No full-text available
To read the full-text of this research,
you can request a copy directly from the authors.
A Low-Complexity Adaptive Co-Channel Interference Removal Scheme for COFDM Systems
Abstract
Coded orthogonal frequency division multiplexing (COFDM) broadband systems may coexist with other wireless systems and suffer from co-channel interferece (CCI). CCI severely affects soft decision decoding and degrades the performance of COFDM systems. In this paper, we propose a novel CCI removal scheme which is adaptive and low in implementation cost. The proposed scheme has three important features: 1) It can detect the pilots affected by CCI and amend the incorrect channel frequency responses (CFRs); 2) It calculates soft decisions by only using the amended CFRs; 3) It can detect the soft decisions affected by CCI and erase them. We compare the proposed scheme with the conventional one presented in [1]. Simulation results indicate that the proposed scheme eliminates CCI more effectively than the conventional one. Moreover, the proposed scheme requires lower computation complexity and memory
... A multicarrier system with messagedriven idle subcarriers (MC-MDIS) has been introduced in [20] for power efficiency and higher spectral compared with OFDM. Alternatively, coded-OFDM (COFDM) [21]- [23] has been proposed to combat channel nulls. The critical dilemma in coding for an OFDM scheme with a large number of subcarriers is that it, either encounters design difficulties or the coding rate becomes prohibitively low. ...
... Fig. 8 compares the BER performance of the oversampled OFDM system, the (15, 3) Reed Solomon (RS) coded OFDM system, and the (511, 130) BCH coded OFDM system with the same bandwidth. In (21) and (23), it is analytically proved that c can be completely recovered from s using the convergent iterative receiver. Note that if sufficient redundancy is provided, perfect reconstruction is guaranteed. ...
In high-speed digital wireless communication applications, the intersymbol interference (ISI) channel may have spectral nulls or erasures, which may degrade the orthogonal frequency division multiplexing (OFDM) bit error rate (BER) performance. In this paper, we prove that the BER performance of the OFDM system is considerably refined if the erased symbols in erasure channel (or received symbols on the deep-faded subcarriers in Rayleigh fading channel) are estimated based on the rest symbols in each OFDM signal. Also, an oversampled OFDM scheme which is a particular form of the pre-coded OFDM schemes is considered to guarantee high enough sampling rate. Furthermore, a new iterative receiver is proposed for perfect symbol recovery. A comprehensive Monte Carlo simulation study is performed to validate the theoretical results and affirm the superior performance of the oversampled OFDM scheme with the iterative receiver, compared to the typical detector for the OFDM scheme.
Narrowband interference from other systems degrades greatly the performance of Orthogonal Frequency Division Multiplexing (OFDM) systems in heterogeneous networks. This paper proposes a cyclostationarity-based narrowband interference suppression algorithm to estimate and suppress the interference, and derives the Signal to Interference plus Noise Ratio (SINR) gain. Simulation results show that the proposed algorithm outperforms the conventional linear prediction filtering method in estimating and suppressing interference, therefore improves the system performance.
The issue of Co-Channel Interference (CCI) in the Orthogonal Frequency Division Multiplexing (OFDM) systems is studied in this paper. A time-domain estimation and elimination scheme of sinusoidal-modeled CCI is proposed based on the Minimum Mean Square Error (MMSE) criterion. It is proved by simulation that the proposed algorithm can suppress the CCI effectively. Compared with the existing scheme, the proposed time-domain method can avoid the interference leakage because of the time-to-frequency transformation, the dependence on channel estimation and the loss of useful data caused by directly erasing.
Orthogonal frequency-division multiplexing (OFDM) effectively mitigates intersymbol interference (ISI) caused by the delay spread of wireless channels. Therefore, it has been used in many wireless systems and adopted by various standards. In this paper, we present a comprehensive survey on OFDM for wireless communications. We address basic OFDM and related modulations, as well as techniques to improve the performance of OFDM for wireless communications, including channel estimation and signal detection, time- and frequency-offset estimation and correction, peak-to-average power ratio reduction, and multiple-input-multiple-output (MIMO) techniques. We also describe the applications of OFDM in current systems and standards.
A comparison in the performance between hard- and soft-decision Viterbi decoding, with application to the HIPERLAN/2 standard, is presented. The results show that when channel state information (CSI) is included in the generation of the soft-decision information, the soft-decision method greatly outperforms the hard method. Moreover, a simplified algorithm for the soft-output demapper for the 16-QAM and 64-QAM constellations is developed, which allows the complexity of the demapper to be maintained at almost the same level for all the possible modes of HIPERLAN/2.
The efficiency of a coded orthogonal frequency division
multiplexing (COFDM) receiver can be improved by use of a pre-FFT
equaliser (PFE). This technique is known as combined OFDM-equalisation.
This paper considers the noise amplification effect of the PFE and its
effect on the performance of a combined COFDM-equalisation modem and
proposes a method to improve this performance. It first reviews and
compares the conventional COFDM and combined COFDM-equalisation
techniques. The PFE is then described and the conditions required to
allow it to take the form of a decision feedback equaliser (DFE) are
discussed. It is shown that these conditions often cannot be met. In
these cases the PFE must take the form of a linear transversal equaliser
(LTE). The performance of the LTE is inferior to that of the DFE due to
significant noise amplification. Hence, if the performance of combined
COFDM-equalisation is to match that of conventional COFDM, a method for
mitigating the noise amplifying effect of an LTE type PFE is required.
One such method is proposed. This technique exploits channel state
information (CSI) available in a COFDM receiver in order to enhance the
performance of a Viterbi convolutional decoder. This `CSI modified'
Viterbi algorithm is capable of mitigating the noise amplification
occurring in the equaliser. To demonstrate this, the performance of
conventional COFDM and combined COFDM-equalisation are compared by means
of software simulation using both standard and CSI modified Viterbi
decoding. The results for the standard Viterbi decoding illustrate the
performance penalty due to noise amplification. The results for the CSI
modified Viterbi algorithm demonstrate the effective mitigation of noise
amplification and the comparable performance of conventional COFDM and
combined COFDM-equalisation
This paper investigates the soft decision decoding techniques for wireless COFDM systems. A new method to estimate the channel state information (CSI) for the Viterbi decoder is proposed, which overcomes the problem of reliability metric of transmitted signals in wireless as well as mobile channels. The CSI for each subcarrier is calculated as a properly defined squared Euclidean distance. On the basis of which, a soft decision decoding scheme to improve reception performance under multipath fading interference and co-channel interference (CCI) from existing analog broadcasting signals is also proposed. The simulation results show that the scheme can provide a compatible improvement to the terrestrial digital video broadcasting (DVB-T) system. The proposed scheme is also suitable for other wireless COFDM transmission systems.
The results of an extensive study on analog PAL signals interfered with by digital DVB-T signals are presented. The research work covers field measurements, laboratory measurements, a theoretical evaluation of the interference problems and a national survey among TV installation companies. The main objective is the identification and explanation of the interference problems reported since the start of DVB-T broadcasting. The report is organized in two parts. This paper, part I, describes the field measurements, whereas the laboratory measurements and the poll results are described in part II (see Acuna, M.A. et al., ibid., p.116-22). It has been found that the frequency plan is part of the source of the interference problems, particularly in the case of cochannel interference. Another source of problems is use of different radiation elements at the same site for broadcasting DVB-T and analog TV signals. As stated in ETSI documents (see ETSI TR-101-190, 1997), the use of a different antennas for DVB-T may require ERP restrictions to protect existing analog services. The power ratios between digital and analog TV signals at the broadcasting station should be adjusted to meet the protection ratios everywhere within the coverage area. The required power ratio depends on the specific shape of the radiation pattern of each antenna. Finally, it has been found that an important part of the interference problems is due to the amplifiers of the master antenna television (MATV) systems in buildings. The results of the laboratory tests and the theoretical analysis conducted to determine the increment of the protection ratio required by the presence of these amplifiers are presented in part II.
For pt.I, see Sanchez, M.G. et al., ibid., p.111-15. The results of an extensive study on analog PAL signals interfered with by digital DVB-T signals are presented. The study covers a theoretical evaluation of the interference problems, field and laboratory measurements and a national survey among TV installation companies. Field measurements are reported in part I. These measurements explain the origin of cochannel and some of the adjacent channel interference problems. However, interference was reported in areas where the field measurements showed that the protection ratios for PAL signals interfered with by DVB-T adjacent channel signals were met. As these reports came from multistorey buildings with master antenna TV (MATV) installations, the study was extended to analyze the role of these installations in the interference problems. Part of the interference problems are due to the MATV system amplifiers which are unable to cope with adjacent channel DVB-T signals. The nonlinear amplifiers give rise to intermodulation problems. The ITU-R recommendations (see ITU-R BT.1368, 2000) recognize that such nonlinear amplifiers could be a source of problems and that higher protection ratios may be needed, but no value is given for the increase. Our laboratory tests of commercial amplifiers found that an increase in the protection ratios of between 12 to 16 dB is needed to avoid a significant degradation of the PAL TV signal in the amplifiers. It has also been found that most of the interference situations are not due to the interfering signal being a DVB-T format, but that any significant signal amplitude in the passband of the filter causes interference. Therefore, MATV system distribution needs planning factors associated with the distribution of any signals near the main signal. The results are presented of a survey conducted among companies who install communal telecommunication infrastructures in Spain.