K Giridhar

To ensure robust communication in doubly dispersive channels, Orthogonal Time Frequency and Space (OTFS) modulation has been proposed to exploit the channel in the Delay-Doppler domain. OTFS is expected to outperform conventional Orthogonal Frequency Division Multiplexing (OFDM) especially in high Doppler environments. In this work, we revisit Multi-Carrier Code Division Multiple Access (MC-CDMA) waveforms and compare them with OTFS in practical downlink Single Input Single Output (SISO) scenario with channel estimation and Peak to Average Power Ratio (PAPR) input back-off. Both the modulation schemes are compared for the same information rate, transmit power per symbol and block size MN, where M and N represent the number of Delay and Doppler bins in OTFS respectively. Simulation results highlight the fact that with channel estimation of OTFS, for constant M N with larger M and N, OTFS performs better than MC-CDMA in medium SNR links with less PAPR. In less noisy environments, both OTFS and MC-CDMA perform similarly, with OTFS having an upper hand at very high Dopplers. However, in low SNR links, MC-CDMA with grid boosting outperforms OTFS for any Doppler. Our work shows that both MC-CDMA and OTFS are good contenders for the present needs of high-speed wireless communication links, and the choice of waveform can be made based on the operating environment and system requirements.

In multicarrier systems, cyclic prefix (CP) is introduced between two symbols to mitigate inter block interference (IBI). The CP length should be equal or more than channel impulse response (CIR) to ensure IBI free system. However, in the interest of spectral efficiency, if the CP is chosen to be less than the (worst case) CIR length, then a time domain channel shortening prefilter (CSP) can be used to truncate or shape the CIR. In [1], we proposed a null-space exploiting CSP (NE-CSP) which is has a lower complexity when compared to the linear minimum mean square error (LMMSE) [2]. In this paper, we propose a symbol-by-symbol adaptive null-space exploiting CSP (SANE-CSP) scheme based on the NE-CSP structure, which is computationally inexpensive. The order of complexity of SANE-CSP is O(n c2), whereas the computational complexity order for NE-CSP [1] is O(n c3) and that of the decoding-delay optimised LMMSE [2] is close to for O(n c4). The SANE-CSP for n c = 100, has a block error rate performance 3 dB poorer than the decoding-delay optimised LMMSE, but with only 0.01% computational complexity. © 2011 National Institute of Inform.

Multiple receive antennas are known to provide receiver diversity but typically require considerable separation between them. We introduce the concept of space sampling at the receiver where antennas are placed relatively close to each other. Since the antennas are close, the samples are highly spatially correlated and does not help in performance improvement in a typical wireless system with flat fading. However, this concept is very useful particularly in an OFDM system with frequency selective fading which has an inherent multipath diversity. Under these circumstances, the required space diversity for performance improvement is obtained by the transformation of multipath diversity to useful space diversity in frequency domain inherently by an OFDM system (G. V. Rangaraj et al., June 2003). The minimum separation required between the antennas under such circumstances is derived analytically and we have show that even with a separation of only 0.44λ, the required spatial correlation in the frequency domain becomes sufficiently low.

Spatial correlation is introduced when antennas are not well separated, which typically leads to performance degradation in space diversity systems for flat fading wireless channels. However, in a frequency-selective environment with orthogonal frequency division multiplexing (OFDM), multipath diversity can help in overcoming this performance degradation. This is due to transformation of a highly spatially correlated channel impulse response to less spatially correlated channel frequency response inherently by ah OFDM system in the presence of multipath diversity [6]. In this letter, we numerically evaluate the minimum antenna separation required for a target spatial correlation in the frequency domain in situations with low multipath diversity. Performance results for two highly spatially correlated receive antennas with such spacing are provided for low multipath diversity channels and it is found to be within 1 dB (at BER = 10 -3) of spatially uncorrelated reception for QPSK modulation. © 2004 IEEE.

Frequency Hopping(FH) is an effective way to ensure resilience to jamming and eavesdropping. We consider an ultra-fast FH system where a contiguous set of subcarriers of an OFDM block which hold data are altered periodically based on a random code pattern. Since only a small set of subcarriers of the OFDM symbol are used in any one hop, receiver synchronization for timing and frequency is difficult since the time-domain auto-correlation properties degrade. In this paper, we compare existing preamble and processing methods using auto-correlation for synchronization. We then propose a new preamble with a two-step improved timing estimate. Simulation and results are presented which show the effectiveness of the proposed preamble and processing scheme, especially at low Signal-to-Noise Ratios as evident from the plots where a 3-dB improvement in the probability of detection at -6 dB SNR is seen.

We describe an algorithm for sparse channel estimation applicable to orthogonal frequency division multiplexing systems. The proposed algorithm uses a least squares (LS) technique for channel estimation and a generalized Akaike information criterion to estimate the channel length and tap positions. This effectively reduces the signal space of the LS estimator, and hence improves the estimation performance as demonstrated using computer simulations. For example, the proposed modified LS with sparse channel-estimation algorithm has a 5-dB lower mean square error in channel estimation when compared to the conventional approach [1], which translates to approximately 0.5 dB improvement in signal-to-noise ratio at the receiver. © 2005 IEEE.

High-Speed Vehicle-to-Everything (V2X) communications is an important aspect of 5G NR technology that requires Ultra-Reliable Low-Latency Communication (URLLC) under highly dispersive channel conditions. In this context, Orthogonal Time Frequency and Space (OTFS) modulation is known to achieve time and frequency diversity, especially in time-dispersive channels with large Doppler frequencies. Literature suggests that Multi-Carrier Code Division Multiple Access (MC-CDMA) can also achieve high diversity gain due to spreading in frequency and time. In this paper, we consider two MC-CDMA schemes, one in which the symbols are spread only along frequency (MC-CDMA-FS) and one where the spread sequence is mapped over both time and frequency (MC-CDMA- TFS). Also, two different precoders based on Walsh-Hadamard (WH) and Zadoff-Chu (ZC) sequences are considered for both of these schemes. We present a comparative performance study of OTFS with MC-CDMA-FS and MC-CDMA-TFS using both the precoders for a single user case, in doubly dispersive channels. Our simulation results show that MC-CDMA-TFS and MC-CDMA-FS schemes with ZC precoding resembles OTFS and even performs better in channels with larger delay and Doppler spreads in this framework.

Multiple antennas are useful in orthogonal frequency division multiplexing (OFDM) systems for providing transmit and receive diversity to overcome fading. Typically, these designs require considerable separation between the antennas. Spatial correlation is introduced when antennas are not well separated, and it often leads to performance degradation in a flat fading environment. However, in frequency selective fading channels with rich multipath diversity, OFDM receivers can overcome this performance degradation due to antenna correlation. This is due to transformation of a highly spatially correlated channel impulse response to a less spatially correlated channel frequency response inherently by an OFDM system in the presence of rich multipath diversity. We illustrate this for a simple receive diversity OFDM system and hence introduce the concept of space sampling at the receiver where antennas are placed relatively close to each other. The minimum separation required between the antennas under such circumstances is derived analytically, and it is shown that even with a separation of only 0.44λ, the required spatial correlation in the channel frequency response becomes sufficiently low. Simulated performance results with such spacing for various multiple antenna OFDM systems corroborate the analytical results. © 2005 IEEE.

Orthogonal frequency division multiplexing with index modulation (OFDM-IM) is a multi-carrier transmission scheme in which a set of sub-carriers is chosen and modulated according to the incoming bit-stream. It is already established that for the same average bits per channel use, OFDM-IM can provide a better error rate performance when compared to conventional OFDM. In this letter, we propose a novel method to further improve the error rate performance of OFDM-IM with the aid of lower order signal constellations, by using fewer bits for symbol selection and using the remaining bits to select the index combinations. As the increased number of indexing bits demand more index combinations for mapping them, certain index combinations are reused, with the reuse made unambiguous by rotating lower order constellation. This approach provides significant signal-To-noise ratio gains without compromising spectral efficiency when compared to existing OFDM-IM schemes.

We consider the uplink channel estimation of a multipath wireless channel used for orthogonal frequency division multiple access (OFDMA) transmission, where the uplink uses a pseudo-random "tile" allocation pattern. A tile is made of small number of physically adjacent data subcarriers along with a few embedded pilot subcarriers and an uplink sub-channel allocated to an user in OFDMA systems such as IEEE 802.16d/e wireless MAN consists of several such pseudo-randomly chosen tiles. While the embedded pilots enable intra-tile channel interpolation, such an estimation will have an error floor which degrades performance substantially for highly frequency selective channels. We propose a parametric channel estimation method applicable to such irregular and sparsely spaced pilots, that does not exhibit an error-floor over the nominal operating range of signal to noise ratios, even for highly selective channels. The proposed algorithm exploits the pilot structure in each tile in estimating the delay subspace corresponding to the parametric channel description. Although this algorithm is more computationally complex when compared to the intra-tile linear interpolator, it offers a greatly enhanced bit-error probability (BEP) performance with a significantly lower pilot overhead. The uncoded BEP expression for the proposed estimator are analytically derived. Simulation results provided compares the mean squared error performance of this parametric channel estimator with the Cramer-Rao bound and also illustrates the significantly improved BEP performance over the existing methods. © 2007 IEEE.