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Adaptive Filters for Blind Equalization
Published in Vijay K. Madisetti, The Digital Signal Processing Handbook, 2017
When the channel is nonideal, its impulse response h(n) is nonzero for n ≠ 0. Consequently, undesirable signal distortion is introduced as the channel output x(n) depends on multiple symbols in {a(n)}. This phenomenon, known as intersymbol interference (ISI), can severely corrupt the transmitted signal. ISI is usually caused by limited channel bandwidth, multipath, and channel fading in digital communication systems. A simple memoryless decision device acting on x(n) may not be able to recover the original data sequence under strong ISI. Channel equalization has proven to be an effective means of significant ISI removal. A comprehensive tutorial on nonblind adaptive channel equalization by Qureshi [1] contains detailed discussions on various aspects of channel equalization.
System Design Considerations
Published in David R. Goff, Kimberly Hansen, Michelle K. Stull, Fiber Optic Reference Guide, 2002
David R. Goff, Kimberly Hansen, Michelle K. Stull
Time Jitter and Intersymbol Interference: Time jitter is a short-term variation or instability in the duration of a specified interval. Intersymbol interference is the result of other bits interfering with the bit of interest. Where receiver noise is directly proportional to bandwidth, intersymbol interference is inversely proportional to the bandwidth. The eye diagram is a good way to see the effects of time jitter and intersymbol interference.
Overview of digital communications
Published in Marcio Eisencraft, Romis Attux, Ricardo Suyama, Chaotic Signals in Digital Communications, 2018
Ivan R. S. Casella, Aline de O. N. Panazio, Murilo B. Loiola
The filtering effect may cause the transmitted waveforms to widen, possibly resulting in an overlap between pulses sent in different time instants if these pulses are not sufficiently apart. This overlapping, known as intersymbol interference (ISI), is one of the main factors limiting the performance of a digital communication system.
Interference Cancellation in Wireless Communications: Past, Present, and Future
Published in IETE Journal of Education, 2022
S. M. Zafaruddin, Pranay Bhardwaj
High bit-rate transmissions over multi-path channels may cause inter-symbol interference (ISI) if the symbol duration becomes comparable or less than the delay spread of the channel. A naive approach may be to lower the data rate transmissions sufficiently to account for the ISI. However, channel equalization is a potential technique to suppress the ISI and render the demand for high data rates in modern communication systems [11]. Channel equalization compensates for the distortion in the transmitted signal due to the frequency-selective channel. There are many equalization techniques for ISI mitigation such as linear equalizer, adaptive equalizer, decision feedback equalizer, and maximum-likelihood sequence estimator [12]. On the other hand, multicarrier modulation divides a wideband channel into multiple low-rate orthogonal sub-channels with negligible ISI [13]. A popular implementation of multicarrier modulation for wireless systems is orthogonal frequency division multiplexing (OFDM). However, OFDM is sensitive to the carrier frequency offset (CFO), which may be caused due to differences in the carrier frequencies of transmitter and receiver.
Performance analysis of concatenated BCH and convolutional coded OFDM system
Published in International Journal of Electronics, 2020
Ozan Yavasoglu, Nursel Akcam, Tayfun Okan
High Frequency (HF) channels are widely used to provide communication over long ranges for many years. The propagation of signals in HF wireless communication takes place by reflection of signals from the ionospheric region of the atmosphere. This causes the HF communication to be among the type of multipath fading channel. Orthogonal Frequency Division Multiplexing (OFDM) (Kapoor & Kohli, 2018), which is used in many indoor or outdoor wireless applications, is a bandwidth efficient technique used to convert frequency selective channel for a single carrier into flat fading channel. The delays and challenges that might occur in this type of channel can cause different types of signal distortions, one of which is Inter-Symbol Interference (ISI). Besides ISI, Multiple-access Interference (MAI) is another problem that might occur in OFDM systems, as the orthogonality between the subcarriers are lost due to the arising Carrier Frequency Offset (CFO) effect. Different methods are analysed in Lim, Choi, and Im (2006) and Tsai, Lin, and Kuo (2005) to eliminate the effect of MAI and to prevent the degradation in the quality of received signals caused by that interference. Moreover, an OFDM system can also be used by adding a cyclic prefix to each subcarrier to minimise the aforementioned ISI and MAI effects of the channel (Singh & Rathore, 2014).
Performance of modified and low complexity pulse shaping filters for IEEE 802.11 OFDM transmission
Published in Journal of Information and Telecommunication, 2019
Tulsi Pawan Fowdur, Louvi Doorganah
Inter Symbol Interference (ISI) is a problem that is encountered in most communication systems including 802.11a/g. ISI occurs when the symbols in OFDM overlap with each other in bandlimited channels. As a result, the probability of error due to distortion effects increases and the performance of the OFDM system degrades. Pulse shaping filters are therefore used to limit the effects of ISI and improve the error performance of the OFDM system. Pulse shaping filters have several conditions to satisfy to curb the ISI effects (Nyquist, 1928). The main condition is that the equivalent impulse response of the transmitting and receiving filters should have zero crossings at multiples of the symbol period, T (Nyquist, 1928). Several works have proposed enhanced pulse shaping filters to solve the problem of ISI and to improve the performance of OFDM. An overview of the current pulse shaping filters is given next.