1. Field of the Invention
This invention generally relates to speech vocoders, and, more specifically, to flexible variable rate vocoders for wireless communication systems.
2. Background
In digital wireless communications systems, a speech vocoder is the device which compresses the digitized the speech signals prior to communication thereof over the wireless channel. Many different compression schemes for speech signals are currently used, including PCM (pulse code modulation), ADPCM (adaptive differential pulse code modulation), and EVRC (Enhanced Variable Rate-Coder). At present, the rate at which a vocoder functions is subject to constraints imposed by the network designer.
For example, in current commercial CDMA (code division multiple access) systems, the rate at which the vocoder functions is limited to four possible rates: full rate, half rate, quarter rate, or eight rate. At full rate, speech is encoded at 170 bits per frame, at half rate, 80 bits per frame, at quarter rate, 40 bits per frame, and at eighth rate, 16 bits per frame. With a 20 ms frame duration, these figures translate into a rate of 8.5 kbit/sec. for full rate, 4 kbit/sec. for half rate, 2 kbit/sec. for quarter rate, and 0.8 kbit/sec. for eighth rate. The rate which is chosen at a time depends on the level of speech activity. For a high level of activity, the full rate is chosen, while for no activity (e.g. silence background), the eighth rate is chosen. For intermediate levels of activity, the half or quarter rates are chosen. Rates greater than 8.5 kbits/sec. or less than 0.8 kbits/sec. are not possible. Since the bit-rate for a particular speech signal varies depending upon the nature of that speech signal, only the average bit-rate is of interest. The average bit-rate is also called average data-rate (ADR).
Recently, a proposal has been made for a selectable mode vocoder (SMV) which is capable of operating in one of three modes. See Test Plan and Requirements of the Selectable Mode Vocoder, version 8.0, 3GPP2-C13-19991912-007R1, which is hereby fully incorporated by reference herein as though set forth in full. In mode 0, the target ADR of the vocoder is the same as the EVRC (Enhanced Variable Rate Vocoder) rate defined in IS-127 standard. In mode 1, the target ADR is roughly 0.7 that of the EVRC. In mode 2, the target ADR is 0.55-0.6 that of the EVRC, depending on the input signal characteristics, such as speech without background noise or speech with background noises. It is contemplated that the mode of operation of a SMV will be determined by the operator based upon the tradeoff between the network capacity which is available in the system and the desired quality of service.
A problem with this approach is that, if the network cannot accommodate a demand for services at the desired level of quality, the user will be denied access to the system. Consider, for example, a user who is seeking access to the system in the case in which the network is near saturation. If the desired level of speech activity requires mode 0, and mode 0 cannot be accommodated by the system, the user will be denied access to the system.
A related problem is loss of flexibility due to the limited number of discrete modes of operation which are available. As detailed above, this loss of flexibility results in users who are denied access to the system. It will also result in lost profit opportunities as there may be users willing to pay for a higher quality of service than available in the limited number of discrete modes of operation.