An Experimental WiMAX based Passive Radar Study

Qing Wang #*1, Yilong Lu #2, Chunping Hou *3

# School of Electrical and Electronic Engineering, Nanyang Technological University 50 Nanyang Avenue, 639798, Singapore

1wqelaine@tju.edu.cn 2eylu@ntu.edu.sg

* School of Electronic and Information Engineering, Tianjin University 92 Weijin Road, Tianjin, 300072,China

3hcp@tju.edu.cn

Abstract — Singapore has launched the world’s first

maritime WiMAX service network which also has introduced a new illuminator of opportunity for passive radar study. This paper presents a new study of passive radar using WiMAX signals. In this paper, the WiMAX standards, WiMAX-based passive radar demonstrator design based on a MIMO-OFDM testbed, and a primary field measurement for detecting ground vehicle are described. Interesting field measurement results may help us to understand the potential capabilities of WiMAX-based passive radar for range and Doppler detection and measurement of moving targets.

Index Terms — Passive radar, WiMAX, moving target detection.

I. INTRODUCTION

Passive radar is, essentially, receiver-only radar that usually dissociates the receiver antenna away from the transmitter. Containing no transmitter, the benefits that passive radar can offer are numerous. Most importantly, passive radar is virtually undetectable to surveillance receivers and there is also no constraint in spectrum allocation. In most cases, passive radar is smaller, more portable and of lower cost compared to conventional active radar. Over the past decade or more, there have been increasing interests in employing various illuminators of opportunity for passive radar, such as FM radio [1], television [2]-[3], digital radio and television (DAB, DVB) [4], cellular phone downlink (GSM) [5], wireless local area network (WiFi) [6] and satellite broadcast [7].

Singapore has launched the world’s first maritime WiMAX network (WISEPORT) [8] in 2008 to offer mobile internet access to ships in south waters of Singapore. This introduced a new illuminator of opportunity for passive radar study.

This paper intends to study the potential capabilities of WiMAX-based passive radar for the detection and tracking of various moving targets, in both range and Doppler detection. To facilitate the study, an experimental WiMAX-based passive radar system was integrated at Nanyang Technological University, Singapore. This paper describes evolution of WiMAX standards, an overview description of WiMAX-based passive radar in addition to primary experimental results for moving target detection and tracking.

II. WIMAX AND IEEE 802.16 STANDARDS

WiMAX is a telecommunications technology that provides wireless transmission of data using a variety of transmission modes, from point-to-multipoint links to portable and fully mobile internet access.

A. The WiMAX Standards Evolution

The WiMAX Forum is an industry consortium promoting the IEEE 802.16 family of standards for broadband wireless access systems.

802.16-2004 standard [9] (also known as 802.16d) addresses only fixed systems. It replaced IEEE standards 802.16-2001, 802.16c-2002 and 802.16a-2003. 802.16d vendors point out that fixed WiMAX offers the benefit of available commercial products and implementations optimized for fixed access. It is a popular standard among alternative service providers and operators in developing areas due to its low cost of development and advanced performance in a fixed environment. Fixed WiMAX is also seen as a potential standard for backhaul of wireless base stations such as cellular, or Wi-Fi.

The mobility enhancements provided by the later 802.16e amendment [10] further enhanced operation of nomadic, portable and mobile wireless access, and was published by IEEE at the beginning of 2006. The 802.16e specification (aka 802.16-2005) provides improved support for intercell handoff, directed adjacent-cell measurement, and sleep modes to support low-power mobile station operation.

Since early 2007, the WiMAX Forum and the IEEE 802.16 Working Group have started separation evolution projects to improve the performance of the current release of mobile WiMAX and keep the momentum of evolving mobile WiMAX as a leading mobile broadband wireless communication solution. The project and the associated future standard are known as 802.16m. The target for 802.16m is to meet the requirements of IMT-Advanced, the fourth generation (4G) successor of IMT-2000. In other word, 802.16m will be the 4G mobile WiMAX evolution [11].

B. Fixed vs. Mobile WiMAX

WiMAX promises to be a single technology that brings wireless broadband connectivity to both fixed and mobile

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users. However, it is now clear that there will be two distinct flavors of WiMAX: 802.16d (or 802.16-2004) WiMAX for fixed services and 802.16e (or 802.16-2005) WiMAX for both fixed and mobile services. Although they are both based on the same IEEE 802.16 standard, 802.16e WiMAX will not be backwards-compatible with 802.16-2004 WiMAX, as was initially expected, each best suited to different market segments and with distinct performance profiles.

Fixed WiMAX, based on the IEEE 802.16-2004 Air Interface Standard, have proven to be a cost-effective fixed wireless alternative to cable and digital subscriber line (DSL) services. Fixed WiMAX supports non-line-of-sight (NLOS) mode in radio bands between 2-11 GHz. The bandwidth of fixed WiMAX is from 1.25 MHz to 20 MHz. A 256-point fast Fourier transform (FTT) OFDM PHY mode and a 2048-point FTT orthogonal frequency-division multiple access (OFDMA) PHY mode are adopted. The PHY specification also support for multi-antenna operation including optional advanced antenna subsystem (AAS) mode, open-loop space time coding (STC) mode (supporting two-four transmit antennas), closed-loop multiple-input multiple-output (MIMO) modes, uplink coordinated space-division multiple access (SDMA), and variable frame sizes (e.g. 2 ms, 2.5 ms, 5 ms).

The first release of mobile WiMAX profiles [12] will cover 5, 7, 8.75, and 10 MHz channel bandwidths for licensed worldwide spectrum allocations in the 2.3 GHz, 2.5 GHz, 3.3 GHz and 3.5 GHz frequency bands. Mobile WiMAX introduces 128, 256, 512 and 1024 in addition to the original length 2048 FFT sizes into OFDMA PHY. This permits so-called scalable deployment, wherein the OFDM symbol duration and inter-subcarrier separation is constant regardless of carrier bandwidth. Besides scalable OFDMA (SOFDMA), mobile WiMAX also supports various frequency permutation schemes and multiple antenna technologies. By taking advantage of flexible resource allocation on frequencies utilizing OFDMA, mobile WiMAX supports full frequency reuse, partial frequency reuse, and even a mixture of full and partial frequency reuse with one TDD frame. In addition, mobile WiMAX supports various multiple antenna system (MAS) technologies. The MAS technologies can generally be classified into two categories: open loop MIMO and closed loop MIMO. The inclusion of MIMO antenna techniques along with flexible sub-channelization schemes, advanced coding and modulation all enable the mobile WiMAX technology to support peak downlink data rates up to 63 Mbps per sector and peak UL data rates up to 28 Mbps per sector in a 10 MHz channel.

According to the system requirement document (SRD) of 802.16m [13], several new techniques are under discussion for 802.16m [11], including: improved and backward compatible frame structure and system protocol; smaller frame/sub-frame size to reduce latency; new multi-antenna technologies; improved interference coordination and management schemes for both downlink and uplink; new control channel design with better system coverage and

reduced overhead; persistent scheduling for VoIP and real-time video services; optimized handover, more efficient paging and random access, and so on. These new techniques cover almost all areas of cellular system design, and will significantly improve the performance of the current WiMAX system while maintaining backward compatibility.

With the rapid development of WiMAX techniques, it is urgent to analysis the feasibility of using WiMAX signal as one of the illuminations of opportunity for passive radar application. The broadband and large coverage area properties make WiMAX signal feasible and attractive for passive radar application.

III. WIMAX-BASED PASSIVE RADAR DESIGN

WiMAX-based passive radar consists of two parallel receiver channels from NI PXI5610 MIMO-OFDM Testbed for Mobile WiMAX. One channel is used to receive the direct path signal while the other is used to receive the target echo signal. The function block diagram of the experimental WiMAX-based passive radar system is shown in Fig. 1.

Fig. 1. WiMAX based passive radar hardware architecture.

In practical applications, both the direct path and target echo channels will contain undesired multipath interferences that distort the direct path and target echo signals significantly. Therefore, signal correction is necessary for both channels before the cross-ambiguity coherent processing. The overall signal processing associated with the WiMAX-based passive radar is illustrated in Fig. 2.

Fig. 2. WiMAX-based passive radar signal processing schemes.

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IV. EXPERIMENTAL RESULTS

Benefitting from the wide bandwidth, the adopted WiMAX waveform has a good range resolution of 25 m. Thus, all moving targets are tracked in two dimensions, both in time delay, i.e. range cell shift, and Doppler frequency shift. The adopted coherent integration time (CIT) is 0.5 s.

A. Measured WiMAX Signal

The spectrum of the measured WiMAX transmitter signal is shown in Fig. 3. For there are multiple basestations and each basestation is configured as a MIMO system, a lot of illuminations can be seen in the spectrum. The adopted signal for this bistatic passive radar application is with 2.118 GHz center frequency and 4 MHz bandwidth, however, the adopted sampling frequency is 6 MHz in this experiment.

Fig. 3. Measured WiMAX spectrum.

B. Geometrical Configuration

In order to evaluate the performance and capability of the WiMAX-based passive radar hardware system and associated signal processing scheme, field experiments were conducted using a selected operational WiMAX basestation in Fuji tower, about 2 km from field trial site. The field trial geometrical configuration is shown in Fig. 4. One antenna points to the WiMAX basestation and the other points to the moving target.

C. Moving Target Detection Results

The ground truth for bus coming tracking is a bus coming from approximately 0.45 km. The experimental range Doppler map tracking results for bus coming is shown in Fig. 5. It should be noted that the CIT time is 0.5 s, i.e., 20 range cells correspond to 10 s. From the first range cell and the 20th range cell comparison, the bus passed about 0.15 km from the beginning. So the speed of the bus coming is about 15 m/s.

Fig. 4. Geometrical configuration for moving target detection.

(a) The 1st range cell

(b) The 21st range cell.

Fig. 5. Moving target range-Doppler map tracking results.

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The detected Doppler frequency for bus coming is about 160 Hz, and the corresponding moving speed can be calculated by

2 2d d

c

f f cvf

λ⋅ ⋅= − = − (1)

where c is the speed of light, fd is the Doppler frequency, is the WiMAX signal wavelength and fc is the WiMAX signal carrier frequency.

Thus, 160 Hz Doppler frequency corresponds to a velocity of about 11.3 m/s, i.e., 41 km/h, which accurately correspond to the actual speed of the bus.

V. CONCLUSION

This paper describes the WiMAX standards evolution, WiMAX-based passive radar system design and primary field experiments of moving target detection. According to the processing results, WiMAX-based passive radar has the capability of detection and tracking the range-Doppler map of the moving target. However, in the WiMAX communication system, there are multiple base stations and each transmitter can be configured as a MIMO system. This feature of such an abundance of illuminators of opportunity enables the feasibility for design and development of a multistatic passive radar network. Undoubtedly, the reliability and performance of target detection and tracking will benefit greatly from the multistatic radar network using multiple transmitters / receivers and the MIMO configuration.

ACKNOWLEDGEMENT

The authors would like to thank Chinese Scholarship Council, China, for the financial support and Nanyang Technological University, Singapore, for host the exchange program. The authors acknowledge Dr. D. W. Liu and Mr. L. Liu from Positioning and Wireless Technology Centre (PWTC), and Dr. H. B. Sun, Dr. H. C. Feng, Dr. W. X. Liu, Mr. W. Li from Communication Lab 1, Nanyang Technological University for their help in using of the MIMO Testbed and the field trial.

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[13] IEEE 802.16 Broadband Wireless Access WG, “IEEE 802.16m System Requirements,” October 2007.

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