Interleaved Frequency Division Multiple Access Research Articles

Efficient FFT Computation in IFDMA Transceivers

Interleaved Frequency Division Multiple Access (IFDMA) has the salient advantage of lower Peak-to-Average Power Ratio (PAPR) than its competitors like Orthogonal FDMA (OFDMA). A recent research effort of ours put forth a new IFDMA transceiver design significantly less complex than conventional IFDMA transceivers. The new IFDMA transceiver design reduces the complexity by exploiting a certain correspondence between the IFDMA signal processing and the Cooley-Tukey IFFT/FFT algorithmic structure so that IFDMA streams can be inserted/extracted at different stages of an IFFT/FFT module according to the sizes of the streams. Although our prior work has laid down the theoretical foundation for the new IFDMA transceiver’s structure, the practical realization of the transceiver on specific hardware with resource constraints has not been carefully investigated. This paper is an attempt to fill the gap. Specifically, this paper puts forth a heuristic algorithm called multi-priority scheduling (MPS) to schedule the execution of the butterfly computations in the IFDMA transceiver with the constraint of a limited number of hardware processors. The resulting FFT computation, referred to as MPS-FFT, has a much lower computation time than conventional FFT computation when applied to the IFDMA signal processing. Importantly, we derive a lower bound for the optimal IFDMA FFT computation time to benchmark MPS-FFT. Our experimental results indicate that when the number of hardware processors is a power of two: 1) MPS-FFT has near-optimal computation time; 2) MPS-FFT incurs less than 44.13% of the computation time of the conventional pipelined FFT.

COMPARISON OF PERFORMANCES BETWEEN SC-FDMA AND OFDMA SYSTEMS UNDER DIFFERENT SUBCARRIER MAPPING SCHEMES

To improve the uplink performance of wireless network for long term evolution (LTE) single carrier frequency division multiple access (SC-FDMA) and orthogonal frequency division multiple access (OFDMA) are used. Two major concerns of these SC-FDMA and OFDMA are high peak-to-average power ratio (PAPR) and efficient subcarrier mapping scheme. This paper presents a performance comparison of SC-FDMA and OFDMA under different subcarrier mapping schemes which includes localized FDMA (LFDMA), distributed FDMA (DFDMA) and interleaved FDMA (IFDMA). Quadrature amplitude modulation (QAM) with different constellation sizes is used. The symbol error rate (SER) in SC-FDMA is minimized using appropriate mapping scheme. The PAPR is less in SC-FDMA compared to OFDMA. SC-FDMA with IFDMA shows better performance for the PAPR reduction compared to the OFDMA. Compared to OFDMA, using SC-FDMA with 8-QAM and IFDMA, PAPR is reduced by 78%, using SC-FDMA with 16-QAM and IFDMA, PAPR is reduced by 80%, using SC-FDMA with 64-QAM and IFDMA, PAPR is reduced by 68%.

Intersymbol interference resilient interleaving architecture for multi‐stream interleaved frequency division multiple access system

SummaryA multi‐stream cyclic interleaving architecture for an interleaved frequency division multiple access (IFDMA) system is proposed. The proposed scheme exploits the full benefits of maximal length sequence ( ‐sequence), in contrast to single data stream transmission scheme proposed in earlier works, thereby allowing simultaneous transmission of multiple data streams by each user. The cyclic spreading allows the separation of multiple data streams for each user in addition to intersymbol interference (ISI) minimization. The assignment of orthogonal subcarriers to each user compensates multiuser interference (MUI). The increase in the number of data stream transmission improves the data rate of the system. The proposed scheme employs maximal ratio combining (MRC) to maximize the instantaneous signal‐to‐noise ratio (SNR) of the multipath signal. The performances of the proposed interleaving scheme are compared with the minimum mean square error frequency domain equalization (MMSE‐FDE)‐based conventional interleaving scheme. Despite the simultaneous transmission of multiple data streams, the proposed scheme retains the performance of single data stream transmission.

A cyclic frequency division non‐orthogonal multiple access system

SummaryIn this paper, we have developed a downlink power domain non‐orthogonal multiple access (NOMA) system based on cyclic frequency division, where interleaved frequency division multiple access (IFDMA) is cyclically combined with NOMA. The newly developed cyclic frequency division NOMA (CFD‐NOMA) system has the benefit of both NOMA and IFDMA. The concept of superposed multiuser principle for non‐orthogonal resource allocation is used in each NOMA cluster, and the users' data associated with different clusters are separated based on IFDMA. Extreme channel conditions are considered among the users in each cluster. Based on channel gain, the users are differentiated as strong and cell edge users. At the strong user terminal, successive interference cancelation (SIC) is applied to decode the strong user data with the aid of the cell edge user data found from a hard decision, whereas the cell edge user terminal detects its signal only through the hard decision. The near‐far effect of the users present in a cluster, the amount of difference among their channel gain, and transmission power play an important role in detecting the cell edge and the strong users. The CFD‐NOMA system inherits an effective use of multipath channel, thereby reaps an increased cell edge user performance in terms of bit error rate (BER).

Efficient Technique for PAPR and BER Reduction in Downlink Large Scale MU-MIMO-OFDM System

Background & Objective: The Orthogonal Frequency Division Multiplexing (OFDM) based large scale multiuser multiple input multiple output (LS-MU-MIMO) system is a preferred choice for signal transmission because it can substantially enhance the spectral and energy efficiency of the next-generation wireless systems. However, there are several practical issues in implementing large multiuser-MIMO-OFDM systems and one of the critical issues is the high Peak-to-Average- Power Ratio (PAPR). Due to the use of a large number of antennas at the Base Station (BS) of an LSMU- MIMO-OFDM system, it requires a large number of Power Amplifiers (PAs) which are needed to be connected to each antenna element. The high PAPR in an OFDM based LS-MU-MIMO system results in the decrease in the efficiency of PAs and enhancement in nonlinear signal distortions. This work is concerned with the reduction in PAPR of LS-MU-MIMO-OFDM systems using the SCFDMA scheme for the downlink transmission. The reduction in PAPR is achieved on the basis of discrete Fourier transform spreading (DFT-S) scheme known as DFT-SC-FDMA. The SC-FDMA scheme can achieve the reduction in PAPR of MIMO-OFDM signal up to the level of single-carrier transmission. Further, the reduction in PAPR is achieved with the sub mapping schemes such as localized FDMA (LFDMA) and distributed FDMA (DFDMA), Interleaved FDMA (IFDMA). Result: In this work, we propose to perform jointly multiuser MMSE precoding, SC-IFDMA and antenna reservation technique to reduce the PAPR and Bit Error Rate (BER) of the large MU-MIMO system. To show the effectiveness of the proposed scheme in the reduction of PAPR with different modulation techniques, subcarriers, and subcarrier mapping schemes, the results are compared with the existing OFDM based large MU-MIMO system. Conclusion: The MATLAB simulation results confirm that our proposed combination of MMSE MUprecoding, SC-IFDMA and antenna reservation technique is able to reduce PAPR and BER to a significant extent for LS-MU-MIMO-OFDM systems.

High Accuracy Uplink Timing Synchronization for 5G NR in Unlicensed Spectrum

Sub-microsecond high accuracy clock synchronization is essential in time critical applications such as factory automation. In 3GPP, 5G New Radio in unlicensed spectrum (NR-U) is being discussed to support time critical applications. In 5G NR-U uplink, synchronization is achieved by using block interleaved frequency division multiple access (B-IFDMA) reference signals (RS). Based on the B-IFDMA-structured RS, conventional methods can only obtain the frequency domain channel at non-evenly spaced frequency points. When this channel is transformed from frequency domain to time domain, the resultant channel impulse response (CIR) will contain additional “pseudo” paths. In multipath channel, such “pseudo” paths may cause aliasing effects, affecting accuracy of CIR-based timing synchronization algorithms. In this letter, we propose two methods to address the above problem, namely, the frequency-domain subspace (FDS) based algorithm, and FDS-aided algorithm. Simulation result shows that our proposed methods counteract the aliasing effects effectively and significantly outperform the conventional algorithms.

Flexible Subcarrier Allocation for Interleaved Frequency Division Multiple Access

Interleaved Frequency Division Multiple Access (IFDMA) and Orthogonal FDMA (OFDMA) belong to a class of signal modulation and multiple-access techniques in which information of multiple users are multiplexed and carried on subcarriers within a shared spectrum. Compared with OFDMA, IFDMA has lower Peak-to-Average Power Ratio (PAPR). However, IFDMA poses two rigid constraints on subcarrier allocation: 1) the subcarriers occupied by a user must be evenly-spaced among the available subcarriers. 2) the number of subcarriers used by a user must be a divisor of the total number of subcarriers. This paper investigates how to overcome these constraints to allow flexible and fine-grained subcarrier allocation in IFDMA. Specifically, we put forth 1) a bit-reversal subcarrier allocation scheme whereby the problem of allocating evenly-spaced subcarriers is transformed to a more intuitive problem of filling contiguous bins; 2) a multi-stream IFDMA scheme whereby a user can have an arbitrary number of subcarriers. For the synchronous scenario in which user requests arrive in a synchronous manner, we show that IFDMA can achieve the same level of flexibility and granularity as OFDMA in subcarrier allocation. For the asynchronous scenario in which user requests arrive and depart asynchronously, we show that the blocking probability of IFDMA is only slightly worse than that of OFDMA: specifically, the gap between the blocking probabilities of IFDMA and OFDMA is only 2:56% at a moderate offered load.

New Transceiver Designs for Interleaved Frequency-Division Multiple Access

This paper puts forth a class of new transceiver designs for interleaved frequency division multiple access (IFDMA) systems. These transceivers are significantly less complex than conventional IFDMA transceivers. The simple new designs are founded on a key observation that multiplexing and demultiplexing of IFDMA data streams of different sizes are coincident with the IFFTs and FFTs of different sizes embedded within the Cooley-Tukey recursive FFT decomposition scheme. For flexible resource allocation, this paper puts forth a new IFDMA resource allocation framework called Multi-IFDMA, in which a user can be allocated multiple IFDMA streams. Our new transceivers are unified designs in that they can be used in conventional IFDMA as well as multi-IFDMA systems. Two other well-known multiple-access schemes are localized FDMA (LFDMA) and orthogonal FDMA (OFDMA). In terms of flexibility in resource allocation, Multi-IFDMA, LFDMA, and OFDMA are on an equal footing. With our new transceiver designs, however, IFDMA has the following advantages (besides other known advantages not due to our new transceiver designs): 1) IFDMA/Multi-IFDMA transceivers are significantly less complex than LFDMA transceivers; in addition, IFDMA/Multi-IFDMA has better Peak-to-Average Power Ratio (PAPR) than LFDMA; 2) IFDMA/Multi-IFDMA transceivers and OFDMA transceivers are comparable in complexity; but IFDMA/Multi-IFDMA has significantly better PAPR than OFDMA.

Cyclic interleaving scheme for an IFDMA system

In this paper, a new interleaving scheme for an interleaved frequency division multiple access (IFDMA) system is proposed. The proposed scheme combines the advantages of code division multiple access (CDMA) with maximal length sequence (m-sequence) and orthogonal frequency division multiple access (OFDMA). Multiaccess interference (MAI) among users is eliminated by employing orthogonal carriers, and the intersymbol interference (ISI) due to multipath channels is minimized by using cyclic interleaving. Diversity gain is achieved in the proposed method with maximal ratio combining (MRC) in the case of a Rayleigh environment. Compared to complex multiuser (MU) detection, the detection process in the proposed method is relatively simple. Multipath orthogonality is achieved in the proposed method, avoiding the need for channel equalization. The proposed method is compared with the conventional interleaving employing well-known minimum mean square error frequency domain equalization (MMSE-FDE). The Monte Carlo simulation results prove the superiority of the proposed interleaving over conventional interleaving with MMSE-FDE, showing that the present scheme is suitable for both uplink and downlink mobile radio transmissions.

Blind Algorithm Development for Peak to Average Power Ratio Reduction in OFDM Systems under Frequency Selective Channels

One major drawback of orthogonal frequency division multiplexing (OFDM) system is peak to average power ratio (PAPR). This effect causes high power amplifier (HPA) to introduce intermodulation and out of band radiation as the signal goes through, thus degrades the performance of OFDM systems. This paper proposes blind algorithms which takes advantage of signal transformation technique and signal distortion technique. Simulation results show that at complementary cumulative distribution function (CCDF) level of 10-3 , the proposed algorithm achieved 3.2 dB PAPR improvement compared to discrete Fourier transform with interleaved frequency division multiple access (DFT-IFDMA) based algorithm. The bit error rate (BER) performance has degraded by 2 dB compared to the original OFDM signal with no distortion under frequency selective channel (FCS) at BER of 10-4 . These presented results, mark this algorithm as a better candidate for PAPR reduction algorithm in long term evolution (LTE) network. Under AWGN channels, the proposed algorithm performs better both in low and high signal power values. Under frequency selective channels, the existing and proposed algorithm converges after 10 dB of signal to noise power values. The low BER transmissions at low signal power values signify energy efficiency, ideal for portable wireless devices with limited battery power.

Multisatellite Cooperative Random Access Scheme in Low Earth Orbit Satellite Networks

Multisatellite cooperative random access (MSC-RA) scheme is proposed in low earth orbit (LEO) satellite networks. In the scheme, a packet structure based on single carrier interleaved frequency division multiple access (SC-IFDMA) is designed to overcome the effect of users’ propagation delays on the received signals at satellite nodes, which ensures the synchronization of received signals. The transmission model from multiple terminals to multiple satellite nodes in a slot can be equivalent to a virtual multi-input multioutput model, and MSC detection is employed to decode multiple collided packets. The MSC detection can be applied to slotted ALOHA (SA), contention resolution diversity slotted ALOHA (CRDSA), CRDSA-3, and irregular repetition slotted ALOHA (IRSA) protocols, which are dubbed MSC-SA, MSC-CRDSA, MSC-CRDSA-3, and MSC-IRSA, respectively. At the gateway station, the MSC detection and iterative interference cancellation processing are alternately conducted for multiple collided packets at satellite nodes. The performance of these MSC-RA schemes is evaluated via mathematical analysis and computer simulation in terms of throughput, stability and energy efficiency. Simulation results show that MSC-SA, MSC-CRDSA, MSC-CRDSA-3, and MSC-IRSA schemes significantly outperform conventional ones, and the MSC-CRDSA scheme is superior to MSC-SA, MSC-CRDSA-3 and MSC-IRSA schemes.

Performance Analysis of DST-Based Intensity Modulated/Direct Detection (IM/DD) Systems for VLC

The dual functionality of assuring ‘illumination’ and “communication” contemporaneously by relying on sustainable and energy-efficient opto-electronic devices like light emitting diodes (LEDs) has enabled visible light communication to garner the attraction of research era. This paper proposes a discrete sine transform (DST)-based optical orthogonal frequency division multiplexing (OFDM) system bestowing as the simplest and alternative multicarrier scheme complying the requirements of intensity modulated and direct detection systems. Moreover, this paper addresses the most vital issue like peak to average power ratio (PAPR) that emerges principally due to the limited dynamic range of LEDs when exploiting DC-biased optical OFDM (DCO-OFDM). Therefore, this paper imparts DST spreading to DST-based DCO-OFDM (DST-S-DCO-OFDM), and consequently, the PAPR performance is compared with other PAPR reduction schemes for the same system. Furthermore, in order to serve multiple subscribers with high-data rates as well as to significantly reduce the amount of PAPR of the multicarrier system, this work investigates a DST-based single carrier frequency division multiple access (DST-SC-FDMA) and derives the mathematical analysis of the time-domain signal formats for different subcarrier mapping strategies like DST-based optical interleaved frequency division multiple access (DST-OIFDMA) and optical localized frequency division multiple access (DST-OLFDMA). The simulated results evidences that the reduction in PAPR for DST-S-DCO-OFDM is more predominant when compared with other PAPR reduction schemes and among the multiple access schemes, the DST-OIFDMA is more power efficient upon comparison with DST-OLFDMA. However, both the schemes exhibit a significant reduction in PAPR when compared with DST-based optical OFDM (DST-OOFDMA).

43.63-Gbit/s Multiple-User SC-FDMA PON With Sub-Nyquist Receiver and PAPR Reduction

In this letter, we propose a novel delay-division-multiplexing (DDM) scheme to enable SC-FDMA PON with a sub-Nyquist sampling rate ADC and lower PAPR, named time-interleaved FDMA (TI-FDMA). The proposed pre-processing method reduces the sampling rate of the ADC to 1/2 ~ 1/32 of the Nyquist sampling rate. The FFT size can be reduced to 1/32 (i.e., from 1024 to 32) due to lower sampling ratio and each user just needs to recover and demodulate their corresponding data without any additional DSP, which greatly reduces the complexity of the receiver. Meanwhile, the proposed TI-FDMA has lower PAPR than traditional localized FDMA (L-FDMA), interleaved FDMA (I-FDMA), and OFDM. Such the PAPR reduction makes TI-FDMA support the modulation format of 1 order higher compared to OFDM, thereby, leading to 12.5% capacity improvement. Employing the proposed scheme, up to 43.63-Gbps PON is experimentally demonstrated, and each of 32 users requires only 0.5-GSample/s ADC.

Efficient PAPR Reduction in DCT-SCFDMA System Based on Absolute Exponential Companding Technique with Pulse Shaping

Discrete Cosine Transform based Single carrier frequency division multiple access (DCT-SCFDMA) is a prominent technique for providing high data rates in multimedia services. It also provides high Quality of Service to the users by mitigating the fading of signals. But High peak-to-average power ratio (PAPR) is a technical challenge which reduces the efficiency of RF power amplifiers. In this article, a novel joint scheme of PAPR reduction method is proposed and analyzed based on pulse shaping and absolute exponential companding (AEXP) technique. Our method proposes the use of standard raised cosine filter and square root raised cosine filter along with the AEXP companding process separately in DCT-SCFDMA system for different subcarrier mapping techniques e.g. interleaved frequency division multiple access and localized frequency division multiple access. Through extensive simulations, the numerical analysis presents that proposed architecture achieves significant improvements over the existing standard pulse shaping methods when used alone in DCT-SCFDMA system as far as the lowest PAPR is concerned.

Carrier Interferometry Coded Single Carrier FDMA (CI/SC-FDMA) for Next Generation Underwater Acoustic Communication

Underwater acoustic (UWA) communication suffers from large propagation delays, limited bandwidth, frequency dependent attenuation and severe multipath fading. Low power consumption, acceptable bit error rate and minimal complexity are three critical factors for UWA sensor network design. Orthogonal frequency division multiple access (OFDMA) is an attractive option because of its robustness against frequency selective underwater channel. But, OFDMA suffers from high peak-to-average power ratio (PAPR) and high sensitivity to frequency offsets. Single carrier FDMA (SC-FDMA) has not been well investigated over UWA channel in literature. Two variants of SC-FDMA namely interleaved FDMA (IFDMA) and localized FDMA (LFDMA) are considered here. Firstly, through simulation it has been observed that IFDMA performs significantly better than OFDMA, in terms of SER and PAPR. However, IFDMA outperforms LFDMA in terms of PAPR, but SER performance of IFDMA is poor as compared to that of LFDMA. In case of power limited uplink UWA communication, IFDMA serves as a feasible solution. But for downlink, where lower SER is an important performance parameter, LFDMA is an efficient multiple access scheme. The use of carrier interferometry (CI) codes with SC-FDMA further enhances the SER and PAPR performance over highly frequency selective UWA transmission. We have also evaluated the effect of multi-user diversity for CI/SC-FDMA over 5-tap frequency selective UWA channel by applying scheduling algorithm. The algorithm selects the sensor mote with best channel condition for transmission.

Experimental demonstration of block interleaved frequency division multiple access for bidirectional visible light communications

We propose a power efficient multiple access scheme for visible light communications (VLC) based on the block interleaved frequency division multiple access (B-IFDMA) which provides large frequency-diversity, flexible bandwidth allocation, low complexity of channel equalization, and user separation. Bidirectional B-IFDMA VLC transmission is experimentally demonstrated to verify its feasibility. The impact of the number of subcarriers per block on the transmission performance under wireless optical channel is investigated.

Experimental Demonstration of IFDMA for Uplink Visible Light Communication

In this letter, we propose an experimental demonstration of uplink visible light communications based on interleaved frequency division multiple access (IFDMA) with high spectral and power efficiencies, low complexity of the transmitter, high tolerance against multipath-induced distortion, and synchronization error. We show that IFDMA with a lower peak-to-average power ratio offers improved bit error rate (BER) performance compared with the orthogonal frequency division multiplexing access. For the uplink, the increase in the transmission span is <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sim 4$ </tex-math></inline-formula> cm at a BER of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$10^{-5}$ </tex-math></inline-formula> .

A New Linearly Combined Nyquist Pulses for PAPR Reduction in IFDMA

Single Carrier Frequency Division Multiple Access (SCFDMA) is proposed for uplink transmission of Long Term Evolution Advanced (LTE-A) due to its lower Peak to Average Power Ratio (PAPR) compared to Orthogonal Frequency Division Multiple Access (OFDMA). In this paper, a comparison is introduced between its two subcarrier mapping schemes, Localized-FDMA (LFDMA) and Interleaved-FDMA (IFDMA), in terms of PAPR. Moreover, the paper addresses the performance of SC-FDMA with Pulse Shaping (PS). Since conventional Raised Cosine (RC) PS introduces a trade-off between spectrum efficiency and low PAPR in IFDMA, the results show that some newly combined Nyquist pulses can reduce PAPR of IFDMA significantly while maintaining the same Bandwidth (BW) compared to conventional RC pulse.

PAPR Reduction in LFDMA Using Hadamard Transform Technique

The power consumption is an essential issue for designers of mobile devices. Orthogonal Frequency Division Multiplexing (OFDM) suffers from its Peak – to – Average Power Ratio (PAPR) problem. The Discrete Fourier Transmission Spread OFDM (DFTS-OFDM) based on Single Carrier Frequency Division Multiple Access (SC-FDMA) has been widely adopted due to its lower peak-to-average power ratio (PAPR) of transmits signals compared with OFDM. There are three types of mapping in SCFDMA. These namely Localized FDMA (LFDMA), Distributed FDMA (DFDMA), and Interleaved FDMA (IFDMA). In this paper, we propose a modified SCFDMA. This is based on using Hadamard transform instead of Fast Fourier Transform (FFT). Utilizing LFDMA Mapping techniques this is proposed for uplink scenario of LTE. Simulation results show that the proposed scheme reduces PAPR compared to the conventional SCFDMA while the BER does not degraded.

SC-FDMA user pairing and frequency allocation with imperfect channel state information

We consider user pairing for a single-carrier frequency-division multiple-access (SC-FDMA) transmission in the uplink of a Long Term Evolution (LTE) system. Two single-antenna users each are multiplexed to the same time and frequency resource within one cell. A virtual multiple-input multiple-output (V-MIMO) system arises for each user pair if multiple receive antennas are employed at the base station. For user pairing and frequency allocation, knowledge about the channel state is necessary. According to the LTE standard sounding reference signals are used for channel acquisition and transmitted with interleaved frequency-division multiple-access (I-FDMA). In channel state acquisition, the subcarriers not included in the channel sounding procedure should be taken into account by interpolation in frequency direction in a first step. Then a prediction in time direction for the time instant of the considered subframe is required. Channel interpolation and prediction are revisited in this paper, and the influence of channel interpolation and prediction errors on the achievable data rate of the system with user pairing and frequency resource allocation is studied. Simulation results show that joint user pairing and frequency allocation combined with channel interpolation and prediction is always beneficial for static or slowly moving users. However, for high user velocity the performance approaches that of random pairing and frequency allocation or becomes even worse.