Increased air traffic coupled with the proliferation of sophisticated avionics can be expected to stimulate increased demand for air traffic control (ATC) services over the next few decades. New challenges for the design of communication, surveillance, and navigation subsystems is an anticipated consequence. Since it is unlikely that significantly larger frequency bands will become available, it will be necessary to exploit more efficient multiple-access techniques. The design of a multiple-access technique for a particular application will depend on a broad range of system issues. A predetermined procedure based on general characteristics of the various techniques will have but limited value. An example of an ATC satellite surveillance system is used to illustrate some of the broad ranging systems issues that can be expected to influence the selection of the multiple access technique.

It is shown that use of a two-stage decoding procedure consisting simply of an inner stage of block decoding and an outer stage employing a single sequential decoder does not result in an improvement in the computational overflow problem for the sequential decoder. Improvement can, however, result from use of multiple sequential decoders or use of a single sequential decoder with appropriate scrambling. Although the performance improvement resulting from application of these techniques to the additive white Gaussian noise channel is not significant, implementation and rate advantages make iterative sequential decoding techniques worth pursuing. Ia particular, with these techniques, a sequential decoder for a binary symmetric channel can be used regardless of the physical channel characteristics. Such a "universal" decoder is expected to be both simple and capable of high rate operation.

The author of the above paper, which appeared in this Transactions, vol. IT-12, pp. 189-195, April 1966, has called the following to the attention of the Editor. The symbol d should have read d(c): in the first line after (3); in the right-hand side of (4); in the first and seventh lines of the last paragraph; and on the same page, and in (8) and (11). The following definition should be inserted after (4): 'The symbol d is the set of decoding rules d(c) for each c.' Finally, (34) is corrected.

We consider the problem of communication over a channel which is selected 1) in a fashion unknown to the communicator, 2) as a function of past history, 3) from a given set of discrete memoryless channels, and 4) for the purpose of maximizing the degradation in performance. An upperbound on error probability insurable with block coding is obtained which exponentially approaches zero with block length for rates less than capacity. Techniques for the communicator to achieve these results are considered. The design of relatively simple communication techniques for channels with memory and jammed channels are discussed.

A channel which is selected for each use (without knowledge of past history) to be one of a given set of discrete memoryless channels is to be used by an ignorant communicator, i.e., the transmitter and receiver are assumed to have no knowledge of the particular channels selected. For this situation an upper bound on the insurable average error probability for block codes of length n is obtained which exponentially approaches zero for all rates less than capacity. Communication design techniques for achieving these results are discussed.