 polling PDU. Due to the possibility of HARQ NACK R ACK errors that happen to the retransmitted PDU, other triggers of polling/status report are still needed (if no indication from the MAC exists to handle such errors) but could be reduced to a minimum extent. The reason is that the probability of RLC retransmission equals that of the HARQ residual PDU error rate, which is about 10-3 [8]. The probability of NACK ® ACK is normally about 10-3, SO the probability of a HARQ NACK ® ACK error happening to an RLC retransmission PDU is only about 10-6. Thus, the periodic triggers of polling/ status reports are largely unnecessary and could result in spurious retrans- missions. A window-based polling could be used because it results in very few additional status reports. © 2009 by Taylor & Francis Group, LLC Long Term Evolution: 3GPP LTE Radio and Cellular Technology 9.5 Cellular Network with Relay For the future broadband RAN, ubiquitous high data rate transmission is a demand- ing requirement. Because the RAN is still likely to inherit the cellular structure, inevitably the transmission quality at the cell edge area would be inferior to that at the cell center. Relay stations may be a good candidate to enhance the cell edge data rate and to provide homogeneous quality of service (QoS) experience for the users. We investigate the relay-enhanced cell (REC) with the aim of enhancing the data rate. As shown in Figure 9.2, a three-sector site is assumed in which each sec- tor still takes the hexagonal form. For each sector, one relay node (RN) is placed at the direction of the antenna bore sight. The UT (user terminal) could directly connect to the BS or be connected via the RN. No diversity from the BS and the RN is assumed for a single UT. It is expected that the RN should not only act as an analog amplifier, as it has traditionally, but also follow the "decode and forward" paradigm. The RN has a full-fledged MAC layer with the HARQ functionality. Also, it has to take the responsibility of scheduling and resource allocatio