Maximizing data rate by adjusting codes and code rates in CDMA system
View Patent ↗The present invention provides for making code rate adjustments and modulation type adjustments in a pseudonoise (PN) encoded CDMA system. Coding rate adjustments may be made by changing the number of information bits per symbol, or Forward Error Code (FEC) coding rate. A forward error correction (FEC) block size is maintained at a constant amount. Therefore, as the number of information bits per symbol are increased, an integer multiple of bits per epoch is always maintained. The scheme permits for a greater flexibility and selection of effective data rates providing information bit rates ranging from, for example, approximately 50 kilobits per second to over 5 mega bits per second (Mbps) in one preferred embodiment.
1. A method for coding channels in a wireless communication system in which a digital signal is communicated from a transmitting station to a receiving station, the method comprising the steps of:
grouping the bits of the input signal into blocks, a size of each block being adjustable according to a predetermined block size parameter;
forward error correction (FEC) coding the bits of the blocks, a rate of the FEC code selected such that a number of FEC symbols in a transmitted block remains constant, even if a number of information bits in a block changes;
symbol modulating the FEC symbols of the blocks with a predetermined number of bits per symbol, again such that the number of FEC symbols in a transmitted block remains constant;
channel coding the modulated symbols with a spreading code and a channel code to produce a transmit signal; and
transmitting the transmit signal over a wireless communication link.
2. A method as in claim 1 wherein the number of FEC symbols in each transmitted block remains the same, even if the rate of FEC code is changed for a given connection.
3. A method as in claim 1 wherein the symbol modulating is selected from a group consisting of Quadrature Phase Shift Keyed (QPSK), eight level Phase Shift Key (PSK), sixteen level Quadrature Amplitude Modulation (16 QAM) and 64 QAM.
4. A method as in claim 1 wherein the number of FEC symbols per transmitted block is selected from the group consisting of 2, 3, 4, and 6 bits per symbol.
5. A method as in claim 1 additionally comprising the step of:
sending a message to the receiver station from the transmitter station, the message including an indication of the rate of FEC code used in generating the encoded symbols, thereby permitting the receiver station to determine a symbol decoding rate required to properly decode the symbols of a received frame.
6. A method as in claim 1 wherein the channel coding further comprising the step of:
coding each of the modulated symbols with a spread code and a channel code to permit separation of an encoded symbols from other encoded symbols transmitted on a given radio carrier frequency signal intended for other channels.
7. A method as in claim 1 wherein the communication link is a forward link transmitted from a base station transmitter in a direction towards an access unit receiver station.
8. A method as in claim 1 wherein the communication link is a reverse link channel transmitting information from a remote subscriber unit stationed towards a receiving base station.
9. A method as in claim 1 wherein the rate of the FEC code is chosen based upon observed link quality conditions in a radio channel.
10. A method as in claim 9 in which radio channels experiencing bit error rates cause selection of the rate of the FEC code which is lower.
11. A method as in claim 1 wherein the rate of the FEC code for different receivers on a given radio carrier frequency have different symbol and framing rates.