ks, and that no padding bits are needed in the LDPC coding. The quantization also results in the same transport block size being obtained, even if there are small variations in the amount of resources allocated, a property that is useful when scheduling retransmissions on a different set of resources than the initial transmission (see Table 4.17). In the case of a retransmission, the transport block size by definition, is unchanged and there is no need to signal this information. Instead, the reserved entries represent the modulation scheme (QPSK, 16QAM, 64QAM, or if config- ured 256QAM), which allows the scheduler to use an (almost) arbitrary combination of resource blocks for the retransmission. The use of the reserved entries assumes that the UE properly received the control signaling for the initial transmission, if this is not the case, the retransmission should explicitly indicate the transport block size [14]. 4.1.8 HARQ Operation and Protocols 4.1.8.1 HARQ Principles While 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 retransmissions, 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. 540 Chapter 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, hybrid ARQ (HARQ) scheme was designed to combine ARQ error-control mechanism and FEC coding. A HARQ system consists of a FEC subsystem contained in an ARQ system. In this approach, the average number of retransmissions is reduced by us