A comparison of QAM and VSB for hybrid fiber/coax: digitaltransmission

There are two competing transmission systems for downstream digital transmission on hybrid fiber/coax cable networks: quadrature amplitude modulation (QAM), and vestigial sideband modulation (VSB). Both QAM and VSB are bandwidth efficient, and have the same bandwidth efficiency. Detailed calculations and simulations of QAM and VSB transmission on hybrid fiber/coax are reported here. It is shown that, since VSB has a higher symbol rate, it has at most 1/2 dB less received SNR than QAM at low frequencies because of dispersion and symbol timing jitter. This difference is negligible. Vendors proposing VSB recover the carrier with a pilot tone and a PLL, and vendors proposing QAM use all-digital data-directed carrier recovery. Simulations reported here show that QAM and VSB have very similar carrier recovery performance. It is concluded that for hybrid fiber/coax, VSB and QAM have practically the same overall performance, and the choice between these transmission systems should be based on considerations other than performance     

Optimal reception of digital data over the Gaussian channel with unknown delay and phase jitter

Asymptotically optimum (in the sense of minimum per-symbol error rate) receiver structures for data communication over the white Gaussian channel with unknown time delay and carrier phase jitter are developed. The receiver structures apply to the following suppressed-carrier modulation systems: double sideband (DSB), quadrature amplitude modulation (QAM) with an arbitrary constellation, vestigial sideband (VSB) and single sideband. The resulting minimum error probability receivers are asymptotically equivalent to maximum-likelihood digital {em sequence}-estimating receivers. The optimum structures implicitly derive joint maximum-likelihood estimates of the unknown parameters and of the sequence of data symbols. It is shown that the parameter estimates can be obtained from two data-directed stochastic approximation algorithms. Unlike traditional theoretical treatments of this communication situation, which have separated the highly important carrier phase and timing recovery problem from the detection problem, a unified theory is presented from which the complete ideal receiver structure can be deduced.     

A Unified Approach to Pulse Design in Data Transmission

The problem of designing optimum pulseshapes for data transmission over randomly selected channels is examined using a meansquared error criterion. Earlier work has focused principally on SSB or VSB systems in the presence of timing and carrier phase jitter, but we extend the results here to any two-dimensional signaling scheme (including as special cases SSB, VSB, combined AM-PM, QAM, and staggered QAM) and to any type of channel dispersion. By imposing certain mild constraints on the transmitter and receiver filters, it is seen that one can solve for optimum pulses more easily than with earlier approaches, and yet the resulting system is still essentially optimum. These constraints leave design freedom only in the rolloff bands of the pulse spectra, and a major thurst of the work is to design for optimal utilization of whatever excess bandwidth is available. The approach used involves immunity to channel distortions: only the type of channel distortion to be encountered is known, and series expansions of mean-squared error are used to find sensitivity coefficients. These coefficients are then minimized by proper signal design. Closed form expressions are found for the optimum pulses, and these results are compared with previous work. A second approach, which uses as its criterion the mean-squared error averaged over all possible channel characteristics, is also discussed.     

Envelope-Derived Timing Recovery in QAM and SQAM Systems

The problem of continuously recovering timing phase and frequency in a high-rate carrier-modulated data communication device is addressed. The communication link is assumed narrowband with respect to the baud rate of the device, thus necessitating the use of some type of equalization in the receiver. The receiver demodulator is assumed coherent in nature. A timing recovery technique based on narrow-band filtering and squaring of the received demodulated waveform(s) is examined. Analysis of the first moment of the waveforms so generated reveals a convenient indicator of sampling instants. If the timing recovery filters are properly designed, then these sampling instants exactly satisfy a requirement previously derived, ensuring efficient operation of the equalizer. Design criteria for the filters are given. Two principal types of linear modulation techniques are considered. These are quardrature-carrier amplitude modulation (QAM) and staggered QAM (SQAM). A set of experimental waveforms observed in a QAM modem signaling at 2400 Bd is included for illustration. Several conclusions are drawn as a result of the study. First, the mean timing waveforms in QAM and SQAM systems bear fixed time relationships to suitable receiver sampling instants. Second, adequate timing filter design is attained with very narrowband filters centered on frequencies equal to half the baud rate and the full baud rate. Third, recovery circuits used in QAM systems may be designed so as to have negligible timing jitter. In SQAM systems and, consequently, in vestigial sideband-amplitude modulation (VSB-AM) systems, this advantage is generally not found.     

Multidimensional Signal Constellations for Voiceband Data Transmission

A significant improvement in noise immunity can be achieved for digital transmission over band-limited channels by the use of multidimensional signal constellations. Conventional 16-point QAM signaling, such as that used in many digital transmission systems, is a two-dimensional modulation scheme where in each signaling interval a group ofNbits is used to determine the amplitudes of the in-phase and quadrature dimension or coordinate, i.e.,N/2bits are conveyed per dimension. In a2M-dimensional QAM system, a group ofMNbits is used to determine the in-phase and quadrature ampllitudes forMconsecutive, symbol intervals whereN/2bits are still conveyed by each dimension. It is the purpose of this paper to describe the basic theory and implementation for a particular 2 bits/dimension four-dimensional (two-symbol interval) encoding which readily lends itself to simple encoding and decoding. For this encoding, theory predicts a 1.2 dB gain in noise margin over conventional 16-point (two-dimensional) QAM signaling. Experimental results agreed with the theoretical predictions, and have demonstrated an order of magnitude reduction in block error rate. Extension to eight-dimensional signaling offers a theoretical gain of 2.4 dB over conventional 16-point QAM.     

Synthesis of multidimensional signal constellations with quadrature amplitude modulation

The multidimensional constellations of quadrature amplitude modulation (QAM) signals are proposed. Each symbol incorporates the blocks of QAM signals not only carrying the same information content but also composed of the symbols of different higher order constellations (in contrast to the simple 1D modulation where each QAM symbol carries its intrinsic information). The multidimensional constellations of QAM signals are synthesized using the optimization algorithm based on the criteria for enhancing the energy or spectral efficiency of digital data transmission systems. The characteristics of interference immunity of the proposed signals are estimated with the help of analytical calculations and simulation. The relationships making it possible to consider faster-than-Nyquist signaling technology as a particular case of the multidimensional constellations of QAM signals are derived. It is demonstrated that the obtained signals provide an energy gain with respect to traditional 1D signals, in particular, ensure the operability at SNRs smaller than those inherent to binary phase-shift keying signals.     

The multitone channel

The maximum bit rate of multitone QAM (quadrature amplitude modulation) over a general linear channel is found. First, the overall bit rate for an AWGN channel with a two-level transfer function is maximized, using a multitone QAM system. The power distribution between the tones and the number of bits/symbol per tone is optimized for a given symbol error rate. Extending these results to the general channel, it is shown that the optimum power division for multitone signals is similar to the water-pouring solution of information theory. Furthermore, multitone QAM performance is about 9 dB worse than the channel capacity, independent of the channel characteristics. The multitone results throughout are compared to those of an equivalent single-tone linearly equalized system. The comparison shows that the multitone system is useful for some channels, e.g. those with deep nulls in the transfer function. The maximum bit error rate over a twisted-pair channel which is performance dominated by near-end crosstalk (NEXT) is also found     

The effects of time-delay spread on QAM with nonlinearity switchedfilters in a portable radio communications channel

A spectrally efficient quadrature amplitude modulation (QAM) scheme with a simple nonlinearly switched filter is shown to provide a signaling waveform that is free from both intersymbol interference and zero-crossing timing jitter in a flat-fading channel. This modulation with coherent detection is evaluated by a computer simulation in a frequency-selective fading channel. Timing offset between in-phase and quadrature rails is varied. The effects of receiver filter bandwidths are also studied. It is found that this modulation is resistant to delay spread with moderate spectral efficiency and low complexity in a portable radio system, as compared to other spectrally efficient modulations     

Wavelet packet modulation for orthogonally multiplexedcommunication

Utilizing multidimensional signaling techniques, a generalized multirate wavelet-based modulation format for orthogonally multiplexed communication systems is presented. Wavelet packet modulation (WPM) employs the basis functions from an arbitrary pruning of a dyadic tree structured filter bank as orthogonal pulse shapes for conventional quadrature amplitude modulation (QAM) symbols. This generalized framework affords an entire library of basis sets with increased flexibility in time-frequency (T-F) partitioning. The bandwidth efficiency and power spectral density figures of merit for the general signal are derived and shown to be that of standard QAM     

SQAM: A New Superposed QAM Modem Technique

A spectral and power efficient modulation techniquesuperposed quadrature amplitude modulation (SQAM-is introduced. In SQAM, the premodulation baseband signal is a double-interval (2T_{s}) raised-cosine pulse superposed with weighted single-interval (Ts) raisedcosine pulses. Our results indicate that SQAM has spectral advantages over OQPSK, QBL, and MSK, and betterP(e)performance than MSK, IJF-OQPSK (or SQORC), and TFM.     

Enhanced architecture for residue number system-based CDMA for high-rate data transmission

This paper presents an advanced architecture for residue number system (RNS)-based code-division multiple-access (CDMA) system for high-rate data transmission by combining RNS representation, phase shift keying/quadrature amplitude modulation (PSK/QAM) and orthogonal modulation. The residues obtained from a fixed number of bits are again divided into spread code index and data symbol for modulation. The modulated data symbol is spread using the indexed orthogonal codes and transmitted through a communication channel. The proposed system uses a lower number of orthogonal codes than conventional RNS-based CDMA and the performance is comparable. The computational complexity of the proposed system is compared against alternative schemes such as M-ary CDMA and conventional RNS-based CDMA. The modified system is simulated extensively for different channel conditions and the results are discussed.     

Asynchronous Layered Modulation for High Spectral Efficiency Transmission with Lower Sensitivity to Phase Errors

In this paper, an asynchronous double layered modulation scheme is proposed to achieve high spectral efficiency transmission but has lower sensitivity to phase errors. By intentionally introducing a delay offset to the transmitted signals at each layer, the asynchronous double layered format can use lower level quadrature amplitude modulation (QAM) to achieve the same spectral efficiency as the conventional high level QAM modulation. The error probability performance of the asynchronous double layered format is analyzed in the presence of phase errors and the symbol error rate and the bit error rate are derived in the closed-form expression. Both the theoretical analysis and the simulation results show that the asynchronous layered format can achieve the same spectral efficiency as the conventional high level QAM modulation while has lower sensitivity to the phase errors.     

Cutoff rates for coordinate interleaved QAM over Rayleigh fadingchannels

Cutoff rates for (perfect) coordinate interleaving over flat Rayleigh fading channels are computed for some representative two-dimensional (2-D) and four-dimensional (4-D) quadrature amplitude modulation (QAM) schemes and compared to the standard symbol interleaved approaches. It is shown that for optimized coordinate interleaving [i.e., using optimum rotation parameters that maximize the cutoff rate for a given signal-to-noise ratio (SNR)], coding gains can be achieved vis-a-vis symbol interleaving that increases with the dimensionality of the signal constellation. Perfect coherent detection, as well as perfect channel state information are assumed     

具有低相位偏差敏感度的异步双层QAM调制

高阶QAM(Quadrature Amplitude Modulation)调制是提高频谱效率的一个有效途径,但它对相位偏差容忍度较差.文章提出了一种异步双层QAM调制方法,可在发射端用较低的调制阶数实现和传统高阶QAM调制相当的频谱效率,并分别针对提出的异步双层QAM调制和传统QAM调制,推导了平坦瑞利块衰落信道下固定相位偏差的误符号率闭合解析表达式.理论分析与计算机仿真结果吻合,研究表明,误符号率为10^-1时,异步双层256-QAM调制在相位偏差为0.9度的误符号率性能与无相位偏差情况比较,差异仅为0.3dB;而传统高阶4096-QAM调制的差异为11dB.     

QAM: the modulation scheme for future mobile radio communications?

With spectral congestion increasing, and plans announced to dramatically increase the number of mobile telephone users over the coming decade, many approaches are being considered to reduce the bandwidth requirements of each user. One approach is the use of quadrature amplitude modulation (QAM), a particular form of multilevel modulation where each symbol contains several bits of information. The use of QAM for mobile radio applications is considered and it is shown that this form of multilevel modulation can offer an attractive solution to current bandwidth restrictions in microcellular networks. An advanced variable-rate system is also considered and hardware development of a QAM modem discussed     

Optimum Pilot Symbol Assisted Modulation

Optimum detectors for pilot symbol assisted modulation (PSAM) signals in Rayleigh and Rician fading channels are derived. Conventional PSAM as used on Rayleigh fading channels is also employed on Rician fading channels. It is shown that the conventional PSAM receiver is optimal for binary phase shift keying in Rayleigh fading but suboptimal for Rician fading and suboptimal for 16-ary quadrature amplitude modulation in Rayleigh fading. The optimum PSAM signal detector uses knowledge of the specular component and also jointly processes the pilot symbols and the data symbol. The performance of the optimum detector is analyzed and compared with that of the conventional detector. It is concluded that substantial gains can be achieved by exploiting knowledge of the specular component while joint processing of the data symbol with the pilot symbols may offer small benefits.     

Cochannel interference immunity of high capacity QAM

To achieve very high bandwidth efficiencies in digital-radio large-level quadrature amplitude modulation (QAM) is an attractive modulation scheme. In the letter, the cochannel interference immunity of such high capacity QAM is determined.     

Bandwidth compressive 64-state sqam modems for non-linearly amplified satcom systems

A new class of spectral and power efficient offset modes of the M-ary QAM modulation technique, 64SQAM (superposed QAM), is analysed. The power spectrum of 64SQAM reveals a fast spectral roll-off and a low out-of-band energy. P(e) performance is analysed with different baseband wave shapes of 64SQAM in AWGN linear channels. For the operation of M-ary QAM at the highest power efficiency (i.e. at the saturation mode of HPA), a new bandwidth compressive NLA (non-linearly amplified) 64SQAM is investigated. Retaining a compact power spectrum and allowing a simple filtering strategy, NLA-64SQAM operates within 0–75 dB C/N degradation at P(e) = 10^?6.     

Application of classical cosine series window functions to fullresponse signaling offset quadrature binary modulation systems

It is proposed that classical cosine series window functions be applied to finite symbol duration quadrature binary modulation systems as pulse shapes in the interest of increasing the spectral confinement of the resulting modulated signals. The respective modulation systems are analyzed in terms of modulated signal envelope uniformity, spectral confinement, and bit error rate in the presence of white Gaussian noise with varying degrees of modulated signal amplitude compression. Even in the presence of moderate modulated signal compression, classical cosine series window function pulse shapes offer spectral confinement for quadrature binary modulation systems that is equal to or better than that provided by the conventional pulse shapes corresponding to QPSK, MSK, SFSK, and DSFSK. It is shown that moderate levels of modulated signal compression have negligible effects on the system bit error rate. A system implementation using the sum of FSK modulated signal components to achieve spectral confinement based on the classical cosine series window functions is presented