ed correctly, the ARQ feedback IE contains a cumulative acknowledgement. If one or more ARQ blocks in the ARQ window are in error, the ARQ feedback IE contains a selective acknowledgement to indicate the error. If the transmitter or receiver declares an ARQ synchronization loss, the transmitter or receiver may initiate the ARQ reset procedure. 7.2 HYBRID AUTOMATIC REPEAT REQUEST FUNCTIONS 7.2.1 HARQ Principles While the ARQ error control mechanism is simple and provides high transmission reliability, the throughput of ARQ schemes drop rapidly with increasing channel error rates, and the latency due to re- transmissions could be excessively high and intolerable for some delay-sensitive applications. Systems using Forward Error Correction (FEC), on the other hand, can maintain constant throughput regardless of channel error rate. However, FEC schemes have some drawbacks. High reliability is hard to achieve with FEC and requires the use of long and powerful error correction codes that increase the complexity of implementation [4]. The drawbacks of ARQ and FEC can be overcome, if the two error control schemes are properly combined. In order to achieve increased throughput and lower latency in packet transmission, a Hybrid ARQ (HARQ) scheme is designed to combine ARQ error-control mechanism and FEC coding. A HARQ CHAPTER 7 The IEEE 802. 16m MAC CPS (Part II) system consists of an FEC sub-system contained in an ARQ system. In this approach, the average number of re-transmissions is reduced by using FEC through correction of the error patterns that occur more frequently; however, when the less frequent error patterns are detected, the receiver requests a re- transmission where each re-transmission carries the same or some redundant information to help packet detection. The HARQ uses FEC to correct a subset of errors at the receiver and rely on error detection to detect the remaining errors. Most practical HARQ schemes utilize CRC codes for error detection and convolutional or Turbo codes for error correction [5]. The hy