Millimeter wave radio channel characterization for 5G vehicle-to-vehicle communications
Introduction
Future vehicles could run safer due to their ability to communicate with their surroundings. They will cooperate by exchanging information through vehicle-to-vehicle (V2V) and vehicle-to-infrastructure communications to increase situation awareness, to avoid hazardous situations and to mitigate traffic accidents [1].
Safety will also be increased with fully-autonomous cars further reducing the potential for human error [2]. Providing data with a command-response time close to zero would be crucial for their safe operation. In addition a fully ‘driverless’ car would need to be driverless in all geographies, and hence would require full road network coverage with 100% reliability to be a viable proposition [2].
Mainly two wireless technologies are considered for these cooperative systems [1]: short range communications under IEEE 802.11p [3] and cellular networks. Most efforts have focused on IEEE 802.11p communications at 5.9 GHz because cellular networks, while providing wider coverage areas, are not adequate to meet the low latency requirements. Continental, Deutsche Telecom AG, Fraunhofer ES, and Nokia Networks have demonstrated real-time communication between vehicles using LTE [4]. However, LTE stations had to be upgraded with plug-in modules so the end to end latency time could be cut to 20 ms and the system could comply with the fast data transmission required by these road safety applications.
This situation will change with upcoming 5G mobile networks. Autonomous driving, together with augmented reality, virtual reality and tactile internet, have been identified as one of the use cases whose requirements cannot be achieved by current mobile networks but will be met by future 5G networks [2]. However 5G communications are not standardized yet, as their performance requirements are currently being defined. Industry has identified a set of eight requirements [2], [5]:
- (a)
1–10 Gbps connections to end points in the field
- (b)
1 ms end-to-end round trip delay (latency)
- (c)
1000× bandwidth per unit area
- (d)
10–100× number of connected devices
- (e)
(Perception of) 99.999% availability
- (f)
(Perception of) 100% coverage
- (g)
90% reduction in network energy usage
- (h)
Up to ten year battery life for low power, machine-type devices
These requirements come from different applications and services that would not need all these capabilities simultaneously. For example, sensor networks may require low power consumption but may not need high data rate connections nor low latency times. Car safety applications will take advantage of the 1 ms latency time and 100% coverage, but may not require high transmission bandwidths.
Several frequency bands are being considered for 5G communications. Due to the spectrum congestion below 6 GHz, frequencies in the microwave and millimeter wave frequency bands are being considered [6]. This is possible because of the development of semiconductor technology for millimeter bands [7]. Among these candidate bands the ones where a larger bandwidth is available are at 38 GHz and 60 GHz. These are the two frequency bands considered for our study.
The use of millimeter wave frequencies for V2V narrowband systems was proposed several years ago [8], [9], [10] and several experiments were conducted to determine the attenuation and fading, narrowband characteristics of the radio channel. The high free space propagation losses at these frequencies, together with the high noise levels because of the larger bandwidths, would yield to low signal to noise ratios, unless directional antennas are used.
For the same reasons the use of directional steerable antennas at both channel ends has been proposed [11], [12], [13], [14], [15] for 5G systems at millimeter wave frequencies. This represents an important change on mobile communications where traditionally omnidirectional antennas have been used, at least at the mobile terminal. As the radio channel includes the propagation channel plus the antenna effect [16], important radio channel characteristics as delay and Doppler spread will be affected by the use of non-omnidirectional antennas [17].
In order to achieve the required wideband radio channel characterization at millimeter wave frequencies using directional antennas recent propagation studies have been conducted in microcell and macro cell environments [18], [19], [20], [21], [22], [23], [24], but not for V2V communications. V2V radio communication scenario has its own distinctive properties: there is neither an access point nor a base station, and both the transmitter and receiver may move at high speeds [25]. Consequently it requires a specific characterization using several narrowband and wideband functions and parameters that try to model path losses, signal fading and delay and Doppler spreads. These functions should be empirically determined using an adequate wideband sounder.
Wideband V2V channel characterization results in the literature focus on frequencies below 6 GHz [25], [26], [27], [28], [29], [30] and consider the use of omnidirectional antennas. The use of higher frequencies and directional antennas is expected to reduce multipath and consequently it will affect signal fading, and delay and Doppler spreads. In this paper we present the results of a V2V millimeter wave propagation experiment conducted to determine the characteristics of this channel.
Section snippets
Experimental setup
We used two wideband radio channel sounders based on the sweep time delay cross-correlation (STDCC) technique, one for the 38 GHz and another for the 60 GHz measurements. Both sounders are similar, just the RF frontends are different. The sounders have a dynamic range of 39 dB, a delay resolution of 2 ns, and a maximum measurable delay of 16.4 μs. In Fig. 1 we show a block diagram of the transmitter end of the 60 GHz sounder. A 213-1 bit long pseudorandom binary sequence (PRBS) was generated at a bit
Time varying impulse responses
The IF signals resulting from measurements were off line processed using a computer to extract the time-varying impulse responses. Processing included demodulation to baseband, sliding correlation with a replica of the transmitted PRBS and noise suppression. As a result we obtained the complex (randomly) time-varying impulse responses of the radio channel.
In Fig. 7 we show the processing results of one of the measurements that where taken with cars driven at 70 km/h and separated 65 m. The 6450
A simple channel model
We have empirically characterized the 5G V2V radio channel by calculating several channel functions and parameters at two frequency bands. Measurements were taken at both frequency bands along the campus bypass road. Results of all the measurements are summarized in Table 1.
Measurements at both frequency bands where not taken at exactly the same road points or under the same driving conditions (such as vehicle speeds or distances). This may give rise to some of the differences found. The number
Conclusions
The radio channel for 5G V2V communications has been empirical characterized at two millimeter wave frequency bands. Two wideband radio channel sounders have been designed and used for the experimental campaign. These sounders have 1 GHz RF bandwidth and 2 ns delay resolution. Directional antennas have been used at the sounder transmit and receive ends and fixed at a low elevation position on the car bumpers.
The channel impulse response, scattering functions, Doppler spectrum as well as the delay
Acknowledgements
This work was supported in part by Spanish Government, Ministerio de Economía y Competitividad, Secretaría de Estado de Investigación, Desarrollo e Innovación, under project TEC2014-55735-C3-3-R, Xunta de Galicia under project GRC2015/019, AtlantTIC under project TACTICA and the European Regional Development Fund (ERDF).
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