An analytical framework for CDMA systems with a nonlinear amplifier and AWGN

Orthogonal frequency division multiplexing (OFDM) is a multi-carrier digital modulation technique that has been under extensive study and research in recent years, in terms of considering its various advantage in combating the severe effects of the frequency selective fading. However, the OFDM system suffers from the strong sensitivity to the nonlinear distortion, introduced by the transmitting high power amplifier (HPA), since the OFDM system has the high peak-to-average power ratio (PAPR). Clipping is the simple but infective method to low the high PAPR; however, it causes nonlinear distortion too. So in this paper, we propose a new approach to compensate the nonlinearity caused by clipping and HPA. Its performance is verified through computer simulations. Specifically, the enhanced performance in terms of the bit error rate (BER) is documented for both the traveling wave tube amplifier (TWTA) and the solid state power amplifier (SSPA).

OFDM (orthogonal frequency division multiplexing) is usefully applied for DAB (digital audio broadcasting) and DVB (digital video broadcasting) systems due to its high spectral efficiency and robustness to ISI (intersymbol interference) and multipath fading. However, the OFDM signal has a large PAPR (peak-to-average power ratio), which results in significant nonlinear distortion when it passes through an HPA (high power amplifier) such as a TWTA (traveling wave tube amplifier) and a SSPA (solid state power amplifier). We propose a new PAPR reduction method using soft clipping and filtering. Unlike conventional hard clipping, soft clipping has non-zero slope in the clip region. Next, filtering, which uses additional FFT and IFFT processes, is applied in order to reject the out-of-band clip noise. Although the filtering removes the out-of-band noise, it oppositely increases PAPR by a small amount. Since an HPA increases the power consumption according to the linearity range, soft clipping for an SSPA lowers power consumption and is more efficient than the conventional hard clipping method. By the results of the proposed method, the wanted PAPR can be obtained and there is no out-of-band radiation caused by the clipping process.

Improvement of the capacity of the CDMA (code division multiple access) system is a major objective. The capacity of the system is mainly limited by the signal to noise ratio, where the noise comes from the channel background and from MAI (multiple access interference). Usually, the way to improve the capacity is to decrease the MAI; this is why orthogonal codes are adopted by the IS-95 standard. One problem is that the number of orthogonal codes is constrained by the dimensionality of the signal space. So in IS-95, there are only 64 orthogonal codes available when the number chips for each sequence is 64. To solve this problem, a non-orthogonal code called WBE (Welch bound equality) sequences (Massey and Mittelholzer 1991) is used in this paper The criterion to construct the code is to minimize the MAI. Two methods are used to construct the WBE sequences: from a linear cyclic code or from a Hadamard matrix. In order to improve the performance of the WBE sequence set, an iteration receiver (Sari et al. 1999) is applied. The conclusion obtained is that when the number of users is slightly greater than the signal space dimensionality, after the first iteration, the performance is only marginally worse than the orthogonal CDMA system.

This paper presents a broadband predistortion technique using dual-gate FETs to linearize the Traveling Wave Tube Amplifiers (TWTA) and Solid State Power Amplifiers (SSPA) used in satellite transponders. The results for a linearized TWTA operating over 11.7-12.2 GHz and a linearized SSPA operating over 3.8-4.2 GHz are presented. The improvement in linearity has been achieved over a broad dynamic range of output power.

Due to the desire for the next generation of wireless communications systems to be able to support higher data rate, multimedia, traffic, it is of interest to determine if the use of a higher order alphabet size, in conjunction with code division multiple access (CDMA) signaling, will allow enhanced performance. We compare the performance of an asynchronous CDMA system when a 16-QAM alphabet is used with that of a CDMA system employing QPSK as the modulation format.

We propose the random Manchester codes (RMC) to improve the bit error probability (BEP) performance in both synchronous and asynchronous fiber-optic code-division multiple-access (CDMA) systems. The spreading sequences used in the synchronous and asynchronous systems are modified prime sequence codes and optical orthogonal codes (OOCs), respectively. Thermal noise, shot noise, and avalanche photodiode (APD) bulk and surface leakage currents are taken into consideration in the BEP analyzes. The results show that the proposed systems can support a larger number of simultaneous users than other systems with similar system complexity under the same bit-error probability constraint.

For high-speed code division multiple access (CDMA) systems, interchip interference that is caused by multipath propagation becomes severe and cannot be ignored. Together with the multiple access interference due to nonideal conditions in CDMA, they become major obstacles to overall system performance. In this paper, both batch and adaptive algorithms based on linear prediction are proposed for joint blind equalization and multiuser detection for asynchronous CDMA systems. It is shown that the new methods are insensitive to estimation error of propagation delay or chip timing. In addition, the adaptive algorithm is computationally efficient. Simulation results show that the proposed methods are also near-far resistant and compare favorably to many existing methods.

Orthogonal frequency division multiplexing (OFDM) system suffers from a high peak-to-average (power) ratio (PAR) which leads to power inefficiency in radio frequency section and large performance degradation of system due to the nonlinearity of high power amplifiers (HPA). In this paper, a new downlink transmission scheme is designed based on orthogonal periodic function (OPF) precoding to achieve low PAR and investigated for single user and multi-user multiple-input multiple-output wireless systems. It is shown (through mathematical analysis and computer simulations) that the proposed transmission scheme can provide a significant PAR reduction using decorrelation, phase-rotation and orthogonality properties of OPF precoding, while it does not change power spectral levels or error performances due to the unitary property when compared with the normal OFDM systems with linear HPA. The OPF precoding outperforms existing precoding schemes when nonlinear solid state power amplifier (SSPA) is considered with several practical values of input back-off (IBO). Computer simulations show that the OPF precoding can achieve 3.1 and 4 dB PAR gain, while 2.2 and 1 dB SNR gain for IEEE 802.11a and DVB-T systems respectively. Moreover, OPF precoding leads to a total degradation improvement from 0.13 to 1.53 dB for 16 QAM, considering an SSPA with IBO = 8---4 dB respectively. OPF precoding does not increase computational complexity when compared with the existing precoding methods. It is shown that the performance of the proposed precoding is independent of modulation schemes and can work for any number of subcarriers.

In the underwater passive positioning systems (UPPS), it involves a key technology of multi-user communication that the beacons synchronously send positioning messages to the targets. Interferences from other users, referred to multi-access interferences (MAIs), can induce significant bit errors for the desired user. In this paper, we designed an underwater acoustic Code Division Multiple Access (CDMA) system using the coded cyclic shift key (CCSK) modulation, and proposed a hybrid MAI cancellation scheme based on cyclic shift inverse matrix transform (CSIMT) and recursive least square (RLS) in the CDMA system. The hybrid scheme can suppress MAI better than RLS or CSIMT, and meets the requirements of strong stability to support reliable transmission for UPPS. The CSIMT interference cancellation performs well under the far-near effects, which doesn’t depend on the accuracy of the estimation of transmitted data, and the strong MAI can be reduced by transformation of the zero-forcing correlation values using m sequence cyclic shift inverse matrix. The RLS interference cancellation is used to mitigate MAI form the detected users with comparable power, by using the estimated the channel impulse response between the receiver and the detected users, and the regenerated signal of detected users form decoder output, which allows of strong error-correcting codes, and it’s robust in an asynchronous environment. The theoretical analysis and experimental result show that the hybrid MAI cancellation scheme can effectively suppress MAI better. The shallow water sea trial was carried out near Dalian Tiger Beach Ocean Park in China, at a distance of about 5 km, the data rates of 100bps, where three users could communicate asynchronously with low failure ratio and can match the UPPS application.

There has been long-standing debate regarding the comparative reliability of solid-state power amplifiers (SSPAs) versus traveling wave tube amplifiers (TWTAs) for the satellite downlink. High-voltage, thermionic, life-limited emitters operating in a high vacuum enclosure (i.e., a vacuum electronic device) contribute to the TWTA risks and reliability questions. Conventional wisdom espouses the SSPA as an inherently more reliable, solid-state device requiring only low voltages to amplify, with failure resulting in only graceful degradation. However, many trade studies on comparing SSPAs and TWTAs rely on transistor device life test data on the ground for SSPAs due to the lack of statistically significant data on orbit. It is expected that current, on-orbit statistics in the industry would shed relevant light on the debate.

This paper studies an orthogonal frequency division multiplexing (OFDM) - code division multiple access (CDMA) system in the frequency-selective Rayleigh fading channel with the nonlinear distortion caused by high power amplifier (HPA). The bit error rate (BER) has been derived which agrees well with the simulation results. Next, we present a new technique to reduce the large instantaneous envelope variations of the OFDM-CDMA signals. This approach is a simple input-power management processing with just minimal extra bandwidth for side information. It is based on the recursive-least-squares (RLS) algorithm to minimize the mean squared error of input instantaneous power and the average input power to HPA, which is shown to suppress the peak-to-average power ratio (PAPR) significantly and improve the system performance as a result. Finally, from the spectral behavior investigation, the present power management is shown to yield a significant adjacent channel power ratio (ACPR) reduction.

The bit error rate (BER) performance of a turbo‐coded code‐division multiple‐access (CDMA) system operating in a satellite channel is analysed and simulated. The system performance is compared for various constituent decoders, including maximuma posterioriprobability (MAP) and Max‐Log‐MAP algorithms, and the soft‐output Viterbi algorithm. The simulation results indicate that the Max‐Log‐MAP algorithm is the most promising among these three algorithms in overall terms of performance and complexity. It is also shown that, for fixed code rate, the BER performance is improved substantially by increasing the number of iterations in the turbo decoder, or by increasing the interleaver length in the turbo encoder. The results in this paper are of interest in CDMA‐based satellite communications applications. Copyright © 2005 John Wiley & Sons, Ltd.

There have been few papers researching on effects of HPA nonlinearity towards system performance of digital phase modulation systems based on DSSS in satellite communication channel. System performance comparison of four digital phase modulation manner (based on DSSS), BPSK, QPSK, OQPSK and MSK are investigated in our paper. Two types of HPA are used in our simulation, TWTA (Traveling Wave Tube Amplifier) and SSPA (Solid State Power Amplifier). Well-known SALEH model is used for TWTA. RAPP model, which consider AM/AM distortion only and modified RAPP model, which consider both AM/AM and AM/PM distortion are adopted towards SSPA. We mainly focus on eye-diagrams, PSD and BER performance when HPA works on saturation or serious saturation area. Comparisons are made among four phase modulation manners. At last some useful and important conclusions are gotten.

Chapter 2 - Direct-Sequence Spread Spectrum Systems

In this paper, we investigate the impact of nonlinear distortion on the overall system outage probability of simultaneous wireless information and power transfer enabled two-way amplify-and-forward relaying network by employing three different nonlinear power amplifier (NLPA) models such as traveling wave tube amplifier, soft envelope limiter, and solid-state power amplifier at the relay node. We consider a time-switching based protocol at the energy-constrained relay node to harvest energy and information transmission. We derive the closed-form expression of the system outage probability by utilizing the selection combining technique at the source nodes over Nakagami-m fading channels. System throughput and energy efficiency of the network are also investigated. The impact of NLPA, threshold data-rate, fading severity, and time-switching factor are highlighted on the network's performance. Finally, the derived analytical results are validated by the Monte Carlo simulations.