behavior of different scheduling strategies: (a) for full buffers and (b) for web browsing traffic model. emphasizes the need to not only design the scheduler for received packet is in error or not. If no error is detected the full buffer case. in the received data packet, the received data is declared error-free and the transmitter is notified by sending a positive acknowledgment (ACK). On the other hand, if 4.5.3 Advanced retransmission an error is detected, the receiver discards the received schemes data and notifies the transmitter via a return channel by sending a negative acknowledgment (NAK). In response Transmissions over wireless channels are subject to errors, to a NAK, the transmitter retransmits the same for example due to variations in the received signal quality. information. To some degree, such variations can be counteracted Virtually all modern communication systems, in- through link adaptation as discussed above. However, cluding WCDMA and cdma2000, employ a combination receiver noise and unpredictable interference variations of forward error-correcting coding and ARQ, a combina- cannot be counteracted. Therefore, virtually all wireless tion known as hybrid ARQ. Hybrid ARQ uses forward communications systems employ some form of Forward error correcting codes to correct a subset of all errors and Error Correction (FEC), tracing its roots to the pioneering rely on error detection to detect uncorrectable errors. work by Claude Shannon in 1948. There is a rich literature Erroneously received packets are discarded and the re- in the area of error-correcting coding, and a detailed de- ceiver requests retransmissions of corrupted packets. scription is beyond the scope of this book. In short, the Thus, it is a combination of FEC and ARQ as described basic principle beyond forward error-correcting coding is above. Most practical hybrid ARQ schemes are built to introduce redundancy in the transmitted signal. This is around a CRC code for error detection and convolutional achieved by adding parity bits to 