Andrew J. Viterbi

On the capacity of a cellular CDMA system

It is shown that, particularly for terrestrial cellular telephony, the interference-suppression feature of CDMA (code division multiple access) can result in a many-fold increase in capacity over analog and even over competing digital techniques. A single-cell system, such as a hubbed satellite network, is addressed, and the basic expression for capacity is developed. The corresponding expressions for a multiple-cell system are derived. and the distribution on the number of users supportable per cell is determined. It is concluded that properly augmented and power-controlled multiple-cell CDMA promises a quantum increase in current cellular capacity. >

Convolutional Codes and Their Performance in Communication Systems

This tutorial paper begins with an elementary presentation of the fundamental properties and structure of convolutional codes and proceeds with the development of the maximum likelihood decoder. The powerful tool of generating function analysis is demonstrated to yield for arbitrary codes both the distance properties and upper bounds on the bit error probability for communication over any memoryless channel. Previous results on code ensemble average error probabilities are also derived and extended by these techniques. Finally, practical considerations concerning finite decoding memory, metric representation, and synchronization are discussed.

Erlang capacity of a power controlled CDMA system

This work presents an approach to the evaluation of the reverse link capacity of a code-division multiple access (CDMA) cellular voice system which employs power control and a variable rate vocoder based on voice activity. It is shown that the Erlang capacity of CDMA is many times that of conventional analog systems and several times that of other digital multiple access systems. >

Very low rate convolution codes for maximum theoretical performance of spread-spectrum multiple-access channels

A spread-spectrum multiple-access (SSMA) communication system is treated for which both spreading and error control is provided by binary PSK modulation with orthogonal convolution codes. Performance of spread-spectrum multiple access by a large number of users employing this type of coded modulation is determined in the presence of background Gaussian noise. With this approach and coordinated processing at a common receiver, it is shown that the aggregate data rate of all simultaneous users can approach the Shannon capacity of the Gaussian noise channel. >

An intuitive justification and a simplified implementation of the MAP decoder for convolutional codes

An intuitive shortcut to understanding the maximum a posteriori (MAP) decoder is presented based on an approximation. This is shown to correspond to a dual-maxima computation combined with forward and backward recursions of Viterbi algorithm computations. The logarithmic version of the MAP algorithm can similarly be reduced to the same form by applying the same approximation. Conversely, if a correction term is added to the approximation, the exact MAP algorithm is recovered. It is also shown how the MAP decoder memory can be drastically reduced at the cost of a modest increase in processing speed.

Soft handoff extends CDMA cell coverage and increases reverse link capacity

The effect of handoff techniques on cell coverage and reverse link capacity is investigated for a spread spectrum CDMA system. It is shown that soft handoff increases both parameters significantly relative to conventional hard handoff. >

Other-cell interference in cellular power-controlled CDMA

An improved series of bounds is presented for the other-cell interference in cellular power-controlled CDMA. The bounds are based on allowing control by one of a limited set of base stations. In particular, it is shown that the choice of cellular base station with least interference among the set of N/sub c/>1 nearest base stations yields much lower total mean interference from the mobile subscribers than the choice of only the single nearest base station. >

A pragmatic approach to trellis-coded modulation

Since the early 1970s, for power-limited applications, the convolutional code constraint length K=7 and rate 1/2, optimum in the sense of maximum free distance and minimum number of bit errors caused by remerging paths at the free distance, has become the de facto standard for coded digital communication. This was reinforced when punctured versions of this code became the standard for rate 3/4 and 7/8 codes for moderately bandlimited channels. Methods are described for using the same K=7, rate 1/2 convolutional code with signal phase constellations of 8-PSK and 160PSK and quadrature amplitude constellations of 16-QASK, 64-QASK, and 256-QASK to achieve, respectively, 2 and 3, and 2, 4, and 6 b/s/Hz bandwidth efficiencies while providing power efficiency that in most cases is virtually equivalent to that of the best Ungerboeck codes for constraint length 7 or 64 states. This pragmatic approach to all coding applications permits the use of a single basic coder and decoder to achieve respectable coding (power) gains for bandwidth efficiencies from 1 b/s/Hz to 6 b/s/Hz.<<ETX>>

Performance of power-controlled wideband terrestrial digital communication

Performance of a wideband multipath-fading terrestrial digital coded communication system is treated. The analysis has applications to a cellular system using direct-sequence spread-spectrum code-division multiaccess (CDMA) with M-ary orthogonal modulation on the many-to-one reverse (user-to-base station) link. For these links, power control of each multiple-access user by the cell base station is a critically important feature. This feature is implemented by measuring the power received at the base station for each user and sending a command to either raise or lower reverse link transmitter power by a fixed amount. Assuming perfect interleaving, the effect of the power control accuracy on the system performance is assessed. >

Spread spectrum communications: myths and realities

Spread spectrum communication techniques date back to the early fifties. Since the earliest applications, system improvements have been more evolutionary than revolutionary. Like most improvements in electronic systems, these are due primarily to the availability of ever higher speed integrated circuit components, which translate in this case to wider spread spectra. In three decades the achievable spreading factor has grown by about three orders of magnitude’ to the point that we are now limited more by bandwidth allocations than by technology limitations. Before we xamine the quantitative effects of spreading, let us catalog briefly the multiple purposes of spread spectrum communications. First, we note that spreading here refers to expansion of the bandwidth well beyond what is required to transmit digital data. Thus, a system transmitting data at a rate ( R ) of 100 Mbits/s using approximately 100 MHz of bandwidth (W) is not spread at all, while a system transmitting at 100 bits/s spread over a spectrum of about 100 MHz has a factor W/R = 106, or 60 dB of so-calledprocessing gain.