Low-Complexity Iterative Detection for Orthogonal Time Frequency Space Modulation

We elaborate on the recently proposed orthogonal time frequency space (OTFS) modulation technique, which provides significant advantages over orthogonal frequency division multiplexing (OFDM) in Doppler channels. We first derive the input--output relation describing OTFS modulation and demodulation (mod/demod) for delay--Doppler channels with arbitrary number of paths, with given delay and Doppler values. We then propose a low-complexity message passing (MP) detection algorithm, which is suitable for large-scale OTFS taking advantage of the inherent channel sparsity. Since the fractional Doppler paths (i.e., not exactly aligned with the Doppler taps) produce the inter Doppler interference (IDI), we adapt the MP detection algorithm to compensate for the effect of IDI in order to further improve performance. Simulations results illustrate the superior performance gains of OTFS over OFDM under various channel conditions.Comment: 6 pages, 7 figure

Orthogonal time frequency space modulation technology

La presente investigación bibliográfica recopila los conceptos básicos de la tecnología de modulación en el Espacio de Tiempo-Frecuencia Ortogonal (OTFS), que será utilizada para la comunicación móvil de quinta generación (5G), la cual presenta ventajas inherentes frente a la modulación de Multiplexación por División de Frecuencia Ortogonal (OFDM) utilizada en la comunicación móvil de cuarta generación (4G). Entre ellas podemos destacar su capacidad de transformar un canal que se desvanece aleatoriamente dentro del espacio de tiempofrecuencia en un canal estacionario, no aleatorio y sin desvanecimiento, entre el transmisor y el receptor. Además, el hecho de que la modulación OTFS opere en dominios en los que el canal puede caracterizarse en una forma muy compacta tiene implicaciones significativas para la solución de los cuellos de botella de estimación de canales, que afectan los sistemas actuales de antenas múltiples, y puede ser una tecnología clave para abordar problemas similares en futuros sistemas masivos de múltiples entradas y múltiples salidas (MIMO). Finalmente, el beneficio clave de la modulación OTFS es su capacidad para operar fácilmente en canales Doppler extremos; esto no solo es útil en las comunicaciones de alta movilidad, sino que también puede ser una tecnología que habilite los sistemas en la banda de frecuencia de onda milimétrica significativamente, para su operatividad en condiciones donde los efectos Doppler se amplifican.This bibliography research gathers together the basics of the Orthogonal Time Frequency Space (OTFS) modulation technology, that will be used in the fifth generation (5G) mobile communications; it has inherent advantages in the face of the Orthogonal Frequency Division Multiplexing (OFDM) modulation technique, used in the fourth generation mobile communication (4G). One of these profits is its skill to transform a channel that randomly fades within the timefrequency space into a stationary, deterministic and non-fading channel between the transmitter and the receiver. In addition, there is the fact that OTFS modulation operates in a domain in which the channel can be characterized in a very compact way; this has significant implications for tackling the channel estimation bottlenecks that affect current multi-antenna systems and can be an implementing key technology for addressing similar problems in future massive MIMO systems. Finally, the main benefit of OTFS modulation is its ability to easily operate in extreme Doppler channels; this is not only useful in high mobility communications, but can also be an enabling technology for mmWave systems, where Doppler effects will be significantly amplified.Universidad de Costa Rica/[805-B9-609]/UCR/Costa RicaUCR::Vicerrectoría de Docencia::Ciencias Básicas::Facultad de Ciencias::Escuela de FísicaUCR::Vicerrectoría de Investigación::Unidades de Investigación::Ciencias Básicas::Centro de Investigaciones Geofísicas (CIGEFI

Extended GFDM Framework: OTFS and GFDM Comparison

Orthogonal time frequency space modulation (OTFS) has been recently proposed to achieve time and frequency diversity, especially in linear time-variant (LTV) channels with large Doppler frequencies. The idea is based on the precoding of the data symbols using symplectic finite Fourier transform (SFFT) then transmitting them by mean of orthogonal frequency division multiplexing (OFDM) waveform. Consequently, the demodulator and channel equalization can be coupled in one processing step. As a distinguished feature, the demodulated data symbols have roughly equal gain independent of the channel selectivity. On the other hand, generalized frequency division multiplexing (GFDM) modulation also employs the spreading over the time and frequency domains using circular filtering. Accordingly, the data symbols are implicitly precoded in a similar way as applying SFFT in OTFS. In this paper, we present an extended representation of GFDM which shows that OTFS can be processed as a GFDM signal with simple permutation. Nevertheless, this permutation is the key factor behind the outstanding performance of OTFS in LTV channels, as demonstrated in this work. Furthermore, the representation of OTFS in the GFDM framework provides an efficient implementation, that has been intensively investigated for GFDM, and facilitates the understanding of the OTFS distinct features.Comment: Accepted in IEEE Global Communications Conference 9-13 December 2018 Abu Dhabi, UA

Differential space-time block-coded OFDMA for frequency-selective fading channels

Combining differential Alamouti space-time block code (DASTBC) with orthogonal frequency-division multiple access (OFDMA), this paper introduces a multiuser/multirate transmission scheme, which allows full-rate and full-diversity noncoherent communications using two transmit antennas over frequency-selective fading channels. Compared with the existing differential space-time coded OFDM designs, our scheme imposes 10 restrictions on signal constellations, and thus can improve the spectral efficiency by exploiting efficient modulation techniques such as QAM, APSK etc. The main principles of our design are s follows: OFDMA eliminates multiuser interference, and converts multiuser environments to single-user ones; Space-time coding achieves performance improvement by exploiting space diversity available with multiple antennas, no matter whether channel state information is known to the receiver. System performance is evaluated both analytically and with simulations