Time-Modulated OFDM Directional Modulation Transmitters

Abstract

In this paper directional modulation (DM) transmitters, designed for orthogonal frequency-division multiplexing (OFDM) wireless data transfer are proposed using time-modulated arrays (TMAs). It is shown that by properly designing the switch controlling time sequences the proposed time-modulated OFDM DM systems exhibit promising features, such as i) requires only one radio frequency (RF) chain, ii) the conventional efficient OFDM signal construction approach, i.e., by using inverse fast Fourier transform (IFFT) modules, can be adopted, iii) it is 'DM synthesisfree', meaning that there is no need to re-synthesize the array excitation vectors for different secure communication directions, different modulation schemes, and different DM power efficiencies; and iv) the trade-off between secrecy performance and power efficiency can be flexibly adjusted. Index Terms-Directional Modulation (DM), orthogonal frequencydivision multiplexing (OFDM), physical-layer wireless security, timemodulated array (TMA) Readers who are following DM developments may be aware of a number of recent DM works, which claim that the FDA concept (and its variants such as non-linear frequency increments and random subcarrier selections) can be incorporated into DM transmitters, achieving free-space wireless security in range-domain, e.g. the random-subcarrier-selection-based OFDM DM transmitters in [19] and some of the references wherein. However, in those works one important factor was overlooked, which is that 'FDA range-angle dependent beamforming patterns are also functions of time'. This

Three‐state time‐modulated array‐enabled directional modulation for secure orthogonal frequency‐division multiplexing wireless transmission

“…In order to show the effect that the generated orthogonal (in the spatial domain) artificial noise has on the proposed three‐state time‐modulated OFDM DM transmitters, comparison examples depicting 1/$$\sqrt N $$ ∙| V ( m , N , $$\tau _n^{{s_1}}$$, ∆ τ (1) , ∆ τ (2) , ρ , t , θ )| in dB, denoted as Γ m , are given in Figure 3 for various parameters that satisfy (). In the examples in Figure 3, it is assumed that the linear transmit array has N = 7 equally spaced ( λ 0 /2) elements and the desired communication direction θ 0 equals 60° in both the proposed three‐state DM systems and the On–Off time‐modulated OFDM DM systems [17]. It is noted that θ 0 is defined as the desired secure transmission direction, which can be selected among all spatial directions from 0° to 180°.…”

“…This multi‐beam characteristic endows TMA transmitters with the ability to achieve space division multiple access [21]. Specifically, as we have mentioned, in [17], the TMA enables OFDM DM transmitter to achieve physical‐layer secure wireless communications. However, since there is little freedom to design the orthogonal artificial noise, it is not able to span the entire null‐space of the legitimate channel.…”

“…The resulting systems exhibit promising features which are summarised as below The positive properties of the previously reported On‐Off time‐modulated OFDM DM are well preserved in our scheme. In other words, the proposed OFDM DM systems here enjoy single RF‐chain, inverse fast Fourier transform (IFFT) compatibility and are DM synthesis‐free [17]; Unlike earlier schemes the distribution (in spatial domain or in null‐space of the legitimate channels) of the orthogonal artificial noise can be fully manipulated; Narrower bit error rate (BER) main beam and suppressed BER sidelobes can be achieved in the static OFDM DM system variant through injecting more interference power, which can be further enhanced in the dynamic counterpart. …”

Physical Limitation of Range-Domain Secrecy Using Frequency Diverse Arrays

“…Therefore, the authors in [26] made the first effort in combining FDA with DM so that an OFDM modulated DM system was constructed. This work was recently extended to the time modulated arrays in [27]. Here, the FDA transmits signal waveforms continuously in time, different to the pulsed signals for FDA radar applications.…”

“…Here, the FDA transmits signal waveforms continuously in time, different to the pulsed signals for FDA radar applications. No range-domain security was claimed in [26], [27].…”