A new method of iterative interference cancellation for mobile OFDM system

Discrete multitone modulation allows for a simple receiver structure if the impulse response of the communication channel is shorter than the cyclic prefix. In ADSL, a time-domain equalizer (TEQ) is used to fulfil this requirement. Without a TEQ, unwanted intercarrier interference (ICI) and intersymbol interference (ISI) occur. In this paper, quadratic forms are developed to calculate the contributions of ICI and ISI to the desired signal. Based on these expressions, a new method for calculating the TEQ coefficients, which minimizes the effects of ICI, ISI, and additive noise, is derived. Simulations in the context of ADSL compare the performance of this new algorithm with one that is state of the art.

The multiuser detection is a signal processing tech- nique which enhances the performance and capacity in DS- CDMA technology. Since its complexity issue was too expensive to handle, there was focus on the suboptimal detectors. In this paper we propose a hybrid method called adaptive differential interference cancellation method (ADIC) which is a hybrid of the differential interference cancellation method and which is then combined in a adaptive way to determine a weight matrix which will map the outcome of the differential interference cancellation method to come out with a better performance. This paper also looks into the computational cost of different methods using the floating point operation (flops) which is a good estimation of the comparative computational cost if not the exact one. I. INTRODUCTION The conventional DS-CDMA match filter detector fails to combat these issues due three prominent drawbacks: unrealis- tic ucorrelated code, near-far problem (even with strict power control (1)), and tough multiple access interference (MAI). Many advanced signal processing approaches have been made to improve the performances of these kinds of detectors. In this paper we will study the different approaches that were made in this field and implement these methods to reach a conclusion about the detection methods. As the name of the topic suggests the focus of our work will be study and propose cost effective better performing multiuser detection method. We will explore Multiuser Detection Techniques in Synchronous CDMA environment. Multi-user detection deals with the development and ap- plication of joint demodulation and interference cancellation cancellation techniques for improved detection of a desired set of digital signals in the presence of multiple access interfer- ence, intersymbol interference and noise. Since veterbi forward dynamic algorithm (2) is used in case of Optimum multiuser detection (3) the complexity grows in with the increase in the number of users. Hence the complexity is immense which increases exponentially with the increase in the number of users. Looking ahead for lower complexity hence lesser cost of implementation, the paper proposes the use of sub-optimal linear detectors (4). The sub-optimal detector the paper next focuses on is Decorrelator type of Detector (5) which is optimal in least squares. This detector out performs the optimal detector in many respects. Then the Minimum Mean Square Error (MMSE) linear detector is proposed which tries to minimize the squared error from the required transformation. It is one of the most popular detectors in use due to its easiness to implement and its lower complexity. But then in the above detection methods the decisions made were based on certain linear transformation criterion and there are missing efforts to check the performance in between the detection process to improvise the transformation method better detection. So then the paper proposes Adaptive multiuser detection. Then the paper focuses on Interference cancellation methods which utilize the feedback to reduce Multiple Access Interference (MAI) in the detected bits. Under the Interference Cancellation umbrella the Successive Interference Cancellation (SIC) (6), (7) method is explored and simulated to observe the results. Then the next method which performs better than SIC and more commonly used in real life, called Parallel Interference Cancellation (PIC) (6), (8) is also implemented. Before going ahead to implement the final proposed detection method, this paper implements Differential Interference Cancellation (DIC) (9) method. And then this paper finally proposes an Inter- ference Cancellation method called the Adaptive Differential Interference Cancellation Multiuser Detection method which is cost effective in comparison to other Interference Cancellation methods as we will require lesser hardware (multipliers as in the SIC or Parallel Interference Cancellation (PIC)) for that matter. More ever since we are using adaptive method the desired BER can be further reduced. This proposed method will be discussed in detail in the following section.

In this paper, a cyclic-prefixed (CP) system with pulse amplitude modulation (PAM) for uplink transmission in light-fidelity (LiFi) is discussed. LiFi supports bi-directional transmission, i.e., downlink and uplink transmissions. However, there have been only a few studies on uplink LiFi despite the fact that the uplink traffic demand increases due to frequent use of, for example, video call and live streaming services. The uplink transmission is designed such that a transmitter has a low peak-to-average ratio as well as a lower computational complexity using a CP in conjunction with PAM. We investigate three different schemes, namely linear equalization (LE) where only a feedforward filter is applied, decision feedback equalization (DFE) where a feedback filter is added at the receiver and Tomlinson-Harashima precoder (THP) where a feedback filter is added at the transmitter. Both feedforward and feedback filters are jointly optimized. In addition, we consider an optical wireless channel with reflections, randomly-located and randomly-oriented user equipment, a human body acting as a reflector and a limited linear dynamic range of a light emitting diode. We conclude that a DFE system is suitable in a low spectral efficiency region, where, compared to an LE system, up to 4 dB gain is achieved. A THP system is suitable for a high spectral efficiency region, where up to 5 dB gain compared to the LE and DFE systems is achieved.

Orthogonal frequency-division multiplexing (OFDM) is robust against frequency selective fading because of the increase of the symbol duration. However, the time-varying nature of the channel causes inter-carrier interference (ICI) which destroys the orthogonal of sub-carriers and degrades the system performance severely. To alleviate the detrimental effect of ICI, there is a need for ICI mitigation within one OFDM symbol. We propose an iterative Inter-Carrier Interference (ICI) estimation and cancellation technique for OFDM systems based on regularized constrained total least squares. In the proposed scheme, ICI aren’t treated as additional additive white Gaussian noise (AWGN). The effect of Inter-Carrier Interference (ICI) and inter-symbol interference (ISI) on channel estimation is regarded as perturbation of channel. We propose a novel algorithm for channel estimation o based on regularized constrained total least squares. Computer simulations show that significant improvement can be obtained by the proposed scheme in fast fading channels.

This paper proposes an equalization method based on multi-dimensional lattice filter for OFDM system and investigates the optimum reflection coefficient of such a multi dimensional lattice filter. It is known that OFDM transmission can decrease the effect of Inter Symbol Interference (ISI) by parallel transmission of orthogonal sub-carriers. However, conventional OFDM can’t compensate for Inter Carrier Interference (ICI). Proposals of 2 dimensional equalization using 2 dimensional filters for removing ISI and ICI have been made. However, those schemes are complex because the large number of taps necessary. This paper proposes 2 dimensional equalization using multi dimensional Infinite Impulse Response (IIR) lattice filters. These filters can be convergence faster and decrease iteration. The optimisation problem of inverse the characteristic function of path propagation is also dealt with. It is shown that the proposed system performs better than conventional systems making use of other equalization methods.

Ever since the adoption of the single carrier frequency division multiple access (SC-FDMA) scheme for the uplink in the Long Term Evolution (LTE) standard, there has been much interest in improved receiver algorithms for the same. Successive interference cancelation (SIC) based turbo receivers (Turbo SIC) have been proposed to tackle the multi-stream interference (MSI) for transmission modes such as spatial multiplexing (SM) and multiuser (MU) multiple input multiple output (MIMO). In addition, decision feedback equalizers (DFE) have been discussed as an alternative to the simple linear minimum mean square error (LMMSE) based receivers to suppress intersymbol interference (ISI) caused by the frequency selective multi-path channel. This paper proposes a DFE receiver variant based on parallel interference cancelation (PIC) with maximum likelihood detection (MLD) and ordered SIC (OSIC) principles to suppress ISI and MSI. Simulations show that the proposed receiver performs better than the existing SIC and DFE based receivers and achieves the matched filter bound (MFB) for both coded and uncoded systems.

Interpolated decision feedback equalization for long intersymbol interference channels

Optical Orthogonal Frequency Division Multiplexing (O-OFDM) is now used by a number of researchers because it provides major advantages in mitigating Group-Velocity Dispersion (GVD) in Single Mode Fiber (SMF). Unfortunately, when the uncompensated long-haul transmission ranges become very large, substantial dispersion is accumulated. Due to the large accumulated dispersion, the Cyclic Prefix (CP) duration will occupy a substantial fraction of the OFDM frame. This effect sets some limitations on the overall throughput and the spectral efficiency. Moreover, the transmission is inefficient because of the energy wastage contained within the CP. In the case where the Channel Impulse Response (CIR) is larger than the CP, the system performance is limited by the Inter Symbol Interference (ISI) and Inter Carrier Interference (ICI). In this paper, we propose a Least Mean Square Time Domain Equalizer (LMS-TEQ) immediately after the channel. It can cancel the residual ISI and ICI caused by both the GVD and the CP length being shorter than the CIR.

A decision-feedback equalizer (DFE) is the basis of a recent development of a quadruple diversity troposcatter modem which can operate up to a data rate of 12.6 Mbit/s in a 99% bandwidth of 15 MHz. In this paper a theoretical approach is developed for the calculation of average bit error rate (ABER), including the effects of intersymbol interference due to multipath and the finiteness of the transversal filters used to realize the DFE. By omitting the intersymbol interference effect, the calculation provides a lower bound which can be used to assess the intersymbol interference penalty for a particular DFE structure. The paper includes calculations of a DFE configuration which has a three tap forward filter with tap spacing equal to one-half a symbol interval. Measured performance results from fading channel simulator tests of a three tap forward filter DFE are presented for data rates from 1.5 to 12.6 Mbit/s and for a wide range of multipath statistical conditions. The results for this DFE configuration show (1) excellent agreement between calculated and measured ABER, (2) a small intersymbol interference penalty when the 2σ multipath spread is less than approximnately one-half the data symbol interval, and (3) successful operation at values of multipath spread up to twice the data symbol interval. In a sequel to this paper, the results of a field test of the DFE modem are presented. These live links test results are consistent with both the calculated and simulator measured data presented here.

As the data rate has been increased over 10 Gb/s with copper interconnect, the intersymbol interference (ISI) caused from the channel loss should be compensated. While a decision feedback equalizer (DFE), which is widely used in the receiver can compensate the ISI, its ability to enhance the signal-to-noise ratio (SNR) is limited especially for high frequency data patterns (alternating data patterns). Even a DFE with a large number of taps, which has powerful compensation capacity for ISI, can improve only limited amount of SNR. To improve SNR, this brief presents a 1/4th baud-rate continuous-time linear equalizer (CTLE) and a two-tap DFE. The 1/4th baud-rate CTLE recovers data to be adequate for the DFE, and remaining ISI is removed by the DFE. To boost the high frequency gain of the DFE, exclusive or merged adders are introduced, so the proposed equalizer is more tolerant against channel noise. It compensates 21.7-dB channel loss and operates with 11.5-Gb/s data rate as it consumes 25.35 mW from 1.3 V supply in a 110-nm CMOS technology.

Optical Orthogonal Frequency Division Multiplexing (O‐OFDM) provides major advantages in mitigating Group‐Velocity Dispersion (GVD) in Single Mode Fiber (SMF). Unfortunately, when the uncompensated long‐haul transmission ranges become very large, substantial dispersion is accumulated. Owing to the large accumulated dispersion, the Cyclic Prefix (CP) duration will occupy a substantial fraction of the OFDM frame. This effect sets some limitations on the overall throughput and the spectral efficiency. Moreover, the transmission is inefficient because of the energy wastage contained within the CP. In the case where the CP length is shorter than the Channel Impulse Response (CIR), energy wastage is reduced but the system performance is limited by the Inter Symbol Interference (ISI) and Inter Carrier Interference (ICI). To overcome this problem, a Time Domain Equalizer (TEQ) immediately after the channel is proposed. It can cancel the residual ISI and ICI caused by both the GVD and the CP length being shorter than the CIR. The simulation results show that, using BER of 10−3 as a reference, the system performance improves by 2.69thinspacedB while considering 6‐feedforward/5‐feedback‐weight Decision Feedback TEQ (DF‐TEQ) and 2.03 dB while considering 10‐tap Least Mean Square TEQ (LMS‐TEQ) even though the CP length is shorter than the CIR. This method reduces size of the CP, and consequently the performance of the system will be improved. Copyright © 2011 John Wiley & Sons, Ltd.

This paper proposes an enhanced decision feedback equalizer (DFE) for filter bank multicarrier (FBMC) systems over offset quadrature amplitude modulation (OQAM) based on a minimal mean square error (MMSE) criterion. For each subcarrier, besides one feedforward (FF) filter and one feedback (FB) filter, we add two intercarrier interference (ICI)-suppressing filters, which feed back detected precursors extracted from neighboring subchannels to enhance ICI suppression in the presence of multipath channels. Our simulation results indicate our proposal makes a considerable improvement comparing with MMSE-DFE under a severe ICI and ISI scenario despite few extra costs in computational complexity.

Inter carrier interference (ICI) is deemed as a critical issue in orthogonal frequency division multiple access (OFDMA) systems. Though ICI caused by Doppler spread is negligible in femtocell systems, frequent reception of signals over misaligned subcarriers will introduce severe ICI. Successive interference cancellation and decision feedback equalizers are some of the techniques proposed to mitigate ICI. However, most of these methods are highly complex, and thus, not suitable for commercial femtocell systems. In this paper, a simple equalization technique to reduce ICI in OFDMA based femtocell networks is proposed. It has been designed particularly considering the femtocell system's operating environment and their limited computational capacity. This technique provides excellent ICI reduction performance for smaller delay spreads of the users.

For digital terrestrial television broadcasting (DT-TB) systems, orthogonal frequency division multiplexing (OFDM) technique is considered to be an effective method to combat the frequency selective channel cause by multi-path effect. In the single frequency network (SFN) scenario, Alamouti space frequency block coding (SFBC) is introduced to provide spatial diversity gain and improve the link stability. However, both OFDM and Alamouti SFBC are sensitive to time-variant during mobile reception. In this paper, we propose a decision feedback equalization algorithm for inter-carrier interference (ICI) can-celation of the dual (PN) padded time domain synchronization OFDM (TDS-OFDM) DTTB system over doubly selective channel. Our algorithm is proven by numerical simulations which show bit error rate (BER) gain over mobile SFN channel and can be adopted in mobile high density television (HDTV) transmission systems.

In recent times, there has been growing interests in integration of voice, data and video traffic in wireless communication networks. With these growing interests, WCDMA has immerged as an attractive access technique. The performance of WCDMA system is deteriorated in presence of multipath fading environment. The paper presents space-time coded minimum mean square error (MMSE) Decision Feedback Equalizer (DFE) for wideband code division multiple access (WCDMA) in a frequency selective channel. The filter coefficients in MMSE DFE are optimized to suppress noise, intersymbol interference (ISI), and multiple access interference (MAI) with reasonable system complexity. For the above structure, we have presented the estimation of BER for a MMSE DFE using computer simulation experiments. The simulation includes the effects of additive white Gaussian noise, multipath fading and multiple access interference (MAI). Furthermore, the performance is compared with standard linear equalizer (LE) and RAKE receiver. Numerical and simulation results show that the MMSE DFE exhibits significant performance improvement over the standard linear equalizer (LE) and RAKE receiver.