The present invention relates to techniques for communicating primary and secondary data streams over a communication channel.
Voiceband modems and other data transmission systems often have the capability of transmitting so-called secondary data in addition to the so-called primary data. The primary data contains "user" information communicated between users while the secondary data typically carries diagnostic and/or maintenance information used within the transmission system itself and not usually passed on to the users.
There are two principal ways of communicating the secondary data.
The first of these is so-called "out-of-band" approach. Here, the secondary data is transmitted in a channel separate from that in which the primary data is transmitted--typically a frequency-division-multiplexed channel.
The out-of-band approach, however, often imposes severe constraints upon the bandsplitting filter required for separating a primary channel from a secondary channel. Such constraints occur in a bandlimited environment when higher and higher primary data rates are desired, and in order to maintain satisfactory error performance, a corresponding increase in the primary channel bandwidth is necessary. As a consequence, the bandwidth available for the secondary channel is accordingly decreased.
The above consideration renders the second, "in-band" approach, more attractive for many high speed communications. This approach, unlike the "out-of-band" approach, requires both sets of primary and secondary data to be transmitted within a single frequency band. The "in-band" approach is conventionally realized by (1) expanding the number of allowed signal points in the signal constellation and (2) adding one or more secondary data bits to the block of primary data during each symbol interval. In an environment where power is normally limited for transmission, this expansion in the number of signal points necessarily results in a loss of noise immunity in the signal transmission.