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The Physical Layer of VANETs

  • Chapter
Vehicular ad hoc Networks

Abstract

This chapter is about the physical layer (PHY) of VANETs and will present all its main features with a bottom-up approach, starting from the relevant physical propagation and reception phenomena; afterwards, the Orthogonal Frequency Division Multiplexing (OFDM)-Wi-Fi standard, as adapted to the vehicular environment, will be introduced; then, the architecture of transceivers will be presented, so to match theoretical solutions with practical implementations and a hands-on perspective; eventually, some open research areas will be introduced, highlighting how much the physical layer could be improved in a standard-compliant way. More innovative solutions will be presented in the following chapters.

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Notes

  1. 1.

    The Fraunhofer distance \(d = D^{2}/\lambda\) is usually adopted as cut-off between the near- and far-field; in far-field, you can also suppose that wave-fronts are spherical and that the field is TEM (transverse electromagnetic (TEM) modes: neither electric nor magnetic field in the direction of propagation).

  2. 2.

    The antenna aperture is measured as a surface (square meters) but is not directly related to the physical size of the antenna.

  3. 3.

    The aperture of the receiving antenna is \(A_{\alpha,R} = G_{R} {\ast}\lambda ^{2}/(4\pi )\), as demonstrated in the antenna theory, for instance in [6]; here, we suppose \(G_{R}(\theta _{T},\phi _{T}) = 1\).

  4. 4.

    The attenuation with power 4 seems to violate the principle of energy conservation but, actually, it does not: due to reflections, the power is only differently distributed; if one considers the power received at a height h T growing with distance, the power would still decay with power 2.

  5. 5.

    In fact \(\alpha ^{2}/2T\int _{-T}^{+T}\cos (t)\cos (t+\tau )dt =\alpha ^{2}/4T\int _{-T}^{+T}\cos (2t+\tau )dt +\alpha ^{2}/4T\cos (\tau )\int _{-T}^{+T}dt\); when \(t\longrightarrow \infty \) the first terms decreases to 0 (due to the divisor 4T) and the second keeps constant.

  6. 6.

    Rayleigh fading defines a PDF which is in the form \(\textit{PDF}(\rho ) =\rho /\sigma ^{2}e^{-\rho /2\sigma ^{2} }\), leveraging the central limit theorem (all the components are independent)—supposing that the two independent components are both gaussian.

  7. 7.

    The Wiener–Kinchin theorem states that the Fourier transform of the autocorrelation function of a wide-sense-stationary random process, represents its power spectrum.

  8. 8.

    The uncertainty principle applied to the Fourier’s transforms states that, if h(t) is a normalized function and H(f) its Fourier transform, then \(\sigma _{h}^{2} \cdot \sigma _{H}^{2} \geq 1/4\pi\). The equality holds only if h(t) is a gaussian function.

  9. 9.

    The Titchmarsh convolution theorem states that if \(\varphi\) and ψ are two functions in the domain \(\mathbb{R}\) whose supports are compact (are, respectively, non-null in that compact sets \(\mathop{\mathrm{supp}}\,\varphi\) and \(\mathop{\mathrm{supp}}\,\varphi\)), then \(\mathrm{supp}\,\phi {\ast}\psi \subset \mathop{\mathrm{supp}}\,\phi +\mathop{ \mathrm{supp}}\,\phi\).

  10. 10.

    In VANETs’ OFDM (Sect. 3.2.3) the spacing between pilots is about 3.2 MHz which may be more than the coherence bandwidth met outside: this may be counteracted by the initial denser pilots held-on over a time shorter than the coherence time, but, indeed, it represents a challenge (see Chap.15).

  11. 11.

    According to the rules of the IEEE Standards Association, there is only one current standard which, for Wi-Fi, is denoted by IEEE 802.11 followed by the date of its publication. At the time of our writing, IEEE 802.11-2012 is the only version in publication and has integrated the previous IEEE 802.11p amendment; next version is expected to be the IEEE 802.11-2015. The standards are updated by means of amendments, which are created by task groups (TG). Both the task group and their finished document are denoted by 802.11 followed by a letter (or a couple of letters), such as IEEE 802.11a and IEEE 802.11ac. For the creation of a new stable version, task group m (TGm) combines the previous version of the standard and all the published amendments not subsumed yet. New versions of the IEEE 802.11 were published in 1999, 2007, and 2012.

  12. 12.

    The EIRP corresponds to the power that an isotropic antenna should radiate in order to produce the peak power density observed in the direction of maximum gain for the antenna.

  13. 13.

    The orthogonality exploits the well-known properties of \(\mathrm{sinc}(f) =\sin (f)/f\), the Fourier transform of the rectangular function in the time domain (rect(t)). The same properties hold also when you take a segment of the sin() function (it can be meant as the product sin(f 0 t) ⋅ rect(t)); in this case, the transform would be sinc(f − f 0).

  14. 14.

    NS-2, the network simulator (Open Source) available at http://www.isi.edu/nsnam/ns/.

  15. 15.

    NS-2, the network simulator (Open Source) available at http://www.nsnam.org/.

  16. 16.

    Omnet\(++\), available at http://www.omnetpp.org/ or (commercial edit.) at http://www.omnest.com/.

  17. 17.

    Qualnet, available at http://web.scalable-networks.com/content/qualnet.

  18. 18.

    The main purpose of network simulators is the testing of protocol and, consequently, they do not focus on physical layer modeling. As a result they typically model physical layer through statistical model, not considering the physical phenomena which may lead to events such as wrong equalization or OFDM misalignment.

  19. 19.

    As shown in Fig. 3.12, during the first \(16\,\upmu \mathrm{s}\) there is a short training sequence where only 12 sub-carriers are used (for pilots) in BPSK; then, after the GI, the long training sequence uses all the 52 sub-carriers as pilots (+DCC); eventually, data follow. The short sequence is used for signal detection and coarse tuning; the long sequence for fine-tuning.

    Fig. 3.12
    figure 12

    Pilots and preamble in the IEEE 802.11p: short training sequence for coarse tuning by 12 sub-carriers; long training sequence (after a double guard interval—2xGI) for fine-tuning, involving all the sub-carriers (including the Direct Current Carrier—DCC components); data symbols follow one another, symbol by symbol, each preceded by a GI, as explained in Chap. 4

    Full size image
  20. 20.

    For instance, the performance of the antenna on the rear-view mirror strongly depended on the position of the transmitter (left or right); the windscreen and the front-fin antennas achieved similar performance under LOS; the rear-fin antenna performed the best for negative coordinates.

  21. 21.

    This applies if the mobile terminals are assumed to be located inside the vehicle and coupled to the vehicle’s roof-top antenna in order to avoid wireless signal degradation due to the penetration loss of the vehicle body.

  22. 22.

    MIMO means multiple-input and multiple-output and refers to the use of multiple antennas to improve communication performance: they may be used just for a diversity gain (counteracting fading) or even to increase the channel capacity by spatial multiplexing (using the different antennas as parallel channels). MIMO techniques have been used by Wi-Fi (802.11n and 802.11ac), LTE and Wimax, for instance.

  23. 23.

    By considering the mean excess time delay (τ MED) which a node can estimate over the multipath channels and thanks to the algorithm UMP (Unitary Matrix Pencil) [20], they can distinguish on the fly between UDP and DDP [23].

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Acknowledgements

The FP7 project GLOVE (joint GaliLeo Optimization and VANET Enhancement Grant Agreement 287175) has partially supported this work.

The authors thank also Giorgio Giordanengo (ISMB-LACE) for the antenna diagrams.

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Authors and Affiliations

  1. Istituto Superiore Mario Boella, Via P.C. Boggio 61, Torino, Italy

    Riccardo M. Scopigno & Alessia Autolitano

  2. University of Michigan-Dearborn, Dearborn, MI, USA

    Weidong Xiang

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  1. Riccardo M. Scopigno
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Correspondence to Riccardo M. Scopigno .

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  1. University Mediterranea of Reggio Calabria, Reggio Calabria, Italy

    Claudia Campolo

  2. University Mediterranea of Reggio Calabria, Reggio Calabria, Italy

    Antonella Molinaro

  3. Istituto Superiore Mario Boella (ISMB), Torino, Italy

    Riccardo Scopigno

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Scopigno, R.M., Autolitano, A., Xiang, W. (2015). The Physical Layer of VANETs. In: Campolo, C., Molinaro, A., Scopigno, R. (eds) Vehicular ad hoc Networks. Springer, Cham. https://doi.org/10.1007/978-3-319-15497-8_3

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