uplink transmission. Such a simple fixed timing relationship is not possible for TDD, since subframe n + 4 may not be an uplink subframe, and consequently other timing relationships have been defined for TDD mode. Another difference concerns the re-transmission of erroneously received data packets. For both downlink and uplink, LTE utilizes HARQ with soft combining, i.e., the terminal and the network can request re-transmission of erroneously received data packets. In case the original trans- mission fails, the receiver buffers the soft information and combines it with the re-transmission. The CHAPTER 7 The IEEE 802. 16m MAC CPS (Part II) fundamental structure used is a set of parallel HARQ processes, where after a fixed time following reception of data, a positive or negative acknowledgement is transmitted. In each downlink subframe, data can be transmitted over one of the HARQ processes in the UE, while the other processes are in various stages of decoding or rescheduling. By selecting the number of processes appropriately, continuous transmission to a terminal can be achieved. In TDD mode, on the other hand, the time from reception of downlink data to transmission of an acknowledgement in the uplink depends on the uplink/downlink ratio, and the number of HARQ processes varies from 4 to 15. This results from the fact that uplink subframes are not available at all times and the feedback must be delayed at least until there is an uplink subframe. The other main difference between 3GPP LTE FDD and TDD modes with respect to HARQ processing is related to the transmission of acknowledgements in the uplink. For frame configurations where the number of uplink subframes is greater than or equal to the number of downlink subframes, each downlink subframe has been associated with an uplink subframe in such a way that acknowl- edgements from at most one downlink subframe need to be transmitted in every uplink subframe. However, in configurations where the number of downlink subframes per radio frame is larger than the numb