Subcarrier Frequency Research Articles

A Multicarrier Multiplexing Method for Very Wide Bandwidth Transmission

The multicarrier orthogonal code division multiplexing (MC-OCDM) introduced here has been designed for very wide bandwidth (VWB) point-to-point and point-to-multipoint transmission. In order to meet VWB transmission requirements, the MC-OCDM design has two components, the basic and the composite. The basic MC-OCDM is a generalized form of the standard orthogonal frequency division multiplexing (OFDM). It has the property of distributing the power of each transmitted symbol into all subcarrier frequencies. Each subcarrier will then carry all transmitted symbols which are distinguished by orthogonal Hadamard sequences. The resulting system is shown to improve the performance of OFDM by introducing frequency and time diversity. As shown, by both analysis and simulation, the basic MC-OCDM combats the effects of narrowband interference (NBI). In particular, the simulation results show that the BER performance of the basic MC-OCDM in the presence of NBI is better than OFDM for both coded and uncoded systems. Furthermore, the composite MC-OCDM is a method of orthogonal frequency division multiplexing (OFDM) basic MC-OCDM channels. This allows us to multiplex more than one basic MC-OCDM channel into a VWB transmission system which can have the performance and spectral efficiency required in fixed wireless transmission environments.

The effects of drive level errors on optical SSB system performance

This paper presents a system performance analysis of an optical single sideband system (OSSB) when drive level errors occur in the modulating signal driving the dual arm Mach-Zehnder modulator (MZM). The performance is evaluated in terms of the optical sideband suppression, frequency response and power penalty measurements. Experiments are carried out and a mathematical model of the OSSB system incorporating the drive level error is developed. The theoretical and experimental results suggest that the drive level error leads to reduced system performance as a result of poor received subcarrier power. The overall frequency response of the system, which is affected by drive level errors, accumulated CD and subcarrier frequency, has a direct influence over the power penalty. However, reduction in power penalty slope is observed at peaks of the OSSB frequency response.

The effects of phase errors on optical SSB system performance

The authors present a system performance analysis of an optical single sideband system (OSSB) when phase errors occur in the modulating signal driving the dual-arm Mach–Zehnder modulator (MZM). The performance is evaluated in terms of the optical sideband suppression, received subcarrier power, frequency response, and power-penalty measurements. Experiments were carried out and a mathematical model of the OSSB system, incorporating the phase error, was developed. Both the theoretical and experimental results show that, although optical sideband suppression reduces with phase errors, the received subcarrier power can increase as high as 3 dB for positive phase errors. However, for transmission over longer fiber, the frequency response of the OSSB system is not only dependent on the phase error, but also on the phase shift due to chromatic dispersion (CD). The power penalty was found to be dependent on the frequency response of the OSSB system. At any frequency-response null point, the power-penalty slope is reversed as the received subcarrier power is reduced. © 2006 Wiley Periodicals, Inc. Microwave Opt Technol Lett 48: 1574–1578, 2006; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.21724

In-band 16-QAM and multi-carrier SCM modulation to label DPSK payload signals for IP packet routing

We present an experimental demonstration of the feasibility of in-band subcarrier multiplexing (SCM) for labeling of differential phase shift keying (DPSK) payload signals. We show that by proper selection of the value of the subcarrier frequency the effect of the superimposed SCM label on the performance of the DPSK signal is minimized. Furthermore, we show experimentally the advantages of using alternative modulation formats such as 16-QAM and multi-carrier SCM for optical labeling of a 10 Gb/s DPSK payload signal.

All-optical subcarrier frequency conversion using an electrooptic phase modulator

All-optical subcarrier frequency conversion using an electrooptic phase modulator in a radio-over-fiber link is proposed and demonstrated. The frequency mixing is implemented by an electrooptic phase modulator, to which a local oscillator frequency is applied. The phase modulation to intensity modulation conversion is realized by using a length of single-mode fiber, which serves as a dispersive device. A subcarrier frequency conversion from 3 to 11.5 GHz performed over a 25-km radio-over-fiber link is experimentally demonstrated.

Novel schemes for local area network emulation in passive optical networks with RF subcarrier multiplexed customer traffic

This paper proposes two novel optical layer schemes for intercommunication between customers in a passive optical network (PON). The proposed schemes use radio frequency (RF) subcarrier multiplexed transmission for intercommunication between customers in conjunction with upstream access to the central office (CO) at baseband. One scheme employs a narrowband fiber Bragg grating (FBG) placed close to the star coupler in the feeder fiber of the PON, while the other uses an additional short-length distribution fiber from the star coupler to each customer unit for the redirection of customer traffic. In both schemes, only one optical transmitter is required at each optical network unit (ONU) for the transmission of customer traffic and upstream access traffic. Moreover, downstream bandwidth is not consumed by customer traffic unlike in previously reported techniques. The authors experimentally verify the feasibility of both schemes with 1.25 Gb/s upstream baseband transmission to the CO and 155 Mb/s customer data transmission on the RF carrier. The experimental results obtained from both schemes are compared, and the power budgets are calculated to analyze the scalability of each scheme. Further, the proposed schemes were discussed in terms of upgradability of the transmission bit rates for the upstream access traffic, bandwidth requirements at the customer premises, dispersion tolerance, and stability issues for the practical implementations of the network.

Performance of Fractionally Spread Multicarrier CDMA in AWGN as Well as Slow and Fast Nakagami-<tex>$m$</tex>Fading Channels

In multicarrier code-division multiple-access (MC-CDMA), the total system bandwidth is divided into a number of subbands, where each subband may use direct-sequence (DS) spreading and each subband signal is transmitted using a subcarrier frequency. In this paper, we divide the symbol duration into a number of fractional subsymbol durations also referred to here as fractions, in a manner analogous to subbands in MC-CDMA systems. In the proposed MC-CDMA scheme, the data streams are spread at both the symbol-fraction level and at the chip level by the transmitter, and hence the proposed scheme is referred to as the fractionally spread MC-CDMA arrangement, or FS MC-CDMA. Furthermore, the FS MC-CDMA signal is additionally spread in the frequency (F)-domain using a spreading code with the aid of a number of subcarriers. In comparison to conventional MC-CDMA schemes, which are suitable for communications over frequency-selective fading channels, our study demonstrates that the proposed FS MC-CDMA is capable of efficiently exploiting both the frequency-selective and the time-selective characteristics of wireless channels.

Adaptive Subcarrier Block Modulation with Differentially Modulated Pilot Symbol Assistance for Downlink OFDM Using Uplink Delay Spread

In AMS/OFDM systems, base station is in control of the modulation level of each subcarrier, and then, adaptive modulated packet is transmitted from the base station to the mobile station. In this case, the mobile station is required the modulation level information (MLI) to demodulate the received packet. The MLI is generally transmitted as a data symbol, therefore, the throughput is degraded. In an OFDM, the channel response at a particular subcarrier frequency is not supposed to be totally different from its neighboring frequencies, and hence, they must have correlation which depends on the coherence bandwidth of the channel Bc. If we could assign the same modulation level for coherently faded subcarrier block, MLI is required only one time for each subcarrier block. Moreover, we can assign the data on the empty space of pilot signals for increasing the total transmission. In this paper, we propose an adaptive subcarrier block modulation with differentially modulated pilot symbol assistance for downlink OFDM using uplink delay spread.

Wavelength-controlled photonic true time delay for wide-band applications

A radio frequency subcarrier modulated wavelength-controlled dense wavelength-division-multiplexing-based photonic true time delay module, with very low phase (/spl plusmn/4/spl deg/) and magnitude (/spl plusmn/0.5 dB) ripples over a bandwidth of several gigahertz, is demonstrated to achieve wide-band linear frequency modulation transmission (600 MHz) with -37-dB sidelobe suppression and signal-to-noise exceeding 80 dB/MHz.

Wavelength reused upstream transmission scheme for WDM passive optical networks

A novel technique facilitating wavelength reused upstream transmission in a WDM passive optical network, eliminating wavelength source at ONUs, is proposed and experimentally demonstrated. A subcarrier frequency is used for downstream transmission and the pure optical carrier is filtered and reused for upstream transmission. The scheme requires only simple baseband receivers at both ONU and OLT terminals.

Blind Subcarrier Frequency Ambiguity Resolution for OFDM Signals Over Selective Channels

Many frequency-recovery algorithms for orthogonal frequency-division multiplexing systems are geared to recover offsets up to half the distance of the subcarriers. If the error is larger, an ambiguity of an integer number of subcarrier spacings is left. This paper develops a blind algorithm for ambiguity resolution that works effectively over frequency-selective radio channels. Simulation results show a sizable improvement over existing techniques, expecially at high signal-to-noise ratios.

Degree of polarization technique used in PMD compensation of optical microwave transmission systems

Polarization-mode dispersion (PMD) can severely degrade the performance of optical microwave transmission systems by inducing a periodic power fading of the received RF signal that depends on the subcarrier frequency and accumulated differential group delay (DGD) along fiber. We derive a compact analytical expression of the degree of polarization (DOP) of optical signal using Jones and Stokes representations based on first-order assumption. Using this expression, we quantify the signal DOP fading induced by PMD by means of numerical simulations for BPSK and ASK modulations. The dependences of signal DOP on subcarrier frequency, accumulated DGD, and modulation formats have been demonstrated. It is found that signal DOP has similar periodic fading with the power of received RF signal, which is caused by DGD. Moreover, if the DOP technique is used in the PMD compensation of the optical microwave transmission systems, the DOP degradation is more sensitive to the DGD in the system modulated by BPSK than by ASK. The performance of this technique is immune to residual chromatic dispersion of the fiber.

Optically Compensating the PMD-Induced RF Power Fading for Single-Sideband Subcarrier-Multiplexed Systems

We demonstrate a novel technique for optically compensating the PMD-induced RF power fading that occurs in single-sideband (SSB) subcarrier-multiplexed systems. By aligning the polarization states of the optical carrier and the SSB, RF power fading due to all orders of PMD can be completely compensated. The 5% RF power fading tail is improved from 13.5 dB to <1.5 dB, as verified from both experimental measurement (300 samples) and computer simulation (10000 samples). This technique is independent of the DGD of the optical fiber link and the subcarrier frequency.

Crosstalk in WDM FM-SSM Optical Receiver utilizing Coherent Carrier Addition

In an optical WDM FM-SCM optical communication system, the crosstalk arising from adjacent optical channels in a non-ideal demultiplexer has been calculated. The adjacent channel crosstalk is found to depend on certain system parameters, like WDM channel separation, number of subcarrier channels multiplexed and conveyed by a single optical carrier, and the isolation factor of the demultiplexer. The analysis shows that when the optical channel separation is greater than five times the highest subcarrier frequency used, the system is free from adjacent channel crosstalk.

Computer‐aided analytical study of a novel de‐multiplexing scheme for SCM optical communication and an experimental scale‐model characterization

AbstractAnalytical study and computer‐aided characterization are conducted for a novel subcarrier demultiplexing scheme, in which a pair of light modulators excited at one of the subcarrier frequencies precede the optical detectors. The baseband signal included in one of the subcarriers is directly detected from the photodiodes. Immunity from subcarrier interference is an important advantage of the scheme. A low‐frequency scale model for this scheme has been experimentally investigated to characterize the performance of the proposed scheme. © 2003 Wiley Periodicals, Inc. Microwave Opt Technol Lett 37: 113–117, 2003; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.10840

PMD-insensitive SCM optical receiver using polarization diversity

Subcarrier multiplexing (SCM) optical systems with high subcarrier frequencies are susceptible to power fading caused by fiber polarization-mode dispersion (PMD). In this letter, an SCM optical receiver free of carrier fading is proposed and demonstrated using polarization diversity. Unlike conventional PMD compensators, this setup does not require a tunable optical delay line.

Dispersion division multiplexing in subcarrier modulated data transmission using channel-specific RF fading

We demonstrate a novel all-optical multiplexing technique for doubling the usable spectral bandwidth and number of channels in subcarrier-modulated data transmission over optical fiber. By taking advantage of the chromatic-dispersion-induced RF power fading of a given subcarrier frequency, we transmit two subcarrier channels that occupy the same frequency space. This is accomplished by designing the dispersion of each subcarrier channel in a system such that one subcarrier channel can be >99% faded at the same time that another subcarrier can be recovered. We have successfully transmitted and received two 155-Mb/s data channels that are both located at 7.1 GHz and on the same optical carrier wavelength.

A novel filter bank for biotelemetry.

In a multichannel biotelemetry system, signals taken from a patient are distributed along the available frequency range (bandwidth) of the system through frequency-division-multiplexing, and combined into a single composite signal. Biological signals that are limited to low frequencies (below 10 Hz) modulate the frequencies of respective sub-carriers. Other biological signals are carried in amplitude-modulated forms. It is recognized that recovering original signals from a composite signal at the receiver side is a technical challenge when a telemetry system with narrow bandwidth capacity is used, since such a system leaves little frequency spacing between information channels. A filter bank is therefore utilized for recovering biological signals that are transmitted. The filter bank contains filter units comprising switched-capacitor filter integrated circuits. The filters have two distinct and opposing outputs (band-stop (notch) and band-pass). Since most biological signals are at low frequencies, and modulated signals occupy a narrow band around the carrier, notch filters can be used to efficiently stop signals in the narrow frequency range. Once the interim channels are removed, other channels become well separated from each other, and band-pass filters can select them. In the proposed system, efficient filtering of closely packed channels is achieved, with low interference, from neighboring channels. The filter bank is applied to a system that carries four biological signals and a battery status indicator signal. Experimental results reinforce theoretical predictions that the filter bank successfully de-multiplexes closely packed information channels with low crosstalk between them. It is concluded that the proposed filter bank allows utilization of cost-effective multichannel biotelemetry systems that are designed around commercial audio devices, and that it can be readily adapted to a broad range of physiological recording requirements.

Statistics of PMD-induced power fading for intensity-modulated double-sideband and single-sideband microwave and millimeter-wave signals

Polarization-mode dispersion (PMD) can severely degrade the performance of millimeter-wave fiber-optic links by inducing a power fading penalty of the received signal that is dependent on the subcarrier frequency and accumulated PMD along the fiber. We experimentally investigate the statistics of PMD-induced power fading as a function of the differential group delay (DGD) for intensity-modulated double- and single-sideband subcarrier-multiplexed signals in the absence of chromatic dispersion. We find a similar susceptibility to PMD-induced power fading for both modulation formats with a subcarrier frequency of 7 GHz using a PMD emulator with a Maxwellian distribution of DGD (average DGD /spl sim/40 ps). A significant improvement in the worst case power fading penalty (/spl sim/20 dB) is achieved by using an electronically controlled polarization controller in combination with a single section of polarization maintaining fiber in a dynamic first-order PMD compensator. Furthermore, the results of numerical Monte Carlo simulations support the measured data and show the scalability of PMD-induced power fading for subcarrier-multiplexed signals in the microwave and millimeter-wave region.

Performance of successive interference cancellation in convolutionally coded multicarrier DS/CDMA systems

This article presents a successive interference cancellation (SIC) scheme for a multicarrier (MC) asynchronous DS/CDMA system, wherein the output of a convolutional encoder modulates bandlimited spreading waveforms at different subcarrier frequencies. In every subband, the SIC receiver successively detects the interferers' signals and subtracts them from that of the user-of-interest. The SIC receiver employs maximal-ratio combining (SIC-MRC) for detection of the desired user, and feeds a soft decision Viterbi decoder. A comparison is made among SIC-MRC, matched filter detection with MRC (MF-MRC), and N-tap minimum mean-squared error (MMSE) receivers with optimal tap coefficients, assuming a slowly varying, frequency selective, Rayleigh fading channel, where N is the processing gain. Analysis and simulation results show that the SIC-MRC can obtain performance close to that of N-tap MMSE receivers, and both of them have better ability to suppress multiple-access interference (MAI) than does MF-MRC. Finally, with timing or phase tracking errors, the results show that SIC-MRC can still retain a performance advantage over MF-MRC.