Frequency Division Multiple Access

Dr. Ilir F. Progri is the President and CEO of Giftet Inc. a privately held company for developing Global Navigation Software, and Web Solutions (Giftet). Dr. Progri's eleven year career in GPS consists of all aspects of signals and system specifications, simulation, software development and implementation of significant new capabilities in GPS and indoor geolocation systems. Dr. Progri has led research and development engineering projects for over eight years. He has over fifty published papers and one patent in all aspects of geolocation systems. Ilir has received over thirty citations from experts, researchers, and scientists of US,

This paper considers the problem of how to quickly and accurately identify spectrum holes from downlink orthogonal frequency division multiple access (OFDMA) signals in frequency selective fading environment. We assume that the subcarrier assignment of primary users (PUs) in an OFDMA system is a priori known to the detector. Under this assumption, we formulate the original problem as the problem of detecting presence/absence of PUs, which requires less computational complexity than its original counterpart. We propose a spectrum sensing algorithm to detect presence/absence of a PU. In the proposed algorithm, we first apply the Thomson's multitaper spectrum estimation (MTSE) method to obtain spectral estimates at certain subcarriers of interest, and then we perform a simple threshold test. We present closed form results for false-alarm and miss-detection probabilities of the proposed algorithm. We study impacts of system/MTSE parameters on the detection performance via Monte Carlo simulation.

Cognitive radio is a promising paradigm for efficient utilization of the radio spectrum due to its capability to sense environmental conditions and adapt its communication and localization features. In this paper, the theoretical limits on time-of-arrival estimation for cognitive radio localization systems are derived in the presence of interference. In addition, an optimal spectrum allocation strategy which provides the best ranging accuracy limits is proposed. The strategy accounts for the constraints from the sensed interference level as well as from the regulatory emission mask. Numerical results are presented to illustrate the improvements that can be achieved by the proposed approach.

The new generation of "user-oriented'' satellites are conceived to involve a dramatic reduction in the earth station size and complexity. Frequency division multiple access (FDMA) and single channel per carrier (SCPC) in the uplink and time division multiplexing (TDM) in the downlink are employed in the system described here. To interface FDMA in the uplink and TDM in the downlink, multicarrier demodulation (MCD) is required on-board the satellite. The operation of the on-board MCD is the separation of each individual channel and subsequent demodulation. The results presented in this paper have been obtained under INTELSAT Contract INTEL-479, which has provided an analysis of the advanced technologies and techniques for on-board frequency demultiplexing and demodulation for low bit rate carriers. The study was constrained by the requirement that new-generation payloads could serve the stations already active in the INTELSAT Business System. A digital hardware design that implements an MCD that can process three channels at 4.4 Mb/s, or 12 channels at 1.1 Mb/s is described in this paper. The test results have confirmed the MCD feasibility, and further improvements are expected from a semicustom implementation.

Orthogonal frequency division multiple access (OFDMA) systems exploit multiuser diversity and frequency selectivity to achieve high spectral efficiencies. However, they require considerable feedback for scheduling and rate adaptation, and are sensitive to feedback delays. We develop a comprehensive analysis of the OFDMA system throughput as a function of the feedback scheme, frequency-domain scheduler, and discrete rate adaptation rule in the presence of feedback delays. Previous allocation methods have been iterative nonlinear methods suitable for offline optimization. In the special high sub-channel SNR case, an iterative radical-jewel method has linear -time complexity in the number of users and N log N complexity in the number of sub-channels. We propose a non-iterative method that is made possible by our relaxation of strict user rate proportionality constraints. Compared to the radical-jewel method, the proposed method waives the restriction of high sub-channel SNR, has significantly lower complexity, and in simulation, yields higher user data rates. Adaptive where, the goal is to redistribute power such that the transmission time of a packet is minimized. While the first two variants decrease transceiver complexity and are simpler, the third is geared towards achieving the maximum throughput possible. We analyze the popular best-n and threshold-based feedback schemes.

178 Abstract—Discrete Fourier Transform (DFT) spread is a modified form of orthogonal frequency division multiplexing (OFDM) system. It promises high data rate for uplink communications with lower peak to average power ratio (PAPR) than the conventional OFDM system. It has similar throughput performance and structural complexity as OFDM. In the present work PAPR performance analysis has been done for its different subcarrier mapping approaches. Among the localized FDMA (LFDMA), distributed FDMA (DFDMA) and interleaved FDMA (IFDMA) the PAPR performance of IFDMA has been found to be better than that of DFDMA and LFDMA. When the DFD spreaded signals are passed through a raised cosine pulse shaping filer IFDMA system shows sharp reduction in PAPR with increase in roll-off factor from 0 to 1. The response of LFDMA is not that much affected by the pulse shaping network. The PAPR performance of DFT spread also depends on the number of subcarriers allocated to each user. The PAPR of LFDMA d...

Since carrier frequency offset destroys user's signal orthogonality in Orthogonal Frequency-Division Multiplexing Access (OFDMA) uplink transmission, resulting in an interference-limited system, its estimation/correction is very important. The residual carrier frequency offset of each user contributes Inter-Carrier-Interference (ICI) and Multiple-User-Interference (MUI) to other users. In this paper, the effect of the carrier frequency offset on OFDMA uplink is analyzed. We first analyze the average uplink capacity losses as well as the Signal-to-Interference-and-Noise Ratio (SINR) reduction due to the carrier frequency offsets, and then discuss the capacity increases by using adaptive power allocation. The averaged Bit Error Rate (BER) performance with the carrier frequency offsets on OFDMA uplink is also analyzed.

Since carrier frequency offset destroys user's signal orthogonality in Orthogonal Frequency-Division Multiplexing Access (OFDMA) uplink transmission, resulting in an interference-limited system, its estimation/correction is very important. The residual carrier frequency offset of each user contributes Inter-Carrier-Interference (ICI) and Multiple-User-Interference (MUI) to other users. In this paper, the effect of the carrier frequency offset on OFDMA uplink is analyzed. We first analyze the average uplink capacity losses as well as the Signal-to-Interference-and-Noise Ratio (SINR) reduction due to the carrier frequency offsets, and then discuss the capacity increases by using adaptive power allocation. The averaged Bit Error Rate (BER) performance with the carrier frequency offsets on OFDMA uplink is also analyzed.

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 transceiver. 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 wellknown 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.

Single carrier frequency domain multiple access (SC-FDMA) system achieves better spectral efficiency when cyclic prefix (CP) is not transmitted. However, the chained turbo equalization (CHATUE) algorithm, an equalizer required to both equalize the multipath fading effect and cancel the inter-block interference of SC-FDMA without CP, requires high computational complexity due to the non-circulant structure of the past and the future interference matrices. This paper proposes efficient hardware architecture based on systolic architecture for practical implementation. The main idea is to minimize the number of required processing elements by utilizing efficient resources sharing method while exploiting concurrency of the processing. A new computation method using masking matrices is introduced to obtain interference matrix from its corresponding circulant channel matrix. The results with fixed point computation show that the computational complexity can be significantly reduced up to 96% for practical implementation without significant degradation in bit-error-rate (BER) performances.

One of the major challenges in deploying femtocells is the management of interference between neighboring femtocells and between the femtocells and the macrocells. This paper explores the use of opportunistic multiuser detection for interference mitigation in the downlink of an orthogonal frequency division multiple access (OFDMA) femtocell network. In particular, we focus on the use of joint decoding (JD) at the receiver, where a macro or femto user may jointly decode both the desired message and the message from a selected interfering user in order to achieve a higher overall transmission rate. It is shown that to take the full advantage of opportunistic multiuser detection, the selection of JD pairs needs to be jointly optimized with scheduling, power allocation, and rate adaptation. This paper adopts a network utility maximization framework and proposes an iterative algorithm for such a joint optimization across the network. Simulation results show that multiuser detection can significantly benefit the femto-users, while maintaining the performance of macro-users. Further, although the lowercomplexity successive interference cancellation (SIC) scheme can already reap significant benefit of multiuser detection, JD can further improve upon SIC.

An assessment of standalone GLONASS RTK performance is provided using its FDMA and CDMA signals. The new integer-estimable GLONASS FDMA model (Teunissen 2019), which guarantees the integer-estimability of its ambiguities, is employed. We introduce the combined integer-estimable GLONASS FDMA+CDMA model and compare its strength against the FDMA model for instantaneous integer ambiguity resolution and positioning. Various combinations of GLONASS signals are considered including FDMA L1, FDMA+CDMA L1+L3, FDMA L1+L2 and FDMA+CDMA L1+L2+L3. To provide insight into the current RTK performance of GLONASS, we used observations of a short baseline to analyze the integer ambiguity resolution success rate and positioning precision, formally and empirically. To provide insight into the future RTK performance of GLONASS, we present a formal analysis of the integer ambiguity resolution success rate and ADOP, assuming that all the GLONASS satellites transmit FDMA L1, L2 and CDMA L3 signals. A forma...

An assessment of standalone GLONASS RTK performance is provided using its FDMA and CDMA signals. The new integer-estimable GLONASS FDMA model (Teunissen 2019), which guarantees the integer-estimability of its ambiguities, is employed. We introduce the combined integer-estimable GLONASS FDMA+CDMA model and compare its strength against the FDMA model for instantaneous integer ambiguity resolution and positioning. Various combinations of GLONASS signals are considered including FDMA L1, FDMA+CDMA L1+L3, FDMA L1+L2 and FDMA+CDMA L1+L2+L3. To provide insight into the current RTK performance of GLONASS, we used observations of a short baseline to analyze the integer ambiguity resolution success rate and positioning precision, formally and empirically. To provide insight into the future RTK performance of GLONASS, we present a formal analysis of the integer ambiguity resolution success rate and ADOP, assuming that all the GLONASS satellites transmit FDMA L1, L2 and CDMA L3 signals. A forma...

Current generation mobile communications require high-quality services. Adopting multiple access (MA) and multi-carrier waveforms potentially enhances the quality of services offered to endusers. However, in the majority of literature, the integration of multi-carrier and multiple-access approaches have not been extensively examined. One possible solution is to review multiple access and multi-carrier waveforms simultaneously to create a favorable foundation for the integration of these schemes. Thus, we consider a comprehensive review of multiple-access systems and multi-carrier waveforms jointly from 1 st to 5 th -generation (1G-5G) cellular networks. Initially, we present orthogonal MA (OMA) schemes called: frequency division multiple access (FDMA), time division multiple access (TDMA), code division multiple access (CDMA), and orthogonal frequency division multiple access (OFDMA) that have been utilized in 1G, 2G, 3G, and 4G, respectively. In addition, 5G wireless non-orthogonal multiple access (NOMA) techniques such as power domain NOMA (PD-NOMA), code domain NOMA (CD-NOMA), and other NOMA multiplexing methods are addressed in detail. On the other hand, we glanced at 5G cellular multi-carrier waveforms such as filter bank multi-carrier (FBMC), universal filtered multi-carrier (UFMC), generalized frequency division multiplexing (GFDM), and filtered orthogonal frequency division multiplexing (f-OFDM) waveforms. The assessment and comparison between different OMA, NOMA, and multi-carrier waveforms are carried out with the parameters: modulation schemes, bit error rate (BER), signal to noise ration (SNR), sum rate, peak-to-average power ratio (PAPR), latency, out of band emission (OOBE), and complexity. The analytical formulas of OMA, NOMA, and multi-carrier waveform schemes are also derived and verified using simulation data. Each multiple access strategy's merits, shortcomings, applications, and factors influencing its performance are also addressed. Eventually, possible recommendations for the integration of multiple-access and modulation technologies for next-generation mobile networks are also included.

This paper provides a simple strategy to reduce peak-to-average power-ratio (PAPR) of single-carrier frequency-division multiple-access interleave-division multiple-access (SC-FDMA-IDMA) scheme employed in underwater acoustic (UWA) communications. The attributes that are attractive for UWA communications are power and bandwidth. High-order modulations are used to make SC-FDMA-IDMA more bandwidth efficient. But as we move towards high-order modulations PAPR of localised SC-FDMA-IDMA signal becomes high. Presented technique aims to further reduce PAPR of SC-FDMA-IDMA signals. Transmitter structure of the presented technique employs random interleaver to differentiate users and power interleaver to generate multiple interleaved sequences for the same user. All power interleaved data sequences are compared and the sequence with minimum PAPR is selected for transmission. Performance has been analysed by PAPR simulations and BER (Bit Error Rate) results in acoustic environment. Analysis confirms the merits of our technique over other competing alternatives in terms of performance, complexity and spectral efficiency.

One limiting issue in implementing high-speed wireless systems is the impairment associated with analog processing due to component imperfections. In uplink transmission of multiuser systems, a major source of such impairment is IQI introduced at multiple transmitters. In this paper, we deal with OFDMA and SC-FDMA which have received attention in recent years as physical layer protocol in WiMAX and 3GPP LTE, and analyze the effect of the transmitter (Tx) IQ imbalances on OFDMA and SC-FDMA receivers. To cope with the inter-user interference problem due to Tx IQ imbalances, we propose a widely linear receiver for OFDMA and SC-FDMA systems and also propose a novel subcarrier allocation scheme, which has high tolerance to such Tx IQ distortion.

System-level simulations of packet-based orthogonal frequency-division multiple access (OFDMA) cellular networks require a proper consideration of the channel-dependent fast fading/scheduling in order to produce meaningful results. However, generating the time and frequency selective fast fading channels and performing detailed channel-adaptive scheduling for each user in a multi-cellular system is computationally too expensive in most system-level simulations. To this end, we propose an efficient mapping technique that allows to accurately characterize the performance of the fast-scheduling system using two parameters: the average effective post-scheduling SINR gain and the ratio of allocated resources, both conditioned on the SINR and the velocity of a user in a scenario defined by the number of co-scheduled users. To derive these parameters for different numbers of users at different velocities and SINR levels, we perform detailed link level simulations including fast scheduling for the downlink of a Long Term Evolution (LTE) system.