1. Field of the Invention
The present invention relates generally to communication systems. More specifically, the present invention is directed to systems and methods for improving digitally encoded transmissions.
2. Background of the Invention
The next generation of military and civilian communications products will incorporate the ability to dynamically adjust their use of spectrum based on measurements of the installed environment. Maintaining the required quality of service (bit error rate, transfer delay, etc.) will usually require the use of various coding methods, including modem concatenated coding and iterative decoding techniques. To obtain the maximum benefit, a given system will require some means of adapting the coding parameters and the transmitted waveform to the time and spectrum constraints indicated by the measurement of the environment.
Consider the idealized communications system 100 represented by FIG. 1. The communication is represented by a three-dimensional box or communications box 120 drawn against time, frequency and power (i.e., spectral density) axes. The time (t) axis has units of seconds, the frequency (f) axis has units of Hertz (“Hz”), and the spectral density (S) axis has units of watts per Hertz (“W/Hz”). Communications box 120 shows that a communication occupies a certain band of frequencies f by transmitting a certain power spectral density S over a certain period of time t. The volume of the communications box represents the total energy. In other words, E=S·f·t, where S is the power spectral density, f is the effective bandwidth, and t is the effective duration of the transmission, as indicated in FIG. 1. Multiplying the units of S,f, and t shows that the energy has units of joules (“J”), as shown below:
            (              W        Hz            )        ×          (      Hz      )        ×          (      s      )        =            W      ·      s        =          J      .      
If there are a total of K information bits transmitted during this process, then the average energy per bit Eb is
      E    b    =            E      K        .  
The average bit- or word-error rate for the K bit information word is a monotonically decreasing function of Eb.
In the usual communications system design process, the maximum acceptable error rate, maximum permissible frequency band, and maximum time duration for a transmission are established as design constraints. Based on these constraints, a communications engineer develops a coding scheme using well-established methods.
Texts such as, S. Wicker, Error Control Systems for Digital Communication and Storage, Upper Saddle River, N.J.: Prentice Hall, 1994; C. Heegard and S. Wicker, Turbo Coding, Boston: Kluwer Academic Publishers, 1999; B. Vucetic and Y. Jinhong, Turbo Codes, Boston: Kluwer Academic Publishers, 2000; S. Lin and D. J. Costello, Error Control Coding: Fundamentals and Applications, Englewood Cliffs, N.J.: Prentice-Hall, Inc., 1983; and R. G. Gallager, Information Theory and Reliable Communication, New York: John Wiley & Sons, 1968 describe conventional coding schemes.
Some communication systems desire to achieve a given error rate performance at a data rate that is adjustable to account for time-varying channel conditions, such as a time-varying signal-to-noise ratio. This requires adaptation of the transmitted codewords to the current channel conditions. Popular techniques for accomplishing this adaptation include Automatic Repeat Request (ARQ) and Hybrid Automatic Repeat Request (HARQ). ARQ and HARQ techniques, which are described in the first four texts mentioned above, are adaptive error-control schemes that choose an error-control code and frame length adaptively based on an estimated channel state or condition. The approach of these techniques is to hold the communications bandwidth, f, and the power spectral density, S, constant and to incrementally increase the time required to complete the transmission, as shown in FIG. 2.
FIG. 2 is a box diagram showing a communication transmitted using ARQ/HARQ adaptive techniques. As shown in FIG. 2, communication 200 occupies a certain band of frequencies f by transmitting a certain power spectral density S over a certain period of time t as three separate transmissions, first transmission 202, second transmission 204, and third transmission 206. The power spectral density S and the effective bandwidth f of the three transmissions are held constant. However, the duration of first transmission 202 t1, the duration of second transmission 204 t2, and third transmission 206 t3 are adaptively chosen based upon an estimated channel state or condition. The sum of durations t1, t2, and t3 for each transmission 202, 204, and 206 equals the total duration t of the transmission of communication 200.
As described above, ARQ/HARQ techniques adaptively adjust along the time dimension, while holding the frequency and power spectral density dimensions fixed. This approach allows a communication system to achieve a given error rate performance at a varying data rate.
Broadly speaking, the error-correcting ability of a linear code increases as the code rate
  R  =      K    N  decreases, where R is the rate, usually expressed as a rational fraction, K is the number of information bits per codeword and N≧K is the number of encoded bits per codeword. In each codeword, there are N−K redundant bits called parity bits. The parity bits provide error correction, such that as the number of parity bits increases (i.e., larger N−K), the error-correction capability also increases. Such a code is referred to as a (N,K) code.
A (N,K) code is designed to achieve the desired error rate performance at a specified channel signal-to-noise ratio. The “maximum” modulation scheme is designed to achieve this signal-to-noise ratio with a given power spectral density. This essentially establishes the (S,f,t) triple illustrated in FIG. 1.
However, once a linear code has been designed and K and N are chosen, the code structure may be adjusted using techniques, such as puncturing and shortening. Other techniques, referred to as lengthening, augmenting, expurgating and truncating are also used to adjust the code rate, but these techniques are not part of the present invention.
Shortening reduces the number of data bits contained in each codeword, while retaining the basic code structure. Of course, reducing the number of bits per code word increases the number of code words in an entire message. By increasing the number of code words, shortening also increases the average transfer delay. However, decreasing the number of bits per codeword provides a higher proportion of parity bits to bits per code word, thereby strengthening error correction performance. Thus, shortening decreases the code rate and strengthens performance.
Puncturing reduces the number of parity bits contained in each codeword. Reducing the number of parity bits, decreases the size of a code word, thereby causing a faster code rate. However, decreasing the number of parity bits decreases error-correcting capability. Thus, puncturing increases the code rate and weakens the performance.
Current shortening and puncturing techniques are limited in that they only adjust in one dimension, namely time or frequency. For the next generation of communication systems, it is highly desirable that time, frequency, and power spectral density are dynamically adjustable based on the measured characteristics of the user environment. Adjustment of the power spectral density by controlling the power axis is regularly performed in current systems, based on measurement of the channel signal-to-noise ratio. However, current techniques do not adjust time and frequency simultaneously and independently. Thus, what is needed is a system and method for adapting conventional puncturing and shortening techniques to adjust the time and frequency dimensions simultaneously and independently.