Ahmed F. Shalash

Multidimensional carrierless AM/PM systems for digital subscriber loops

Carrierless amplitude/phase modulation (CAP) is a viable alternative for digital subscriber loop (DSL) systems such as high-speed DSL, asymmetric DSL, and very high-speed DSL. In this paper, two novel orthogonal-based modulation schemes are introduced for the DSL environment. In the first technique, the conventional two-dimensional (2-D) CAP-16 line code is extended to a three-dimensional (3-D) scheme. The 3-D system is designed so that the new overall transfer matrix maintains perfect reconstruction (PR) of the transmitted information. The system is designed by solving a minimax optimization problem by using the sequential quadratic programming algorithm; this searches the whole space of signals under the PR condition and the DSL constraints. The 3-D CAP system leads to a 50% increase in throughput at the expense of the output signal-to-noise ratio (SNR) degradation by 3-4 dB. It should be noted that this increase in throughput can also be achieved by a CAP-64 system. Nevertheless, the dynamic range of the proposed 3-D CAP approach is 7.4 dB less than CAP-64, which leads to faster equalization. Furthermore, the equalizer performance in the 3-D case is 1-3 dB better, depending on channel and noise considerations. Another application of the multidimensional CAP approach, referred to as orthogonality division multiple access (ODMA) is presented. In this approach, the CAP system is extended to more than three dimensions by maintaining the same overall symbol rate as in the 2-D case. The ODMA system allows multiple-access operation of the DSL communication link with minimal hardware penalty. The performance of the ODMA system for the multiple-access environment is found to match the conventional 2-D CAP, allowing suitable multiple-access topology with minimal complexity overhead. The ODMA system complexity is compared with discrete multitone and is found to have 25% less complexity for the same bit rate. Theoretical analysis of the ODMA system using zero-forcing equalization shows that the equalization process yields better SNR as the number of dimensions increases, asymptotically approaching the intersymbol interference-free matched filter bound.

Comparison of discrete multitone and carrierless AM/PM techniques for line equalization

The design and performance of the Discrete Multitone (DMT) transceiver is compared with the Carrierless Amplitude Phase modulation (CAP) transceiver. The theories of both techniques are summarized and the architecture of each is studied for the unshielded twisted-pair cable environment. It is shown that the DMT system is well suited for equalization at low transmission rates and the CAP system is well suited for high-speed transmission applications. With increase in speed, the hardware penalty increases for the DMT equalizer unlike in the CAP equalizer where the hardware penalty is weakly dependent on speed.

Power efficient FIR folding transformation for wireline digital communications

A new algorithm is introduced for power efficient implementation of the folding algorithm transformation for the adaptive FIR filters. The algorithm is based on the known minimum spanning tree problem which is a modification of the traveling sales person NP-hard problem. A sub-optimal solution can be obtained leading to dramatic reduction in the folded architecture switching activities. The optimum ordering within the folding sets is done in the steady state condition of the filtering operation. For a typical word length, and for large enough folding factor, the projected reduction in switching activity can be as large as 60%. The hierarchical energy analysis too (HEAT) was used to simulate the effect of this reduction on a typical Baugh-Wooley (1973) multiplier and reduction in power consumption for a typical wireline equalization implementation was found to be between 25% and 60%. The new algorithm is based on a simple breadth first search approach and can be easily performed for one time only when the filter is geared to the steady state mode. Dramatic improvement in power consumption can be achieved with minimal overhead.

Theoretical and practical limits of next-generation high-speed digital subscriber loops

For the single-pair HDSL (HDSL2), the concept of having different upstream and downstream power spectral densities is highlighted. The signal-to-noise ratio (SNR) at the input of the central office HDSL terminal unit (HTU-C) can be enhanced by boosting the power output of the remote user HDSL terminal unit (HTU-R). Further improvement in performance can be achieved by reducing the HTU-C NEXT interference. Two techniques are proposed to reach this goal. The first approach is to reduce the HTU-C transmitter power in the downstream direction for shorter reach cables. The second approach is borrowed from the analog frequency-modulation preemphasis/deemphasis frequency tilting in the upstream direction. Combining the three techniques leads to at least 6-dB SNR margin improvement for the full reach carrier serving area loops.

Three-dimensional carrierless AM/PM line code for the unshielded twisted pair cables

The Carrierless AM/PM (CAP) modulation scheme is a viable alternative for high speed digital transmission for the HDSL/ADSL/VHDSL environments. In this paper, the conventional 2-D CAP system is expanded to a 3-D space. A 50% increase in the system throughput is achieved at the expense of added receiver complexity and some performance degradation. The 3-D signaling is obtained through a minimax optimization process to match the characteristics of the original 2-D system and to achieve perfect reconstruction (PR) condition. The 3-D CAP system is simulated over unshielded twisted pair (UTP) cable environment for baud rate of 150 Mbps.

Power Efficient Folding of Pipelined LMS Adaptive Filters with Applications to Wireline Digital Communications

In this paper, a new two-step algorithm is introduced for power efficient implementation of the folding algorithm transformation of the LMS adaptive FIR filters. The first step handles the scheduling within the folding sets to reduce the switching activity using a greedy algorithm solution for the traveling sales person (TSP) NP-hard problem. The greedy algorithm gives a sub-optimal solution of the TSP problem with low implementation cost. For a typical word-length, and for large folding factors, the projected reduction in switching activity can be as large as 50%. HEAT tool was used to simulate the effect of this reduction using a typical Baugh-Wooley multiplier and the reduction in power consumption for a wireline equalization implementation was found to lie between 25% and 60%. The new algorithm is based on a simple breadth-first search approach and can be easily performed for one time only when the filter is geared to the steady-state mode. The second step involves optimal pipelining of the folded hardware elements for minimum power. The tradeoff between the number of pipelining levels and the power consumption is evaluated. To compensate for the LMS degradation due to pipelining, a 1-pole IIR compensator is introduced in the error loop for relaxed LMS. The IIR based relaxation outperforms the relaxed lookahead pipelining by 2-3 dB of output error. Another feature of the IIR relaxation is the “smoothing” nature of the filter, thus reducing the effective switching activities on the multiplier input. The combined effect of the two techniques can lead to power savings up to 65% compared to a normal folded structure. Simulations for the combined techniques are carried out for the digital subscriber loop channel and significant savings in power are demonstrated.

Three-dimensional equalization for the 3-D QAM system with strength reduction

Recently, the 3-D CAP system was introduced as a modification to the conventional 2-D CAP. This paper makes two contributions in the context of the 3-D CAP system. First, the QAM equivalent for the three-dimensional case is introduced by extending the 2-D complex notation into a 3-D vector notation. The definition is a modification of the well known quaternions. Second, the 3-D QAM filtering is redefined using algorithmic strength reduction to reduce the overall complexity by about one third with marginal degradation. The 3-D QAM system using 9 filters performs almost exactly as the 3-D CAP. Further complexity reduction is achieved by exploiting filtering reduction at the expense of only one dB of receiver SNR degradation.

Multiple access over wireline channels using orthogonal signaling

With the emerging high-rate wireline channels, multiple access is inevitable. In this paper the multiple access problem over wireline channels is solved using orthogonal signaling. The 2-D FIR CAP/QAM based line code is extended to a multi-dimensional system to accommodate multiple access topology. Using the well known Orthogonality Division Multiple Access, ODMA, we can achieve better theoretical and practical advantages over Time Division Multiple Access (TDMA) and Frequency Division Multiple Access (FDMA). The problem of increasing the number of dimensions is stated as a minimax optimization problem, then solved using sequential quadratic programming. 3,4, and 6-dimensional systems are introduced to support multiple access applications. The superiority of ODMA over other multiple access techniques is shown in light of the receiver equalizer performance. Computer simulations are carried out to evaluate the performance of the new line code.

Orthogonality division multiple access LTI transmit filters for ISI-channels

In this paper, the problem of multiple access over digital subscriber loops is introduced and defined as FIR LTI filter design over linear dispersive channels. The 2-D FIR CAP/QAM based modulation line code is extended to the multi-dimensional system to accommodate multiple access topology. The concept of orthogonality division multiple access, ODMA, is used as the methodology to achieve that goal. The problem of increasing the number of dimensions is stated as a minimax optimization problem, then solved using sequential quadratic programming. 3, 4, and 6-dimensional systems are introduced to support multiple access applications. Computer simulations are carried out to evaluate the performance and functionality of the orthogonal signaling line code.

Hybrid FIR-IIR Adaptive Echo Canceller for Wireline Applications

In wireline communications, echo is a problem affecting analog front-end dynamic range and received signal to noise ratio. Echo cancellation schemes were devised using adaptive filtering but they are computationally expensive. Traditionally, echo cancellers are implemented using adaptive FIR filters that tend to be long to allow for enough echo rejection. In this paper a new equation-error based IIR-tail connected in tandem with FIR echo canceller is introduced. The new architecture can save up to 50% of the number of FIR taps needed for the echo cancellation. Simplified weight update for the new architecture is examined using block-update LMS for the FIR portion and full update for the IIR portion. The combined savings can be up to 70% compared to regular FIR only adaptive echo canceller. Echo rejection performance is verified via simulations and adaptive IIR stability is confirmed for the equation-error based adaptation