An iterative detection technique for DS-CDMA signals with strong nonlinear distortion effects

Whenever a DS-CDMA signal (direct sequence code division multiple access) is the sum of several components associated to different spreading codes (e.g., the DS-CDMA signal to be transmitted by the BS (Base Station) in the downlink, or any multicode DS- CDMA signal), it has high envelope fluctuations, and a high PMEPR (Peak-to-Mean Envelope Power Ratio) setting strong linearity requirements for the power amplifiers. For this reason, it is desirable to reduce the envelope fluctuations of the transmitted signals. A promising class of nonlinear signal processing schemes, combining a nonlinear operation in the time- domain with a linear, frequency-domain filtering operation, was recently proposed for reducing the envelope fluctuations (and, inherently, the PMEPR) of DS-CDMA signals, while maintaining the spectral occupation of the corresponding conventional DS-CDMA signals. However, the performance degradation due to the nonlinear distortion effects on the transmitted signals can be relatively high, especially when a low PMEPR is intended. This can be especially serious if different powers are assigned to different spreading codes. To avoid significant performance degradation in these situations, we consider improved receiver where there is an iterative estimation and cancelation of nonlinear distortion effects. Our performance results show that the proposed receiver allows significant performance improvements after just a few iterations, even when we have strong nonlinear distortion effects

A technique for reducing the PMEPR of MC-CDMA signals

In this paper we consider the use of MCCDMA (Multi-Carrier Code Division Multiple Access) schemes with a frequency-domain orthogonal spreading. The transmitter structure is especially modified for reducing the envelope fluctuations of the transmitted signals. An analytical approach for the performance evaluation is also included which relies on the Gaussian characteristics of MC signals with a high number of suhcarriers. A set of performance results is presented. Our performance results show that the proposed transmitter structure is adequate for power-efficient MC-CDMA transmission since we can reduce the envelope fluctuations of the transmitted signals, while keeping essentially the same spectral efficiency of the conventional MC-CDMA transmission

Iterative detection of multicode DS-CDMA signals with strong nonlinear distortion effects

Whenever a direct-sequence code-division multiple-access (DS-CDMA) signal is the sum of several components associated with different spreading codes [e.g., the DS-CDMA signal to be transmitted by the base station (BS) in the downlink or any multicode DS-CDMA signal], it has high envelope fluctuations and a high peak-to-mean envelope power ratio (PMEPR), setting strong linearity requirements for the power amplifiers. For this reason, it is desirable to reduce the envelope fluctuations of the transmitted signals. The use of clipping techniques combined with frequency-domain filtering was shown to be an effective way of reducing the envelope fluctuations (and, inherently, the PMEPR) of DS-CDMA signals, while maintaining the spectral occupation of the corresponding conventional DS-CDMA signals. To avoid PMEPR regrowth effects, the clipping and filtering operations can be repeated several times. However, the performance degradation due to nonlinear distortion effects on the transmitted signals can be relatively high, particularly when a very low PMEPR is intended (e.g., when a low clipping level and several iterations are adopted). This can particularly be serious if different powers are assigned to different spreading codes. To avoid significant performance degradation in these situations, we consider an improved receiver where there is an iterative estimation and cancellation of nonlinear distortion effects. Our performance results show that the proposed receiver allows significant performance improvements after just a few iterations, even when we have strong nonlinear distortion effects

Joint multiuser detection and cancelation of nonlinear distortion effects for the uplink of MC-CDMA systems

In this paper we consider the uplink transmission in MC-CDMA systems (multicarrier coded division multiple access). To reduce the envelope fluctuations of the transmitted signals, the MC-CDMA signal associated to each MT (mobile terminal) is submitted to a clipping device, followed by a frequency-domain filtering operation. We define an iterative receiver that jointly performs the MUD (multiuser detection) and the estimation and cancellation of the nonlinear distortion effects that are inherent to the transmitted signals. Our performance results show that the proposed receiver structures allow good performances, even for severely time-dispersive channels and/or when a low-PMEPR is intended for each MT

Turbo multiuser detection for MC-CDMA signals with strongly nonlinear transmitters

In this paper we consider the uplink transmission in MC-CDMA (multicarrier -coded division multiple access) systems. Since MC-CDMA signals are OFDM-like multicarrier signals, they have high envelope fluctuations and a high PMEPR (Peak-to-Mean Envelope Power Ratio) which leads to amplification difficulties. To reduce the envelope fluctuations of the transmitted signals, while maintaining the spectral efficiency, the MC-CDMA signal associated to each MT (mobile terminal) is submitted to a clipping device, followed by a frequency-domain filtering operation. However, the nonlinear distortion effects can be high when an MC-CDMA transmitter with reduced envelope fluctuations is intended. In this paper, we define an iterative receiver that jointly performs a turbo-MUD (Multiuser Detection) and the estimation and cancellation of the nonlinear distortion effects. The set of simulation results presented shows that the proposed receiver structure allows good performances, very close to the linear receiver ones, even for high system load and/or when a low-PMEPR is intended for each MT

Joint turbo equalization and multiuser detection of MC-CDMA signals with low envelope fluctuations

In this paper, we consider the uplink transmission in multicarrier code-division multiple-access (MC-CDMA) systems. As other multicarrier signals, MC-CDMA signals have high envelope fluctuations and a high peak-to-mean envelope power ratio (PMEPR), which leads to amplification difficulties. This is particularly important for the uplink transmission, since an efficient low-cost power amplification is desirable at the mobile terminals (MTs). Moreover, the transmission over time-dispersive channels destroys the orthogonality between spreading codes, which might lead to significant multiple-access interference levels. To reduce the envelope fluctuations of the transmitted signals, while maintaining the spectral efficiency, the MC-CDMA signal associated to each MT is submitted to a clipping device, followed by a frequency-domain filtering operation. However, the nonlinear distortion effects can be high when an MC-CDMA transmitter with reduced envelope fluctuations is intended (e.g., a small clipping level and/or when successive clipping and filtering operations are employed). In this paper, we define an iterative receiver that jointly performs a turbo multiuser detection and the estimation and cancellation of the nonlinear distortion effects. Our performance results show that the proposed receiver structure allows good performances, very close to the linear receiver ones, even for high system load and/or when a PMEPR as low as 1.7 dB is intended for each MT

Iterative detection of multicode DS-CDMA signals with strongly nonlinear transmitters

Whenever a DS-CDMA signal is the sum of several components associated to different spreading codes it has high envelope fluctuations and a high peak-to-mean envelope power ratio (PMEPR) setting strong linearity requirements for the power amplifiers. The use of clipping and filtering techniques (which can be repeated several times) was shown to be an effective way of reducing the PMEPR of DS-CDMA signals. We consider an improved receiver where there is an iterative estimation and cancelation of nonlinear distortion effects. The proposed receiver allows significant performance improvements after just a few iterations, even when we have strong nonlinear distortion effects

Near-Instantaneously Adaptive HSDPA-Style OFDM Versus MC-CDMA Transceivers for WIFI, WIMAX, and Next-Generation Cellular Systems

Burts-by-burst (BbB) adaptive high-speed downlink packet access (HSDPA) style multicarrier systems are reviewed, identifying their most critical design aspects. These systems exhibit numerous attractive features, rendering them eminently eligible for employment in next-generation wireless systems. It is argued that BbB-adaptive or symbol-by-symbol adaptive orthogonal frequency division multiplex (OFDM) modems counteract the near instantaneous channel quality variations and hence attain an increased throughput or robustness in comparison to their fixed-mode counterparts. Although they act quite differently, various diversity techniques, such as Rake receivers and space-time block coding (STBC) are also capable of mitigating the channel quality variations in their effort to reduce the bit error ratio (BER), provided that the individual antenna elements experience independent fading. By contrast, in the presence of correlated fading imposed by shadowing or time-variant multiuser interference, the benefits of space-time coding erode and it is unrealistic to expect that a fixed-mode space-time coded system remains capable of maintaining a near-constant BER

Analytical evaluation of nonlinear effects in MC-CDMA signals

In this paper we present an analytical tool for the per formance evaluation of nonlinear effects in MC-CDMA signals. This tool takes advantage of the Gaussian-like behavior of MC-CDMA signals with a large number of subcarriers and employs results on memoryless nonlinear devices with Gaussian inputs so as to characterize statis tically the signals at the output of the nonlinear device. This characterization is then used for an analytical com putation of the SIR levels (Signal-to-Interference Ratio) and the BER performance (Bit Error Rate). A set of numerical results is presented and discussed, showing the accuracy of the proposed analytical BER per formance analysis

Joint turbo equalization and cancelation of nonlinear distortion effects in MC-CDMA signals

In this paper, we consider low-PMEPR (Peak-to-Mean Envelope Power Ratio) MC-CDMA (Multicarrier Coded Division Multiple Access) schemes. We develop frequencydomain turbo equalizers combined with an iterative estimation and cancellation of nonlinear distortion effects. Our receivers have relatively low complexity, since they allow FFT-based (Fast Fourier Transform) implementations. The proposed turbo receivers allow significant performance improvements at low and moderate SNR (Signal-to-Noise Ratio), even when a low-PMEPR MC-CDMA transmission is intended