Fast Frequency Hopping Code Division Research Articles

Spectral efficiency of a non‐orthogonal FH‐CDMA system

Fast frequency hopping code division multiple access (FH-CDMA) is a low complexity option for multiuser communication in fast, frequency selective fading channels such as in mobile mmWave communications. The original, orthogonal FH-CDMA has low spectral efficiency and up to now lacks a non-orthogonal version. In this work, a soft input soft output, non-orthogonal multiuser detector is designed using factor graphs. Extrinsic Information Transfer (EXIT) chart analysis of this detector allows determination of which multiple access code length maximises system spectral efficiency. Results show that when the access code length is reduced to an ideal length, spectral efficiency is maximised. Another conclusion is that maximum spectral efficiency is similar in value to a system with no multiple access method.

Equalization advantages of OFFH-CDMA over WDM in EDFAs

The gain of an erbium-doped fiber amplifier (EDFA) is in general wavelength-dependent, leading to different amplification levels among wavelength-division-multiplexing (WDM) channels. The use of optical fast frequency-hopped code-division multiple-access (OFFH-CDMA) offers a natural diversity of wavelength, and therefore gain, thus eliminating the requirement of an equalization stage to achieve flat gain on all channels. Computer simulations are developed to analyze and compare the EDFA equalization performance of these two transmission approaches (code-division multiplexing and WDM), especially in situations where the EDFA operating point is subject to change (e.g., channel add-drop, packet-switching environment). We find that WDM requires an equalization stage that fails when traffic characteristics shift, whereas, OFFH-CDMA presents the advantage of more robust, hence truly dynamic, gain equalization.

A Cost-Effective Fast Frequency-Hopped Code-Division Multiple-Access Light Source Using Self-Seeded Fabry–PÉrot Laser With Fiber Bragg Grating Array

We propose a novel light source for the optical fast frequency-hopped code-division multiple-access scheme using a self-seeded gain-switched Fabry-Pe/spl acute/rot laser diode (FP-LD) with the reconfigurable fiber Bragg grating array (FBGA). The embedded FBGA in the external cavity loop encodes the input pulses both temporally and spectrally. Before feeding back into the FP-LD, the encoded optical pulses propagate the FBGA with reverse direction and are again temporally combined. With the corporation of the variable fiber delay line, the limitation of the pulse rate by the fixed external cavity length is removed and makes the scheme more flexible. The output with sidemode suppression ratio of more than 20 dB has been obtained. With three fiber Bragg gratings on the FBGA, we demonstrate the encoded chip rate of 1.425 GHz under the driving radio frequency of 475 MHz. It is proved that a different codeword can be generated by simply reconfiguring the encoder without changing the rest of the experimental setup.

Multimedia applications using optical FFH‐CDMA communication systems

AbstractIn this Letter multimedia transmission in an optical‐fiber network using a fast frequency hopping–code division multiple access scheme is discussed. The multirate one coincidence variable length frequency hop patterns have been generated and their auto‐ and cross‐correlation properties are studied. A series of multiple Bragg gratings in a single optical fiber has been proposed to implement the encoder and decoder. Bit‐error‐rate analysis assuming Gaussian approximation for the error probability has been carried out. The results of the numerical analysis reveal that this system can support up to 30 simultaneous users with two different data rates (e.g., 500 Mbps and 2 Gbps). It can be concluded from the results that the fast frequency hopping–optical code division multiple access system is capable of supporting multimedia applications. © 2002 Wiley Periodicals, Inc. Microwave Opt Technol Lett 34: 259–263, 2002; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.10432

Robust optical FFH-CDMA communications: coding in place of frequency and temperature controls

In wavelength division multiplexing (WDM), transmitters require stringent and complex frequency control loops to avoid wavelength drifts due to temperature fluctuations. This makes the transmitters heavy, bulky, and inappropriate for local- and short-haul communications networks, as well as for manufacturing locales and other open areas where temperature control is not feasible. We propose and analyze a technique we call robust fast frequency hopping code division multiple access (FFH-CDMA), particularly suitable for severe, hostile, noncontrollable environments. This approach avoids all conditioning and frequency stabilization loops in the transmission end. We develop a modified version of extended hyperbolic congruence codes to achieve environment-resistant codes. We present expressions for the auto- and cross-correlation functions for optical implementation of the codes. We simulate the encoding/decoding operations with parameters from real Bragg gratings. We evaluate probability of error for a single user and as an average over all users versus capacity (the number of simultaneous users). Robust FFH-CDMA is an efficient access technique for hostile environments. It avoids the frequency and temperature control problems of WDM and nonrobust FFH-CDMA at the cost of lower overall capacity in terms of number of simultaneous users.

Asynchronous fast frequency-hopping CDMA cellular systems

We address the problem of constructing frequency-hopping codes for use in a fast frequency-hopping code-division multiple-access cellular system with a K-cell frequency-reuse scheme. Different hopping codes are used in different cochannel cells to reduce the cochannel interference. The frequency-hopping patterns possess ideal auto- and cross-correlation properties for use in the intracells and ideal autocorrelation and nearly ideal cross-correlation properties in the cochannel intercells. Additionally the cardinalities (the number of patterns) of the codes for use in the intracells meets the bound and hence are optimal. The maximum number of users which can be accommodated by using the proposed scheme is 20% more than the scheme using the same frequency-hopping code in all cells under a carrier-to-interference ratio of -15 dB.

Forward link capacity limit of coded FFH/CDMA multiuser mobile radios

The theoretical limit of the forward link (base to mobile) user capacity of fast frequency hopping code division multiple access (FFH/CDMA) mobile communication systems with error correction coding is investigated. The channel cutoff rate of coded FFH/CDMA channels is calculated and the optimal code rate that maximises the user capacity is determined. It is shown that with deletion-free transmission, the frequency efficiency of an FFH/CDMA channel coded at the optimal code rate is 47% higher than without coding.