Carl-Erik Sundberg

The performance of rate-compatible punctured convolutional codes for future digital mobile radio

A study is reported of the unequal error protection capabilities of convolutional codes belonging to the family of rate-compatible punctured convolutional codes. The performance of these codes is analyzed and simulated for the fast-fading Rice and Rayleigh channels with differentially coherent 4-phase modulation. Interleaving performed over one or two blocks of 256 channel bits to mitigate the effect of fading. Examples are provided to show that it is possible to accommodate widely different error protection levels within short information blocks. The effect of the code and channel parameters is considered, such as the encoder memory, the code rate, interleaver depth, fading bandwidth, and the contrasting performance of hard and soft decisions on the received symbols. The authors highlight the need for soft decisions and soft channel state information to extract the maximum benefit from Viterbi decoding on a channel as harsh as the one with Rayleigh fading.<<ETX>>

Variable-rate sub-band speech coding and matched channel coding for mobile radio channels

The effect of digital transmission errors on a family of variable-rate embedded subband speech coders has been analyzed. It is shown that there is a difference in error sensitivity of four orders of magnitude between the most sensitive and the least sensitive bits of the speech coder. As a result, a family of rate-compatible punctured convolutional codes with flexible unequal error protection capabilities has been matched to the speech coder. On a Rayleigh fading channel with 4-DPSK modulation, as much as 5-dB gain in channel signal-to-noise ratio can be obtained by using four levels of error protection compared to only two levels in more conventional designs. The gain is achieved at no extra bandwidth requirement and at a negligible complexity increase. Among the results, analysis and informal listening tests show that with a four-level unequal error protection scheme, transmission of 12-kb/s speech is possible with little degradation in quality over speech is possible with little degradation in quality over a 16-kb/s channel with an average bit error rate of 2*10/sup -2/ at a vehicle speed of 60 mph.<<ETX>>

Energy—Bandwidth Comparisons and Shannon Theory for Phase Modulation

Neither energy nor bandwidth consumption alone is a sufficient measure of a modulation system. It is a simple matter to reduce the bandwidth of a scheme if large energy is available, and similarly high energy is not needed for a low error probability if a large bandwidth can be tapped. What is much more difficult is reducing one of these without reducing consumption of the other.

Signal Analysis and an Overview of Modulation Methods

We begin this chapter by setting up ways of handling the signals that appear in a phase modulation system. The concept of signal space is introduced. The simplest phase modulations are then discussed as examples of signal analysis and as introduction to the more general methods that follow in later chapters. Some basic signal processing operations that occur in communications links are introduced, such as baseband conversion, channel filtering, and hard limiting. The chapter requires understanding of the basic ideas of vector spaces and stochastic processes. An excellent treatment of the former from a communications theory point of view appears in Ziemer and Tranter(1) and in many other communications theory texts. References for stochastic processes are Papoulis(2) or Helstrom.(3)

Phase Modulation and Convolutional Codes

In previous chapters we have seen that the memory in the continuous phase of the CPM signal can be utilized to improve the minimum Euclidean distance; so also can the memory introduced by the controlled intersymbol interference in partial response CPM. Even more memory can be built into the signals by means of multi-h coding (see Chapter 3) or convolutional codes. In this chapter we will present some recent results on combinations of many-level PSK and CPM with convolutional codes. Section 11.2 covers binary and quaternary CPFSK with rate 1/2 convolutional codes, and presents combinations with the best free Euclidean distance. Section 11.3 covers eight-level CPM with rate 2/3 codes, 16-level CPM with rate 3/4 codes, etc. Finally Section 11.4 presents some simulations of eight- and 16-level CPFSK with rate 2/3 and rate 3/4 codes. Viterbi detection is used throughout.

Partially Coherent Receivers

As was made clear in the previous chapter on synchronization, an ideal coherent detector with no carrier phase error does not exist in practice. The best we can hope for is a phase error that constantly fluctuates around the mean value zero. In this chapter we will give a statistical description of the steady state phase error and study both optimum and suboptimum detection at high SNR. A parameter called the minimum equivalent Euclidean distance is derived and used. First, however, we will precisely define the notion of partially coherent detection.