Orthogonal Frequency Division Multiplexing (OFDM) is still at 5G (and since 3G) the preferred modulation of choice, due to its robustness to inter-symbol interference associated with multipath channels.
Although the significant out-of-band emission (due to the use of a rectangular window profile) and large peak-to-average power ratio considerably limits its spectral and power efficiencies. These raised the interest for the research of new OFDM-based waveforms, able to keep the main key advantage characteristics of OFDM while surpassing its disadvantages.
Disclosed herein is a method and transceiver apparatus of an OFDM-based multicarrier technique that packs together multiple windowed OFDM blocks with a single zero-pad to deal with the multipath propagation delay of the channel. New proposed windowing assures a compact spectrum of OFDM-based blocks. To keep transmission rate and spectrum occupancy windowed OFDM blocks are partial overlapped at time domain. To improve robustness against time dispersive channels time-interleaving of samples of the packed and overlapped-OFDM-based blocks is employed. Different embodiments of non-iterative and iterative receivers to cancel both channel impairments (at frequency domain) and interference resulting from the overlapping operation (at time domain) are disclosed.
Disclosed techniques allow for spectral efficiency improvement leveraged by windowing and block overlapping, power efficiency improvement resulting from the use of a single zero-padding block per multiple OFDM-based blocks and transmission resilience against time dispersive effects.
Future 5G and beyond wireless communications systems are expected to bring improvements in the way data are transmitted and the waveforms are designed. Such improvements are related to higher data rate, lower latency, and flexibility brought by the need to transmit over hostile channel conditions, as well as higher spectral and power efficiency Orthogonal Frequency Division Multiplexing (OFDM) has been, since the 3rd generation (3G) of wireless communications, the preferred waveform of choice, due to its robustness to inter-symbol interference associated with multipath channels. In order to support transmission of data in several scenarios, with different service qualities, delay requirements and different carrier frequencies, a new radio interface is being developed by 3GPP, in the scope of the 5G. The choice of the waveform for 5G New Radio culminated in the adoption of OFDM with the addition of cyclic prefix for the downlink and uplink transmissions, leaving open the possibility of using new waveform modulation types, as usually designated in the context of 5G.
Thus, the development of new techniques as alternatives to OFDM, with greater spectral and power efficiency, has been the subject of many recent studies, with several techniques being proposed as: filter-bank multicarrier (FBMC); generalized frequency division multiplexing (GFDM); filtered-OFDM; and more recently the Time-Interleaved Block Windowed Burst Orthogonal Frequency Division Multiplexing (TIBWB-OFDM) technique.
In TIBWB-OFDM, OFDM-based blocks are sequentially cyclic extended and windowed, in time domain, through a roll-off dependent window, known as square root raised cosine, granting a much better spectrum confinement by reducing the out of band radiation and improving the spectral efficiency when compared to CP-OFDM systems. An interleaving operation is performed between the time samples of each OFDM-based block, creating a diversity effect at the frequency domain, improving the robustness against deep inband fades. In this waveform, several small size windowed OFDM-based blocks are packed together with a single zero-pad to deal with the multipath propagation delay of the wireless channel, thereby improving power efficiency.
Although the spectral confinement increases for higher window’s roll-off, the length of the TIBWB-OFDM block increases proportionally, due to the juxtaposition of the component symbols. This implies a reduction of symbol rate, which limits spectral efficiency gains. Consequently, the achieved spectral efficiency gain of this modulation technique is limited, by either, improving spectral confinement by reducing out of band (OOB) radiation when using a larger roll-off, or by improving symbol rate when conventional rectangular window is used since a sole ZP prefix is used per the group of packed OFDM-based blocks. Also, the overall average power of the TIBWB-OFDM block is reduced thus implying an increase of the transmitted peak-to-average power ratio (PAPR) which limits achievable power efficiency gains.
The alternative approach regarding the TIBWB-OFDM symbol construction is approached by allowing a partial overlap between the adjacent windowed OFDM symbols, in time domain to keep transmission rate and spectrum occupancy. This new waveform would allow achieving a very high spectral efficiency since there is no temporal expansion of the TIBWB-OFDM block, permitting a spectrum saving when transmitting at a fixed rate. Furthermore, the overlapping operation creates a flatter waveform, diminishing the windowing attenuation effect and opposing the decrease in the average signal power and consequently decreasing the signal’s PAPR. However, the overlapping operation introduces interference between the data transmitted in adjacent sub-blocks. Thus, time domain cancellation algorithms must be developed acting as signal reconstruction methods. A linear forward and backward interference successive cancellation (ISC) algorithm is included to override the self-created interference resulting from this process, based on a sequential process. In this process, the data sent in the first symbol allows to partially recover the information that has been corrupted (superimposed) by the next symbol. In addition, the data sent in the last symbol allows partial retrieval of data that has been corrupted by the previous symbol. Furthermore, to improve robustness against time dispersive channels time-interleaving of samples of the packed and TIBWB-OFDM with windowing time overlapping (WTO) based blocks is employed. Different embodiments of non-iterative and iterative receivers to cancel both channel impairments (at frequency domain) and interference resulting from the overlapping operation (at time domain) are also included. In this context, linear or iterative frequency domain equalization (FDE) techniques, like minimum mean square error (MMSE) or iterative block decision feedback equalizer (IB-DFE), respectively, can be complemented with linear ISC techniques or iterative inter-block interference cancellation (IBIC) algorithms to improve the performance of the new proposed waveform.
F. Conceição, M. Gomes, V. Silva and R. Dinis, “Time Overlapping TIBWB-OFDM Symbols for 740 Peak-To-Average Power Ratio Reduction,” in 11th Conference on Telecommunications (ConfTele 2019), Lisbon, Portugal, June 2019.
F. Conceição, M. Gomes, V. Silva and R. Dinis, “Highly efficient TIBWB-OFDM waveform for broadband 737 wireless communications,” in 2020 IEEE 91st Vehicular Technology Conference (VTC2020-Spring), Antwerp, 738 Belgium, pp. 1-5, May 2020.
F. Conceição, M. Gomes, V. Silva and R. Dinis, “An OFDM-Based Waveform With High Spectral Efficiency,” in IEEE Communications Letters, vol. 24, no. 11, pp. 2614-2618, Nov. 2020
Stage of Development
TRL 4 – technology validated in lab.
Some of the solutions being proposed have drawbacks, such as the difficulty of extending FBMC to Multiple Input Multiple Output (MIMO) consider as a key enabling technology for 5G. This provisional patent addresses the TIBWB-OFDM already showed to be easily extendable to MIMO scenarios. However, the promised spectral and power efficiency increase proposed by the method is limited by the growth of the windowed OFDM-based blocks and also due to their juxtaposition.
The windowing procedure, although improves the spectral confinement of the OFDM-based blocks forming the TIBWB-OFDM signal, causes the block temporal extension and, thus, a reduction of transmission rate. To keep transmission rate, in fact, the spectral occupancy must increase, thus limiting the spectral efficiency gains of the technique. Furthermore, the temporal expansion also increases the PAPR of each OFDM-based block. So, although a PAPR reduction is achieved by the single use of a zero-padding for multiple OFDM-based packed blocks, the overall reduction achieved is limited.
The new technique allows a partial overlap between adjacent windowed OFDM-based blocks, in the time domain, therefore achieving effective spectral and power efficiency improvements. For that, it is necessary to develop equalization algorithms in the receivers capable of overriding the self-created interference resulting from this process.
This new waveform would allow achieving an increased spectral efficiency since the temporal expansion of the TIBWB-OFDM signal is avoided. Furthermore, the overlapping operation diminishes the windowing attenuation effect in time domain, opposing the decrease in the average signal power and consequently effectively decreasing the PAPR of the transmitted signal.
OFDM-based waveform with considerably enhanced power and spectral efficiency suitable for strong frequency dispersive wireless and optical communications, with potential use to be included in beyond-5G wireless communication systems.
Available for exclusive and non-exclusive licensing.
New waveforms for beyond-5G; MIMO systems; OFDM; Hybrid Modulation system; Power and Spectral efficiency; Filtered OFDM; Time-interleaving; Iterative-block frequency domain equalization.
Rui Miguel Henriques Dias Morgado Dinis