Subcarrier Frequency Research Articles

Throughput Performance Analysis of AMC Based on a New SNR Estimation Algorithm Using Preamble

The rapid growth in mobile communication users necessitates the development of reliable communication systems that provide higher data rates. To meet these requirements, techniques such as multiple input multiple output (MIMO) and orthogonal frequency division multiplexing (OFDM) have been developed in recent years. Current research activity is focused on developing MIMO-OFDM systems that combine the benefits of both techniques. In addition, for a fast wireless channel environment, the data rate and reliability can be optimized by setting the modulation and coding adaptively according to the channel conditions, as well as by using sub-carrier frequency and power allocation techniques. The overall system performance depends on how accurately the feedback-based system obtains the channel state information and feeds it back to the transmitter without delay. In this paper, we propose a signal-to-noise ratio (SNR) estimation algorithm in which the preamble is known for both sides of the transceiver. Also, we applied AMC on several channel environments using the parameters of IEEE 802.11n and compared throughput performance using each of the different SNR Estimation Algorithm. The results obtained prove that our proposed algorithm is more accurate than traditional algorithms.

Sensitivity improvement and carrier power reduction in direct-detection optical OFDM systems by subcarrier pairing

This paper introduces subcarrier pairing to optical OFDM systems and shows, using simulations, that the sensitivity of Direct-Detection Optical Orthogonal Frequency Division Multiplexed (DDO-OFDM) systems can be improved by 0.7 dB, without any coding overheads. Subcarrier pairing works because each subcarrier acquires a different electrical Signal to Interference plus Noise Ratio (SINR), which typically increases with the subcarrier's frequency. Pairing the good and bad subcarriers, so that information is split between them, improves the performance of the bad subcarrier more than it degrades the performance of the good subcarrier. This lowers the required Optical Signal to Noise Ratio (OSNR) for the system to give a certain Bit Error Ratio (BER).

Analytical Evaluation of the Performance of Proportional Fair Scheduling in OFDMA‐Based Wireless Systems

This paper provides an analytical evaluation of the performance of proportional fair (PF) scheduling in Orthogonal Frequency‐Division Multiple Access (OFDMA) wireless systems. OFDMA represents a promising multiple access scheme for transmission over wireless channels, as it combines the orthogonal frequency division multiplexing (OFDM) modulation and subcarrier allocation. On the other hand, the PF scheduling is an efficient resource allocation scheme with good fairness characteristics. Consequently, OFDMA with PF scheduling represents an attractive solution to deliver high data rate services to multiple users simultaneously with a high degree of fairness. We investigate a two‐dimensional (time slot and frequency subcarrier) PF scheduling algorithm for OFDMA systems and evaluate its performance analytically and by simulations. We derive approximate closed‐form expressions for the average throughput, throughput fairness index, and packet delay. Computer simulations are used for verification. The analytical results agree well with the results from simulations, which show the good accuracy of the analytical expressions.

Performance Improvement of OFDM Using Subcarrier Frequency Diversity

Orthogonal Frequency Division Multiplexing is a key contender for broadcasting standards, due to its immunity towards multipath fading and ease of implementation. Here a novel scheme to exploit the frequency diversity within OFDM subcarriers has been proposed which further increases the bit error rate performance. The diversity is achieved by circularly shifting the OFDM subcarrier coefficients in pseudorandom manner. Thus we notice a significant performance improvement without any additional bandwidth requirements.

Efficient Channel Delay Estimation for OFDM Systems over Doubly-Selective Fading Channels

In this paper, we propose an efficient channel delay estimation method for orthogonal frequency-division multiplexing (OFDM) systems, especially over doubly-selective fading channels which are selective in both the symbol time domain and subcarrier frequency domain. For the doubly-selective fading channels in single frequency network (SFN), long and strong echoes exist and thus the conventional discrete Fourier Transform (DFT) based channel delay estimation system often fails to produce the exact channel delay profile. Based on the analysis of the discrete-time frequency response of the channel impulse response (CIR) coefficients in the DFT-based channel delay estimation system, we develop a method to effectively extract the true CIR from the aliased signals by employing a simple narrow-band low-pass filter (NB-LPF). The performance of the proposed system is verified using the COST207 TU6 SFN channel model.

Fast S-Parameter Calculation Technique for Multi-Antenna System Using Temporal-Spectral Orthogonality for FDTD Method

This paper proposes an S-parameter analysis method that uses simultaneous excitation for multi-antenna systems. In this method, OFDM (Orthogonal Frequency Division Multiplexing) and CI (Carrier Interferometry) pulse generation schemes are employed for maintaining the orthogonality among the excited signals. In OFDM excitation schemes, the characteristics of the neighboring antennas can be calculated by assigning different frequency subcarriers exclusively. CI enables the simultaneous verification of the antennas distant enough since this method can provide temporal orthogonality. Combining these two methods yields the simultaneous analyses of array antennas with both narrow and wide element spacing. The simulation of a 2×2 multi-antenna shows that the results of the proposed method agree well with those of the conventional method even though its computation speed is more 4 times that of the conventional method.

Generation of full C-band coherent and frequency-lock multi-carriers by using recirculating frequency shifter loops based on phase modulator with external injection

We demonstrate the generation of full C-band coherent and frequency-lock multi-carriers by using two recirculating frequency shifter (RFS) EDFA loops based on phase modulator with external injection-seeding. In our proposed novel scheme, only one phase modulator is used in each loop and two loops are used to generate long wavelength and short wavelength subcarriers, respectively. Full C-band with totally 178 subcarriers and 26 GHz subcarrier frequency spacing covering nearly 36.66 nm from 1527.54 to 1564.20 nm are obtained. The performance of 178 subcarriers with superior flatness less than 3dB and tone-to-noise ratio (TNR) larger than 20 dB after a wavelength selective switch (WSS) shows that this novel scheme is a potential technique for the future 10Tb/s optical communication.

A Fully-Integrated, Short-Range, Low Data Rate FM-UWB Transmitter in 90 nm CMOS

This paper presents a fully-integrated 3-5 GHz-band FM-UWB transmitter implemented in 90 nm bulk CMOS. The front-end consists of an RF current-controlled oscillator (RF-ICO) and class-AB power amplifier. Transmit data modulates a sub-carrier oscillator. The 2-FSK modulated output is amplified by a transconductor, and directly modulates the RF-ICO tune input. A successive approximation register (SAR) algorithm and on-chip all-digital frequency-locked loop (FLL) calibrate the carrier and sub-carrier frequencies. All voltage and current references required by the transmitter are included on-chip. The 0.2 × 0.5 mm2 active area transmitter consumes 900 μW from a 1 V supply. Energy efficiency of the transmitter is 9 nJ/bit running continuously at 100 kbits/s.

Exact Closed-Form BER Analysis of OFDM Systems in the Presence of IQ Imbalances and ICSI

In this paper, we derive an exact closed-form expression for the bit error rate (BER) of orthogonal frequency division multiplexing (OFDM) systems impaired by transmit (TX) and receive (RX) in-phase/quadrature (IQ) imbalances and imperfect channel state information (ICSI), in Rayleigh fading channels. Since the decision metric in this scenario can be expressed as a quadratic form D in Gaussian complex-valued non-circularly symmetric random variables (RVs) with zero mean, we provide a general expression for the calculation of Pr{D <; 0}. In addition, two approximate closed-form expressions for the BER are obtained, which are demonstrated to be extremely accurate for practical values of IQ imbalances. Results show that TX IQ imbalance is in general less detrimental than RX IQ imbalance, and confirm that the correlation between the channel response at the observed subcarrier frequency and the mirror subcarrier has a significant impact in the BER.

The Transporting Property of Hybrid SCM/Ethernet PON

Abstract SCM/Ethernet PON is the hybrid PON, which two wavelengths transport downstream services simultaneously. With the high rate of Ethernet PON and the high subcarrier frequency of SCM PON, the two down-stream wavelengths of hybrid PON are interactional seriously. This paper mainly analysis the impacts of Cross-power Modulation and Dispersion in SCM/Ethernet PON, rewards the relationship of BER and the modulation frequency, modulation index, and the average optical power.

Multi-Wavelength WiMAX–PONs With Overlapping Cells

An access network architecture integrating standard wireless signal formats over multi-wavelength splitter passive optical networks (PONs) based on radio-over-fiber (RoF) technology is demonstrated. Frequency division multiplexing (FDM) is applied to address individual base stations sharing a single wavelength. The extended wavelength band overlay overcomes the need for complex dispersion compensation techniques as it avoids the use of high frequency subcarriers in the FDM window. Also, it achieves enhanced network scalability compared with previously reported converged architectures as well as reduced component cost. Significantly, extended features of wireless networks have been presented through the application of overlapping cells to overcome the wireless spectrum congestion beyond current deployment scenarios and provide resilience for legacy PONs. To evaluate the network transmission characteristics, multiple IEEE802.16d channels, with 70 Mbits/s downstream and 40 Mbits/s upstream data rates, are frequency shifted around the same RF carrier and transmitted on two different wavelengths. The obtained results have demonstrated error vector magnitudes (EVMs) of -31 dB at optical network unit/base-station (ONU/BS) remote antenna inputs and minimum 10-4 bit error rates (BERs) bidirectionally over combined 20 km and faded 330 m overlapping micro-cell circumferences without any error coding or relay techniques. The application of a low-cost long-wavelength vertical cavity surface emitting laser (VCSEL) array in upstream for colorless transmission demonstrated 0.7 dB power penalty to the received worldwide interoperability for microwave access (WiMAX) channels in the optical line terminal (OLT).

Balanced Frequency Reuse with Ordering and Directional Subcarrier Allocation in OFDMA Systems

In multi-cell OFDMA-based networks, co-channel interference (CCI) is inevitable when the frequency reuse scheme is used. The CCI affects the performance of users, especially that of cell edge users. Several frequency reuse schemes and subcarrier allocation algorithms have been proposed to solve the CCI problem. Nevertheless, it is difficult to improve both the cell capacity and the performance of cell edge users since they have a trade-off. In this paper, we propose a new balanced frequency reuse (BFR) as a new frequency partitioning scheme that gives more power to the users in the outer region and allocates more subcarriers to the users in the inner region. In addition, we propose ordering and directional subcarrier allocation (ODSA) for our frequency partitioning proposal to mitigate the CCI effectively when cells have heterogeneous traffic loads. The performance of the proposed BFR with the ODSA algorithm is investigated via analyses and simulations. Performance evaluation shows that the proposed BFR with the ODSA algorithm can increase both the spectral efficiency and the performance of cell edge users if the transmission power is appropriately handled.

Combining Illumination Dimming Based on Pulse-Width Modulation With Visible-Light Communications Based on Discrete Multitone

In the field of indoor wireless networks, visible-light communications is garnering increasing attention. One of the type of emitters used in this technology is white light-emitting diodes, which can synergistically provide both illumination and data transmission. Discrete multitone modulation is attractive for visible-light communications. One of the issues to be addressed in these synergetic use cases is how to incorporate light dimming while not corrupting the communication link. In this paper, the performance of a visible-light communication system combining pulse-width modulation for dimming and discrete multitone for data transmission was investigated. Performance indicators were addressed, i.e., the signal-to-interference ratio due to dimming and the achievable bit-error ratio in the absence of additional noise. By aid of simulations it was shown that practical communication is only feasible when the line rate of the dimming modulation is at least twice the frequency assigned to the largest multitone subcarrier frequency. The results demonstrate that under this constraint and when using a suitably modified demodulation scheme, dimming does not influence the data transmission.

A novel medium access control protocol for passive optical network supporting local area networking capabilities

An alternate solution for Ethernet passive optical networks (EPON) providing local area networking (LAN) capabilities is proposed in this article. Our solution adopts a star coupler-based PON architecture and uses radio frequency subcarrier multiplexed transmission for the LAN traffic delivery. The proposed medium access control (MAC) protocol supports a fully distributed control plane among the optical network units (ONUs) for ONU---ONU communication as well as upstream access to the OLT. The simulation results indicate that the proposed MAC protocol outperforms the others in terms of the average packet end-to-end delay, especially for LAN traffic.

Optically Injected Semiconductor Laser for Photonic Microwave Frequency Mixing in Radio-Over-Fiber

Nonlinear dynamics of an optically injected semiconductor laser for microwave frequency mixing are experimentally investigated. Through optical injection, the laser is driven into the nonlinear period-one (P1) dynamical state and acts as a broadly tunable photonic microwave oscillator. With an external current modulation at an intermediate frequency, the P1 oscillation is modulated accordingly, thus achieving frequency up-conversion beyond the laser bandwidth. Using a 2.5-Gbps-grade laser, binary phase-shift keying (BPSK) data signal is up-converted from an intermediate frequency of 2 GHz to a subcarrier frequency as high as 20 GHz. While the direct modulation bandwidth limits the intermediate frequency, it does not limit the subcarrier frequency. The approach uses only low-cost lasers and requires no high-speed electronic mixers.

Performance enhancement of OFDM-SQ2AM in distorted channel environments

In this letter, a new pulse shaped OFDM-Superposed Quadrature Quadrature Amplitude Modulation (OFDM-SQ2AM) system is proposed to reduce the inter-channel interference (ICI) as well as to maintain the orthogonalities between the OFDM subcarriers in the presence of the subcarrier frequency offset. Time and frequency dispersion properties of the proposed SQ2AM pulse shape are analyzed using the Heisenberg parameter and in-band energy function. Such properties of the proposed pulse are compared with those of a rectangular pulse and IOTA pulse. The BER performance of the pulse shaped OFDM-SQ2AM is compared with those of conventional OFDM-QAM and IOTA/OFDM-OQAM in the 3GPP ITU-R environment. Our simulation results indicate that the proposed OFDM-SQ2AM achieves higher spectral efficiency and power efficiency as compared to the other schemes and gives better robustness of the frequency dispersion.

Double-locked semiconductor laser for radio-over-fiber uplink transmission

The nonlinear dynamics of an optically injected semiconductor laser are explored for radio-over-fiber uplink transmission. Under optical injection locking, the laser at the base station is operated in the period-one oscillation state, where its intensity oscillates at a tunable microwave frequency. When the oscillation is tuned to the subcarrier frequency, it is further locked by the uplink microwave signal. By simply using an ordinary 2.5-Gbps-grade semiconductor laser, uplink transmission of the phase-shift keying (PSK) signal at a subcarrier of 16 GHz with bit-error rate of less than 10(-11) is demonstrated experimentally. Microwave PSK to optical PSK is achieved at the double-locked laser, which allows all-optical demodulation without any high-speed microwave electronics.

Anti-collision backscatter sensor networks

Sensor collision (interference) is studied in a large network of low bit-rate sensors that communicate via backscatter, i.e. modulate the reflection of a common carrier transmitted by a central reader. Closed-form analysis is provided, quantifying sensor collision (interference) in high-density, backscatter sensor networks (BSN), as a function of number of tags and aggregate bandwidth. Analysis is applicable to a broad class of sensor subcarrier modulations, propagation environments and reader antenna directivity patterns. It is discovered that anti-collision performance in high-density backscatter sensor networks is feasible provided that appropriate modulation is used at each sensor. That is due to the round-trip nature of backscatter communication as well as the extended target range, which both impose stringent requirements on spectrum efficiency, not easily met by all modulations. Furthermore, aggregate bandwidth savings for given anti-collision performance are quantified, when simple division techniques on subcarrier (modulating) frequency and space (via moderately directive hub antenna) are combined.

The effects of spatial diversity and imperfect channel estimation on wideband MC-DS-CDMA and MC-CDMA

In our previous work, we compared the theoretical bit error rates of multi-carrier direct sequence code division multiple access (MC-DS-CDMA) and multi-carrier code division multiple access (MC-CDMA). To ensure a fair comparison, we constrained both schemes to the same bandwidth, information rate, and energy-per-bit, and these constraints resulted in a possible trade-off between diversity gain and channel estimation errors between the two schemes. While only a single-input single-output (SISO) system was considered in our previous work, in this paper, we extend the comparison to a multiple-input multiple-output (MIMO) system which employs Alamouti space-time block coding at each sub-carrier frequency to achieve spatial diversity. We consider only those cases where MC-CDMA has higher frequency diversity than MC-DS-CDMA. Since increases in diversity yield diminishing gains, we conclude that the addition of spatial diversity to this multi-carrier comparison benefits MC-DS-CDMA more than MC-CDMA. To determine whether these gains for MC-DS-CDMA are enough to offset the difference in frequency diversity between the two schemes, we derive closed-form expressions for the bit error probabilities of both schemes, and we compare the MIMO results against those of the SISO system for different information rates, number of users, and number of pilot symbols per channel estimate.

A Flexible DSP Architecture for MIMO Sphere Decoding

This paper presents the architecture and circuit design of a sphere decoder for agile multi-input multi-output (MIMO) communication systems. Algorithm and architecture co-design is used to reduce hardware complexity, which enables the proposed sphere decoder to support larger antenna-array sizes and higher order modulations. The proposed architecture is also capable of processing multiple frequency subcarriers for orthogonal frequency-division multiplexing (OFDM) based systems. A 20 times area reduction is achieved, even without interleaving of subcarriers compared to the direct-mapped architecture. The sphere decoder supports multiple configurations: antenna arrays from 2 times 2 to 16 times 16, constellation sizes from binary phase-shift keying (BPSK) to 64-QAM (quadrature-amplitude modulation), and 16-128 subcarriers. The peak estimated data rate exceeds 1.5 Gbits/s of ideal throughput in a 16-MHz bandwidth. The core area is estimated at 0.31 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> in a standard 90-nm CMOS technology. The estimated power consumption is 33 mW in the 16 times 16 64-QAM mode at 256 MHz from a 1-V supply voltage.