The present invention generally pertains to telecommunications and is particularly directed to enhancing the efficiency of packet transmission while using an automatic repeat request (ARQ) routine, in which transmission of a particular packet is repeated automatically when correct receipt of said particular packet is not acknowledged within a timeout interval following transmission of the packet. When the transmission of a particular packet is repeated, the multiple transmissions of the particular packet are combined for processing to decide whether the particular packet ultimately has been correctly received.
There have been a number of different ARQ routines. These routines are broadly characterized by whether the combining of the multiple transmissions of the particular packet uses hard or soft decisions, and whether the signal constellation of the received particular packet is constant. Generally, changing the signal constellation and changing the data transmission rate are synonymous.
An early ARQ routine included a constant constellation (data transmission rate) and hard-combining decisions. However, the bit error rate (BER) typically increases rapidly for a constant data transmission rate below a certain threshold signal-to-noise (S/N) ratio, as exemplified by waterfall BER curves. Catastrophic failure occurs below the threshold S/N ratio without regard to the type of data combining. This method was quickly abandoned on channels that exhibit varying S/N ratios.
A subsequent, second ARQ routine included a changing constellation and hard-combining decisions. This routine changes the data transmission rate as the channel S/N ratio varies. It eliminates the catastrophic failure of the earlier ARQ routine, but it has some practical difficulties. If the data transmission rate is too high, (transient) catastrophic failure may occur. If the data transmission rate is too low, the system is inefficient with respect to what data transmission rate may have been attained. On dynamic channels, optimal control of the data transmission rate may be acutely difficult.
A later, third ARQ routine included a constant constellation, soft-combining decisions and a constellation complexity on the order of two bits/symbol. This routine provided a dramatic improvement over the second ARQ routine. Essentially, the data transmission rate remains constant and is not higher than approximately the bandwidth; and the data combining process attains channel capacity (approximately). This routine does not attain data transmission rates significantly greater than the channel bandwidth.
The present state of the art is to use an ARQ routine that includes a simple and immediate extension of the third ARQ routine. This extension uses a changing constellation, soft-combining and constellation complexity that is greater than two bits/symbol. The data transmission rate is the same as for the third routine for rates that are not higher than approximately the bandwidth. For higher data transmission rates, such as for higher order QAM, more complex constellations are used, as in the second ARQ routine. However, there are difficulties associated with data transmission rate control as described above in relation to the second ARQ routine.