The rapid development taking place in the field of transport and communications in the last decade has led to the emergence of smarter cars which reduce the probability of accident, these cars are able to explore the possibility of danger and obstacles surrounding them by the ability of cars to communication with surrounding such as vehicles and roadside. The researchers and automotive industry are working to develop the dedicated short range communication (DSRC) technology, for use in vehicle to infrastructure and vehicle to vehicle communication. DSRC is described in [DSRC in U.S.]. The physical layer of DSRC is IEEE 802.11p standard which is define in [ieee802.11p] [27], it is divided into two sublayers, the physical layer convergence procedure (PLCP) sublayer and the physical medium dependent (PMD) sublayer. The PLCP defines the mapping between the MAC frame and the basic PHY layer data unit. The PMD interfaces directly with the wireless medium. It utilizes the familiar orthogonal frequency division multiplexing (OFDM) technique.
The basic idea of OFDM is to divide a single wide bandwidth into several narrow bands (subbands) each of subbands undergoes frequency flat fading. The main reasons to merged OFDM in IEEE 802.11 is to increase the robustness against frequency selective fading.
OFDM-IM History
in 2009 Rami Abu-Alhiga and Harald Haas proposed (SIM) in [SIM], SIM is a new transmission approach referred to subcarrier-index modulation (SIM).SIM is proposed to be integrated with the orthogonal frequency division multiplexing (OFDM) systems. The proposed SIM transmission technique employs the subcarrier-index to convey information in an on-off keying (OOK) fashion. SIM OFDM aims at providing either BER performance enhancement or power-efficiency improvement over conventional OFDM by incorporating different power allocation policies. The problem of original SIM-OFDM scheme is a potential bit error propagation which could lead to significant burst errors.
In 2011 Harald Haas proposed a modified technique to SIM-OFDM called Enhanced Subcarrier Index Modulation (ESIM-OFDM) which avoids bit error propagation, A slight modification in the way BOOK is encoded can limit the bit error propagation [ESIM]. Each bit from BOOK can be encoded in the states of two consecutive carriers (pair of carriers) Whenever a “1” is encountered in BOOK, the first carrier of the pair is set as active and the second one as passive. Whenever a “0” is encountered in BOOK, the first carrier of the pair is set as passive and the second one as active. This modification limited the bit error propagation in pair of subcarriers.
finally, In 2013 Ertuğrul Başar proposed a new transmission scheme called OFDM with Index Modulation (OFDM-IM) for frequency selective fading channel [IM]. The error performance of the OFDM-IM scheme is significantly better than that of classical OFDM due to the bits transmitted in the spatial domain. The number of active subcarriers in OFDM-IM is variable unlike ESIM-OFDM (fixed =N/2), therefor the OFDM-IM scheme provides an interesting trade-off between complexity, spectral efficiency and performance by the change of the number of active subcarriers.
In 2014 Xiang Cheng,[x 2014] and others proposed Index modulation OFDM with interleaved grouping for V2X communications, for improving spectral efficiency, high reliability communication, by using interleaved subcarrier grouping.
In 2014 Rui Fan,[gen 2014] and others proposed OFDM with generalized index modulation to improve the spectral efficiency of OFDM-IM. In this scheme, the number of active subcarriers in an OFDM subblock is no longer fixed. Dependent on the input binary string, different number of active subcarriers are assigned to carry constellation symbols. In this approach, a higher spectral efficiency than that of OFDM-IM may be achieved.
One issue of OFDM-IM is not using all subcarrier activation patterns (SAPs) this will produce unused SAPs at the receiver.in 2016 Abdulrahman Siddiq proposed a new scheme to use all SAPs in OFDM-IM to avoid the case of unused SAPs at the receiver [all SAPs].
In this paper proposed the improving of vehicle to infrastructure (V2I) communication by using OFDM-IM with enhancement instead of pure OFDM technique in the physical layer of vehicular communication system. The enhancement is the use of all SAPs with OFDM-IM and including interleaved at the level of subcarriers to improve the performance of error rate and power efficiency. The rest of paper is organized as follows. In section II, describe V2I communication system model (DSRC). In section III, review of proposed system. The implementation of proposed system and scenarios is given in section IV. Section V represent the simulation and results. Finally, section VI concludes the paper.
II. Communication system model of V2I
Dedicated Short Range Communications (DSRC) are one-way or two-way short-range wireless communication channels specifically designed for automotive use, US Federal Communications Commission (FCC) has allocated 75MHz of licensed spectrum in the 5.9GHz band for short-range communications to support wireless communication in vehicular environment including vehicle to vehicle (V2V) and vehicle to infrastructure(V2I). In our paper we focus on physical layer of DSRC model, DSRC employs IEEE 802.11p standard for physical and MAC layers, figure() represent the Layered architecture of DSRC communication in the US. IEEE 802.11p has a 10 MHz frequency bandwidth is used, uses 64 subcarriers OFDM that includes 48 data subcarriers and 4 pilot subcarriers. The 4 pilot signals are used for tracing the frequency offset and phase noise.
III. Review of OFDM-IM and the proposed system
The block diagram of original OFDM-IM is presented in figure (). In general OFDM-IM divide a block of OFDM information into groups of bits, each group is mapped to subblock by split each group of bits into two part, one part for selected the activation of subcarriers and the other part of information is modulated and convey on active subcarriers. Later combine subblocks into one block and apply invers fast fourier transform (IFFT).
A block of information of m bits enter the OFDM-IM transmitter. These m bits are split into g groups of p bits, where p= p1+p2. The number of bits in p1 is
p1= ⌊log2(C(n,k))⌋ which is define the subcarrier activation pattern (SAP) and convey on the indices of subcarriers. SAPs define the k active subcarriers in each subblock that convey the modulated data of p2, where p2=klog2(M). the size of subblock is n, where n is the number of subcarriers in subblock, k is the active subcarriers in subblock, C is number of combinations of total SAP, and M is the modulation index of QAM. in other word, each group of p-bits is mapped to an OFDM subblock of length n subcarriers, For each subblock B, only k out of n subcarriers are employed for carry QAM symbol and they are determined by a selection procedure from a predefined set of active indices, based on the first p1 bits of the incoming p-bits sequence. Later combine g subblocks into one block and applying IFFT with N points (where N is the total number of subcarriers), the result of IFFT is OFDM-IM block and it is ready for transmission.
Note that in OFDM-IM used 2^p1 SAPs out of C(n,k) available SAPs,
where C(n,k)= n!⁄(n-k)!k!
There is ( 〖C(n,k)-2〗^p1 ) wasted SAPs, this will produce unused SAPs at the receiver and led to incorrect mapping of subblock.
The proposed system is to replace the modulation scheme of the vehicular communication system (OFDM) with OFDM-IM and including the modification of used all SAPs to avoid unused SAPs at received [All SAPs] in addition to use interleaved at the level of subcarriers. The result is produce great enhancement in power consuming in addition to improving the bit error rate (BER) of V2I communication system.
IV. Implementation and Scenarios
The proposed system is implemented and simulated in matlab program under different scenarios to evaluate the performance of BER and evaluation of power efficiency by applying two power policies including power saving policy that save the power of inactive subcarriers and power reallocation policy that redistributing the power of inactive subcarriers.
Three scenarios are applied to evaluate the performance of proposed system. Each have different parameters including Delay spread, Doppler shift and Path gain based on measurement campaign was carried out in the metropolitan Atlanta, Georgia area including three scenarios expressway, Urban canyon oncoming and suburban street [17a b] related to V2I. These scenarios are implemented by matlab Rayleigh channel.
V. Simulation and Results
First we simulate Urban scenario, in this scenario the vehicle 100 m away from the infrastructure (roadside) and minimum vehicle speed is 20 km/h and maximum speed is 50 km/h. The parameters of Urban Canyon Scenario are 5-paths with jakes Doppler spectrum, maximum delay spread ( 500 nsec) and average angle of arrival equal to 75 degree and tap power is [0 -11.5 -19.0 -25.6 -28.1] dB.
In Suburban scenario the distance between vehicle and infrastructure (road side) is about 100 m and speed of vehicle reach 80 km/h. The parameters of Suburban Scenario are 5-paths with jakes Doppler spectrum, maximum delay spread ( 400 nsec), average angle of arrival equal to 75 degree and tap power vector is [0 -9.3 -14.0 -18.0 -19.4] dB.
The last simulated scenario is Expressway. It refer to low building density and high speed. In this scenario the vehicle 300 m away from the infrastructure (roadside), the velocity of vehicle is from 120 km/h to 140km/h, high speed producing high Doppler shift. the Scenario parameters are 5-paths with jakes Doppler spectrum, maximum delay spread ( 400 nsec), average angle of arrival equal to 75 degree and tap power vector is [0 -9.3 -20.3 -21.3 -28.8] dB.