1. Technical Field
The present disclosure relates to managing parameters that affect the amount of buffer memory space used to store Log-Likelihood Ratio (LLR) values in a receiver.
2. Background Information
In communication systems, such as cellular telephone systems, information is often to be communicated between devices in the presence of noise and other environmental factors that interfere with the communication. To prevent loss of information, the original information to be communicated is often encoded along with additional information before transmission. After reception, the received information is decoded to recover the original information. Due to the encoding and decoding process, if some of the transmitted information is lost during transmission between transmitter and receiver, the received information can nevertheless often be used to recover the original information.
FIG. 1 (Prior Art) is a simplified block diagram illustrating such an encoding process. A plurality of bits 100 of data undergoes encoding 101 to generate a larger number of encoded bits 102. The encoding process adds redundancy to the original data by producing several encoded bits 102 for each incoming bit 100. The encoded bits 102 are sometimes referred to as “symbols.” The measure of redundancy introduced by the encoder 101 is often referred to as “code rate.” In the illustration, a data packet 103 containing “X” number of data bits is encoded by an encoder 104 with a code rate of 1/5. The code rate of 1/5 means that encoder 104 produces five encoded bits 105 for each data bit in data packet 103. Encoder 104 can be a turbo encoder or a Viterbi encoder. The added redundancy provides protection against noise and disturbances in the transmission of data.
In the art, encoder 104 with a code rate of 1/5 is considered to have a relatively low code rate, and thus provides a relatively high amount of redundancy. An encoder with a code rate of 1/2 would produce only two encoded bits for each data bit in a data packet. An encoder with a code rate of 1/2 would be considered to have a relatively high code rate, and provide a relatively low amount of redundancy. After the encoding process, encoded bits 105 are typically blocked together by a modulator (not shown) and mapped to points on a constellation by a mapper (not shown) before being transmitted to a receiver.
FIG. 2 (Prior Art) is a simplified block diagram illustrating a part of the receiver that receives the transmission of encoded data. The received signal is digitized to produce samples 106. Samples 106 are converted by a Fast Fourier Transform (FFT) circuit 107 into symbols 108. A demodulator circuit 109 outputs an I/Q modulation symbol and a signal-to-noise ratio (SNR) measurement 110 for each symbol. The I/Q and SNR information is then converted by a Log-Likelihood Ratio (LLR) generator 111 into a set of soft-information values referred to as LLR values. Each LLR value 112 produced by LLR generator 111 is stored into an LLR buffer memory 113. The LLR buffer memory 113 is typically a Random-Access Memory (RAM). The number of bits used to represent each LLR value 112 is referred to as the “LLR bit width.” If the LLR bit width is larger, then more LLR buffer memory is generally required to store the LLR values, whereas if the LLR bit width is smaller then less LLR buffer memory is generally required to store the LLR values. Then, in a reversal of the encoding process described above in connection with FIG. 1, the LLR values from LLR buffer 113 are decoded by a decoder 114 to generate data bits 115. Ideally, data bits 115 of FIG. 2 are identical to the original data bits 103 of FIG. 1, despite the presence of interference and the loss of some of the transmission from the transmitter to the receiver.
Ways are sought to reduce the cost of implementing the overall transmitter and receiver system without unduly degrading system performance.