A Sixty GHz Intra-car Multi-media Communications System

Hirokazu Sawada1, Tomohide Tomatsu1, Gen Ozaki1, Hiroyuki Nakase2, Shuzo Kato2,3,

Katsuyoshi Sato3, and Hiroshi Harada3 1Tohoku Institute of Technology, 35-1, Kasumi-cho, Yagiyama, Sendai, 982-8577, Japan

2Tohoku University, 2-1-1, Katahira, Sendai, 980-8577, Japan

3National Institute of Information and Communications Technology, 3-4, Hikarinooka, Yokosuka, 239-0847, Japan sawahiro@tohtech.ac. jp

Abstract— In order to realize intra-car multimedia communications systems composed of entertainment which requires several hundred Mbps to 1 Gbps capacity and monitoring signal transmission between many parts of the car which requires high reliability and low latency, this paper proposes a millimeter wave system which has high capacity and low latency transmission capability. To clarify the feasibility of millimeter wave systems applications to intra-car multimedia communications, propagation characteristics inside a car have been extensively measured and analyzed. The proposed system sets a car navigator as the center of Intra-car multimedia communications with an Omni antenna and receivers are assumed at front seats, back seats and the luggage space behind the back seats in a typical SUV (Sports Utility Vehicle). The measured results show i. Front seat communications are no worry with simple single carrier (SC) modems without any equalizer, ii. Back seats communications may need beam forming capability for better receive signal power (an Omni antenna system still works with a simple SC modem with a simple rake receiver but without any equalizer. iii. “Luggage space communications may require a directive / beam forming antenna for better receive signal power. The maximum delay spread observed is 6 to 7 ns and no equalizer necessity has been found for Omni antenna receivers inside a car. Further advancement to beam forming antenna will have only 1 - 2 ns maximum delay spread inside a car. Thus, this paper has proved the feasibility of millimeter wave intra-car multimedia communications system by employing a simple single carrier modem without any equalizers but with simple rake receivers.

I. INTRODUCTION The wireless applications of the intra-car multimedia

communications include video transmission, hands free phones, wireless harnesses, sensor monitoring, a portable navigation system and many multimedia entertainment equipment inside a car and a car navigator will be the center of this type of applications. As the candidates for these applications, Bluetooth (IEEE802.15.1), ZigBee (IEEE802.15.4), and UWB (IEEE802.15.4a) have been studied [1-5]. However Bluetooth and ZigBee have latency and reliability issues for the wireless harnesses application and UWB is not high enough bit-rate for intra-car multimedia applications composed of multiple video channels. To solve these problems, this paper proposes to use 60GHz millimeter-wave for the intra-car applications, and a propagation measurement campaign has been done.

The wide bandwidth of several hundred Mbps to 1 Gbps capacity will be required for the multimedia applications like multiple video and music streaming. The high capacity/bit rate of millimeter wave systems will provide low latency transmission which is important for wireless harness applications. In addition, the beauty of 60GHz band is that the whole bandwidth (57 to 64 GHz or 59 to 66 GHz depending on the country) is unlicensed and uncontaminated band different from UWB in many countries in the world. The potential problem of millimeter-wave systems is reliability & transmission range which heavily depend on propagation characteristics inside cars.

WPAN (Wireless personal area network) at 60 GHz has been in process of standardization at IEEE (IEEE802.15.3c Task Group [6]) and the standardization process is close to the completion. This WPAN targets indoor communications up to 10 m transmission range and extensive propagation measurements were done in various environments. The measurement environments depend on the usage models such as KIOSK (sync and go) usage model [7], uncompressed video streaming. The TSV propagation model [8-16] was proposed by authors to include a direct path (LOS: Line of sight) and angle of arrival on top of SV model which has been used for Wireless LAN (no antenna directivity). The TSV model was approved as TG3c’s channel model and Matlab codes for all channel models under consideration were developed and uploaded to the IEEE server for public use by the authors. The propagation measurement work presented in this paper is the extension of these extensive propagation measurements and channel model creation done at TG3c for a new and very interesting environment, inside cars. The high level goal of the propagation measurements is clarify whether it is required to use complicated equalizers / OFDM systems for “Intra-car multimedia communications”

II. PROPAGATION MEASUREMENT IN CAR

A. Measurement Set Up The propagation measurement inside a car was performed

in order to investigate receive power and delay spread at the most likely transmission and reception points in a car. The configuration of the measurement system is shown in Fig.1. The transmitter antenna with 0dBi antenna gain (Omni antenna) was set at the position of the car navigator as the

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center of car entertainment and wireless harness, transmitting multiple video channels and collecting monitoring signals to display. The receiver antenna with antenna gain of 22 dBi (Half power bandwidth: 15 degree) was set at front, back seats as well as in the luggage space. The measurement frequency band was set to 61-64GHz and a test equipment (HP8510C) was used as SG and signal analyzer. The measurements were done by rotating the receive antenna 360 degrees (5 degree step), and signals were swept over 3 GHz band. The measured frequency response for the receive signal amplitude was further Fourier transformed for delay spread calculation As for the polarization, vertical polarization was selected to lower the reflected wave power as usual. The calibration was carried out by direct connection of the input and output ports of the network analyzer without antennas. The major measurement parameters are given in Table 1.

B. Antenna Set Up in Car A sport-utility vehicle (SUV) type car was used for the

measurement as shown in Fig.2. The car size is 4455mm (length) x 1765mm(width) x 1875mm(height). The transmitting antenna was fixed at the position of the car navigator as mentioned before while the receiving antenna was set at three different locations and rotated by 5 degree step in a horizontal plane. The window of driver seat was kept open to pass the measuring cables. The whole measurement instruments including human being were located outside the car, and only the antennas, transmitter and receiver modules were put inside the car. The receiving antenna was located on the front seat (Rx1) and back seat (Rx2), and luggage space (Rx3) as shown in Fig.3. The heights of Rx antenna were set at 78cm, 80cm, and 84cm from the car floor respectively. The distances d between Tx and Rx1, Rx2, Rx3 were 55cm, 147cm, and 235cm respectively. This setting created a line of sight (LOS) environment for Rx1, and non-line of sight (NLOS) environments for Rx2 and Rx3.

Figure 1. Measurement set up

TABLE I. MAJOR MEASUREMENT PARAMETER

Instrument HP8510C VNA

Center frequency 62.5 GHz

Bandwidth 3 GHz

Number of frequency points 801

Frequency step 3.75 MHz

Averaging 32 times

Tx antenna Omni antenna (0dBi)

Rx antenna Conical horn antenna (22dBi)

(a)Side view

(b)Top view

Figure 2. Configuration of Tx and Rx antennas in car

(a) Tx: Omni directional antenna

(b) Rx: Directional conical horn antenna

Figure 3. Photograph of Tx and Rx antennas

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(a) Front seat

(b) Back seat

(c) Luggage space

Figure 4. Power delay profile map

C. Measurement Result Fig.4 shows the measured power delay profile at each Rx

antenna angle (15 degree step) with the direction of 0 degree set as shown in Fig.2. As expected, many reflected waves came from various directions. In the cases of front and back seats, one cluster of received waves was observed in each angle at the receiving point. On the other hand, the second cluster coming from rear of the car was observed. Thus, a multi-cluster model with decay factor [17] can be adopted as the channel model in the car. As for the general channel model for the intra-car communications, TSV-model [16] is more suitable since the strong LOS component was observed at the front seat.

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Figure 5. Measured average power at each direction

Fig. 5 shows the measured average receiving power at each direction in the polar coordination. The power is normalized by the maximum averaged power.

(a) At front seat (an LOS environment), the strong direct path component was observed at 315 degrees, and a strong reflected wave came from passenger's door at 60 degree.

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(b) At back seat (a non-line-of-sight environment), the larger components were reflected waves by doors of the car at 50 and 330 degree directions. Although it is NLOS environments, relatively strong wave from Tx antenna direction at 15 degree was received.

(c) At luggage space, large direct wave component was observed and relatively high reflection waves coming from the rear window of the car were confirmed.

As a result, the propagation characteristic was shifted from "Rayleigh fading" to "Rician fading" by moving Rx position from the front toward the backside of the car.

Fig.6 shows the cumulative distribution function of the averaged received power for each Rx position. In front seat case the difference of received power is about 30dB, since direct wave could be received. The variance of the received power decreases by increasing distance from Tx antenna. The other important parameter in propagation characteristics is delay spread, S. The value is defined as following [18]. Here TD is the averaged delay time.

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Fig.7 shows the delay spread performance of three Rx positions for each direction in the polar coordination. The amount of delay spread depends on the Rx antenna direction and the maximum delay spread is around 6-7ns over 360 degree Rx antenna position. However, the minimum delay spreads at the maximum receive power directions are less than 1ns. This indicates no necessity of complicated equalizers or OFDM to overcome fading if directive/beam forming antenna is employed.

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Figure 7. Delay spread in each direction

(a) At the front seat, the delay spread is less than 1ns at 315 and around 60 degrees since a direct or strong reflected wave were received. In other direction at the front seat, the delay spread is less than 4ns. Thus, front seat communications are no worry with simple single carrier (SC) modems without any equalizer.

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Figure 8. Cumulative distribution function of delay spread

(b) At back seat, the delay spread is small as less than 1ns at 50 and 330 degrees which are strong reflected waves while the delay spread at 15 degree (Tx antenna direction) has about 3 ns due to NLOS environment. An equalizer is unnecessary by turning the antenna in the direction of a strong reflected wave by using a directive / beam forming antenna. Thus, back seat communications may need beam forming [19] capability for better receive signal power (an Omni antenna system still works with a simple SC modem with a simple rake receiver but without any equalizer though).

(c) At luggage space, the delay spread is as small at 0 and 180 degrees as 2ns. Luggage space communications may require a directive / beam forming antenna for better receive signal power.

The cumulative distribution function of the delay spread is shown in Fig.8 as a reference for the case in which an Omni antenna is used for the receiver antenna. For all Rx positions the response was almost one cluster profiles as shown in Fig.4, then the energy of delay waves concentrated around the first response. As the results, the maximum delay spread was not large. The difference of three curves at each Rx position is small. Then the distribution model of the delay spread can be assumed as a uniform distribution.

III. CONCLUSION An intra-car multimedia communications system using

60GHz band has been proposed in order to achieve several hundred Mbps to 1 Gbps data rates. Propagation measurements including angel of arrival analysis were carried out to clarify propagation characteristics inside cars. An Omni directional antenna and 22dBi conical horn antenna were used for TX and RX antenna, respectively. Two or three propagation paths with low delay spread of 1 nsec were observed if the receiving antenna was set at the right direction. This suggests deployment of beam forming antenna to reduce delay spread. As a result, WPAN systems using single carrier modems without equalizer which is under standardization process at IEEE802.15.3c has a very high potential to be used for intra-car multimedia communications system s as well.

ACKNOWLEDGMENT This research is partially supported by the Ministry of

Education, Science, Sports and Culture, Grant-in-Aid for Scientific Research (C), 20560346, 2008.

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