSK

200 1 57 10 50

< 1 × 10–6 < 2 × 10–3 < 5 × 10–2 < 1 × 10–6 < 1 × 10–6

3 15.2 30 300 70

pulse pulse pulse TFMCW TFMCW

3000 500 1000 20 100

12.4 63 34 750 165

indicates the simultaneous operation of two functions still needs further investigation. On the contrary, time-domain duplexing provides isolation between these two functions and minimises their mutual interference.

4

Future perspective

With the state-of-the-art research in mind on the joint wireless communication and radar sensing functions with a single transceiver, the following research topics can be envisioned from a perspective of system design and implementation in the near future. 4.1

Frequency up-scaling

Over low-frequency ranges such as the popular wireless bands from 800 MHz to 6 GHz, the spectral congestion has created difficult problems for the development of joint radio and radar applications as the licensed radio bands cannot be shared by radar applications. For most ISM unlicensed bands, the frequency band available for high-resolution radar development is extremely limited. Of course, the governmental regulations contribute to the challenging difficulties of further developing high-performance joint radio-radar systems. The ever-increasing demands for high-resolution ranging and high-speed communication request more and more bandwidth for system operation. Compared with the highly congested low-frequency spectrum, the conventional licensed frequency bands for automotive radar applications, such as 77/79 GHz (or E-band), present a bandwidth of several GHz, and they can effectively be exploited for joint communication and radar applications. However, cost-effective system design and implementation at such high frequencies also present great technological challenges. For example, the performances of millimetre-wave circuits and components are very limited, which are much less available than their low-frequency counterparts. Therefore link budget analysis and Tx/Rx chain budget optimisation should be carefully performed at the stage of system design. Also, there would be trade-offs between the communication mode and the radar mode, such as transmit power, receiver linearity, dynamic range, sensitivity and antennas. Generally, antennas for wireless communication applications usually have an omni-directional or well-defined sectorial radiation pattern to increase the coverage. However, the radar applications may vary from simple ranging parameter detection to complex multiple parameters acquisition, and they generally require high-gain and narrow-beamwidth antennas for compensating the round-trip signal attenuation and achieving high-resolution angle detection. There are several solutions to this dilemma. IET Microw. Antennas Propag., 2013, Vol. 7, Iss. 11, pp. 876–885 doi: 10.1049/iet-map.2012.0450

For instance, two antennas/arrays can be, respectively, dedicated to each application and they can be electronically switched with precise synchronisation between two modes. It is also expected to be able to generate electronically switchable beams (wide and narrow) with a single phased-array antenna system. Also, smart antennas can be employed to change the radiation pattern of the entire antenna array. Such strategies should be acceptable because of the small size of millimetre-wave antennas/arrays. Moreover, system development at millimetre-wave frequencies also offers the opportunity to integrate the entire system or at least the transceiver front-end using various sorts of integration technologies, such as the CMOS technology [50], the SiGe technology [51] and the system-on-substrate scheme [52]. 4.2

Adaptive modulation waveform

Most of the proposed modulation schemes are enabled by the software-defined platforms, which can intelligently or adaptively adjust system functionality according to different application scenarios. For instance, in our proposed trapezoidal modulation waveform, a time-agility or a flexible functional reconfiguration can be achieved through adjusting the duration of the communication and radar cycles in a real-time manner. Also, the chirp rate can be changed adaptively in order to detect multiple moving targets. With the recently-proposed cognitive radio [53] and cognitive radar concept [54], this waveform adaptation can even be performed based on the knowledge of surrounding environment and it will finally lead to cognitive-generation multifunctional communication and radar systems. 4.3

Networking

The joint operation of wireless communication and radar functions is essential to achieve a special kind of WSN, namely the ad-hoc radar networks. The capability of wireless communication enables the mobility of the radar network. Such ad-hoc radar networks can find applications in vehicular and combat scenarios. However, the high mobility of the each system platform or node also presents challenges of maintaining good connectivity and maximising the network lifetime. Therefore research directions will be related to the investigation and development of networking approaches with such requirements as low-cost, real-time, easy implementation and dynamic adaption.

5

Conclusion

Multifunctional systems with wireless communication and radar sensing capability have aroused research interest over 883 © The Institution of Engineering and Technology 2013

www.ietdl.org the past decade, especially in the last five years. Different modulation schemes have been proposed and studied to integrate or fuse these two functions in a single transceiver/ system platform and they have been classified into two categories and four subcategories in this paper. The advantages and disadvantages of each subcategory are discussed and compared. Spread spectrum technique is advantageous in its security and CDMA features, whereas OFDM technique offers no range-Doppler coupling and high spectral efficiency. Both of them suffer from high implementation cost and complex signal processing. Following that, four representative systems have been selected to present the current research progress. The operation principle of each system is discussed individually and finally their system performances are compared for both communication and radar modes. Experimental results have shown that system performance can be highly degraded if there is mutual interference between communication and radar signals. As a result, time-domain duplex scheme is presently a viable solution at low cost. Based on a novel time-agile modulation waveform, we have developed two integrated communication and radar systems with good demonstration of performances at 5.9 and 24 GHz, respectively. Finally, we have anticipated a number of technological challenges and opportunities for future system development towards millimetre-wave frequencies.

6

Acknowledgment

The authors acknowledge the financial support of the NSERC and the FQRNT for many years.

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