169 research outputs found
MIMO signal processing in offset-QAM based filter bank multicarrier systems
Next-generation communication systems have to comply with very strict requirements for increased flexibility in heterogeneous environments, high spectral efficiency, and agility of carrier aggregation. This fact motivates research in advanced multicarrier modulation (MCM) schemes, such as filter bank-based multicarrier (FBMC) modulation. This paper focuses on the offset quadrature amplitude modulation (OQAM)-based FBMC variant, known as FBMC/OQAM, which presents outstanding spectral efficiency and confinement in a number of channels and applications. Its special nature, however, generates a number of new signal processing challenges that are not present in other MCM schemes, notably, in orthogonal-frequency-division multiplexing (OFDM). In multiple-input multiple-output (MIMO) architectures, which are expected to play a primary role in future communication systems, these challenges are intensified, creating new interesting research problems and calling for new ideas and methods that are adapted to the particularities of the MIMO-FBMC/OQAM system. The goal of this paper is to focus on these signal processing problems and provide a concise yet comprehensive overview of the recent advances in this area. Open problems and associated directions for future research are also discussed.Peer ReviewedPostprint (author's final draft
Orthogonal transmultiplexers : extensions to digital subscriber line (DSL) communications
An orthogonal transmultiplexer which unifies multirate filter bank theory and communications theory is investigated in this dissertation. Various extensions of the orthogonal transmultiplexer techniques have been made for digital subscriber line communication applications.
It is shown that the theoretical performance bounds of single carrier modulation based transceivers and multicarrier modulation based transceivers are the same under the same operational conditions. Single carrier based transceiver systems such as Quadrature Amplitude Modulation (QAM) and Carrierless Amplitude and Phase (CAP) modulation scheme, multicarrier based transceiver systems such as Orthogonal Frequency Division Multiplexing (OFDM) or Discrete Multi Tone (DMT) and Discrete Subband (Wavelet) Multicarrier based transceiver (DSBMT) techniques are considered in this investigation.
The performance of DMT and DSBMT based transceiver systems for a narrow band interference and their robustness are also investigated. It is shown that the performance of a DMT based transceiver system is quite sensitive to the location and strength of a single tone (narrow band) interference. The performance sensitivity is highlighted in this work. It is shown that an adaptive interference exciser can alleviate the sensitivity problem of a DMT based system. The improved spectral properties of DSBMT technique reduces the performance sensitivity for variations of a narrow band interference. It is shown that DSBMT technique outperforms DMT and has a more robust performance than the latter. The superior performance robustness is shown in this work.
Optimal orthogonal basis design using cosine modulated multirate filter bank is discussed. An adaptive linear combiner at the output of analysis filter bank is implemented to eliminate the intersymbol and interchannel interferences. It is shown that DSBMT is the most suitable technique for a narrow band interference environment.
A blind channel identification and optimal MMSE based equalizer employing a nonmaximally decimated filter bank precoder / postequalizer structure is proposed. The performance of blind channel identification scheme is shown not to be sensitive to the characteristics of unknown channel. The performance of the proposed optimal MMSE based equalizer is shown to be superior to the zero-forcing equalizer
Applying Frequency-Domain Equalization to Code-Division Multiple Access and Transform-Domain Communications Systems
This research examined the theory and application of using orthogonal frequency division multiplexing (OFDM), or discrete multi-tone (DMT), frequency domain equalization (FEQ) with two communications systems that inherently possess unused carrier frequencies, or null-tones, in their respective transmission frequencies. The fundamental DMT-FEQ property relies on null-tones to equalize a non-ideal channel and mitigate the effects of interchannel interference (ICI), intersymbol interference (ISI), and noise. The two communications systems investigated were a Hadamard encoded code division multiple access (CDMA) communications system with up to 32 synchronous users and a transform domain communications system (TDCS) with only one user. Both communications systems were simulated while operating with real channel data corrupted by noise. Simulation results showed that the Hadamard encoded CDMA system worked well with DMT-FEQ only when the Hadamard code set was used to construct a transmission signal that obeyed DMT-FEQ null-tone conditions in conjunction with a vector estimation method. Simulation results also showed that a TDCS using traditional pseudo-random phase component, and traditional spectral mask with consecutive null-tones, did not work well with DMT-FEQ. Modifications to the TDCS model revealed that a TDCS with a conjugate-symmetric phase component in conjunction with a modified spectral mask with consecutive null-tones and forced null-tones provided acceptable results when equalizing with DMT-FEQ. The DMT-FEQ may be suitable for covert applications, such as TDCS, when modifications to TDCS’ phase component and forced null-tones in its spectral mask are made
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Channel equalization to achieve high bit rates in discrete multitone systems
textMulticarrier modulation (MCM) techniques such as orthogonal frequency division
multiplexing (OFDM) and discrete multi-tone (DMT) modulation are attractive
for high-speed data communications due to the ease with which MCM can combat
channel dispersion. With all the benefits MCM could give, DMT modulation has an
extra ability to perform dynamic bit loading, which has the potential to exploit fully
the available bandwidth in a slowly time-varying channel. In broadband wireline
communications, DMT modulation is standardized for asymmetric digital subscribe
line (ADSL) and very-high-bit-rate digital subscriber line (VDSL) modems. ADSL
and VDSL standards are used by telephone companies to provide high speed data
service to residences and offices.
In an ADSL receiver, an equalizer is required to compensate for the channel’s
dispersion in the time domain and the channel’s distortion in the frequency domain
of the transmitted waveform. This dissertation proposes design methods for linear
equalizers to increase the bit rate of the connection. The methods are amenable
to implementation on programmable fixed-point digital signal processors, which are
employed in ADSL/VDSL transceivers.
A conventional ADSL equalizer consists of a time-domain equalizer, a fast
Fourier transform, and a frequency domain equalizer. The time domain equalizer
(TEQ) is a finite impulse response filter that when coupled with a discretized channel
produces an equivalent channel whose impulse response is shorter than that of
the discretized channel. This channel shortening is required by the ADSL standards.
In this dissertation, I first propose a linear phase TEQ design that exploits symmetry
in existing eigen-filter approaches such as minimum mean square error(MMSE),
maximum shortening signal to noise ratio (MSSNR) and minimum intersymbol interference
(Min-ISI) equalizers. TEQs with symmetric coefficients can reach the
same performance as non-symmetric ones with much lower training complexity.
Second, I improve Min-ISI design. I reformulate the cost function to make
long TEQs design feasible. I remove the dependency of transmission delay in order
to reduce the complexity associated with delay optimization. The quantized
weighting is introduced to further lower the complexity. I also propose an iterative
optimization procedure of Min-ISI that completely avoids Cholesky decomposition
hence is better suited for a fixed-point implementation.
Finally I propose a dual-path TEQ structure, which designs a standard singleFIR
TEQ to achieve good bit rate over the entire transmission bandwidth, and
designs another FIR TEQ to improve the bit rate over a subset of subcarriers. Dualpath
TEQ can be viewed as a special case of a complex valued filter bank structure
that delivers the best bit rate of existing DMT equalizers. However, dual-path
TEQ provides a very good tradeoff between achievable bit rate vs. implementation
complexity on a programmable digital signal processor.Electrical and Computer Engineerin
Power pre-emphasis for suppression of FWM in coherent optical OFDM transmission
Four-wave-mixing (FWM) due to the fiber nonlinearity is a major limiting factor in coherent optical OFDM transmission. We propose to apply power pre-emphasis, i.e. to allocate the transmitted power nonuniformly among subcarriers in order to suppress the FWM impairment. The proposed technique was numerically investigated for both single channel 15.6 Gbs CO-OFDM transmissions and 7-channel WDM transmissions, showing that up to 1 dB improvement in the system's Qfactor can be achieved without considering sophisticated power loading algorithms developed for wireless communications
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High Speed Optical Links Using CAP Modulation and Novel Equalisation Techniques
High speed optical links suffer from inter-symbol-interference (ISI) due to their limited bandwidth. Equalisation is typically used to mitigate ISI and therefore improve the link capacity. This dissertation explores novel equalisation techniques for carrierless amplitude and phase (CAP) modulation based optical communication systems including OM4 based and plastic optical fibre (POF) based links.
An 850 nm VCSEL based OM4 link using CAP-16 scheme is studied. For the first time, the CAP equaliser, is proposed to mitigate both crosstalk channel interference (CCI) and ISI in the link at the receiver side. Performance comparisons are studied between the CAP-16 scheme using CAP equaliser and a conventional equaliser, pulse amplitude modulation (PAM-4) scheme, and discrete multitone (DMT) scheme. CAP based data transmission of 112 Gb/s is achieved over 150 m OM4 fibre with this novel equaliser, while the conventional equaliser can only support over 1 m OM4 fibre and fails to recover the signals at the same data rate. In addition, this novel equaliser provides a 1.2 dB and 1.7 dB improvement in receiver sensitivity over PAM-4 and DMT schemes, respectively, at 112 Gb/s over 100 m OM4 fibre. A novel pre-CAP-equaliser solving CCI at the transmitter side is also proposed. Data transmission of 56 Gb/s over 100 m OM4 fibre is reported experimentally with an improvement of 0.7 dB in receiver sensitivity compared to using the CAP equaliser at the receiver side. A simulation study shows a 2 dB improvement in receiver sensitivity at 112 Gb/s over 100 m OM4 fibre. Furthermore, an artificial neural network (ANN) equaliser in conjunction with the CAP equaliser structure is explored in a VCSEL based OM4 fibre link in order to further mitigate the nonlinear impairments. For 112 Gb/s data transmission over 100 m OM4 fibre, a 2.4 dB improvement of receiver sensitivity is achieved compared to the CAP equaliser.
In addition to the electrical equalisers, a monolithically integrated silicon optical equaliser consisting of three taps is used for 50 Gb/s data transmission. After 10 km standard single mode fibre (SSMF), error free eye diagrams at the receiver are demonstrated.
A μLED based POF link based on an APD receiver is also investigated with the CAP equaliser at the receiver side. Data transmission rates of 4 Gb/s over 25 m and 5 Gb/s over 10 m POF links are demonstrated with this equaliser while the conventional equaliser can only support 4 Gb/s over 10 m and fails to recover the signals for 5 Gb/s data transmission
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