 Subframe FIGURE 10-14 Physical structure of HARQ feedback channel [2] CHAPTER 10 The IEEE 802. 16m Physical Layer (Part II) Table 10-19 Orthogonal Sequences for the Uplink HARQ Feedback [2] Orthogonal Sequence HARQ Feedback Channel Sequence Indexi CioCi1Ci2Ci3 Information and Index [+1 +1 +1 +1] Even-numbered channel ACK [+1 -1 +1 -1] Even-numbered channel NACK [+1 +1 -1-1] Odd-numbered channel ACK [+1 1-1-1+1] Odd-numbered channel NACK 10.3.3 Sounding Channel The sounding channel is used by a mobile station to send sounding signals (alternatively known as a sounding reference signal in 3GPP LTE) for MIMO feedback, channel quality feedback, and uplink channel measurement at the base station. Furthermore, uplink sounding enables sounding-based downlink MIMO in TDD mode, and uplink closed-loop MIMO in TDD and FDD modes. The sounding channel occupies specific uplink sub-bands (narrowband sounding signal) or the entire bandwidth (wideband sounding signal) over one OFDM symbol. It is obvious that the use of narrowband or wideband sounding signals depends on the uplink MIMO mode, which will be dis- cussed in the next section. The sounding signal is transmitted over predefined sub-carriers within the sub-bands. The sounding channel parameters are transmitted in System Configuration Descriptor MAC control message and SFH SP1 broadcast channel. The periodicity of the sounding signal for each MS is configurable. The sounding channel is frequency-division multiplexed (narrowband sounding) or time-division multiplexed (wideband sounding) with other control and data channels. The BS can configure a group of mobile stations to transmit sounding signals on the corresponding sounding channels. The sounding channels from multiple users or multiple antennas per user are orthogonally multiplexed. Power control for the UL sounding channel is supported to improve sounding quality. Each mobile station's transmit power for the sounding channel is individually controlled according to its sounding channel target CINR value. Transmission