Release 5 UMTS (universal mobile telecommunication system) frequency division duplex (FDD) and time division duplex (TDD) modes have incorporated a feature called high-speed downlink packet access (HSDPA) for improving throughput, latency, and spectral efficiency in the downlink (DL).  The principle of HSDPA is to schedule packet transmissions on the air interface to different mobile units as a function of their instantaneous experienced radio and service conditions in a dynamic manner (i.e., fast; for example, every 2 ms in FDD or every 10 ms in TDD).  Key functionalities of HSDPA in both FDD and TDD modes include: fast re-transmissions (hybrid automatic repeat request (ARQ)) of DL packets received in error over the air interface (Uu), fast uplink (UL) notification of DL packets received in error (acknowledgements/negative acknowledgements), fast channel feedback in the UL on a wireless transmit/receive unit's (WTRU's) DL channel state, and fat-pipe scheduling for efficiently servicing many users in the DL.  This functionality, i.e., the fast, dynamic HSDPA packet scheduler, is located in the base station (i.e., the Node B) and operates in a rather autonomous manner from the radio network controller (RNC). 
In a TDD system, the RNC allocates a certain number of timeslots for the usage of HSDPA data channels, i.e., the HS-DSCH (high-speed downlink shared channel), to each cell.  The RNC communicates to the Node B which timeslots and which set of spreading codes in each of the timeslots can be used for the HS-DSCH by means of Iub/Iur signaling.  The RNC subsequently passes control to the Node B on when to send DL packets in the selected timeslots and spreading codes. 
Furthermore, for HSDPA operation in TDD, DL and UL control signaling from the Node B to the WTRU and from the WTRU to the RNC is important.  Two types of HSDPA control channels exist, the HS-SCCH (high-speed shared control channel) for fast DL signaling and the HS-SICH (high-speed shared information channel) for fast UL signaling.  Both the HS-SCCH and the HS-SICH occupy one resource unit (one spreading factor 16 code in one timeslot). 
The DL HS-SCCH is used by the Node B to alert a WTRU in a group of WTRUs that high-speed data is scheduled for it on the HS-DSCH.  One particular WTRU can monitor up to four HS-SCCHs in parallel.  It is noted that more than four HS-SCCHs can be set up in a cell. 
The UL HS-SICH is used by a WTRU to inform the Node B of the outcome of a HS-DSCH decoding attempt, i.e., data reception successful/not successful.  Any HS-SICH is unambiguously associated with the occurrence of a particular HS-SCCH (fixed timing relationship and code mapping) in order to allow the Node B to establish a clear relationship between a WTRU which has been addressed on the HS-SCCH and the same WTRU's corresponding UL transmission after HS-DSCH decoding. 
For HSDPA operation in both FDD and TDD, the RNC maintains a permanent low-rate UL and DL signaling connection to the WTRU by means of a dedicated channel (DCH).  This so-called associated DCH conveys radio resource control (RRC) information (for example, handover commands or measurement data) and is also used in the UL for conveying user plane data, for example TCP/IP acknowledgements.  This associated DCH is identical from a functional point-of-view to conventional UMTS R99 or R4 dedicated channels, even if a much lower data rate (i.e., 3.4 kbps) is needed. 
Channel configuration for HSDPA, i.e., the allocation of the HS-DSCH, HS-SCCH, and HS-SICH channels necessary for HSDPA operation and associated UL and DL DCHs to timeslots and spreading codes, is done by the RNC at connection set-up.  The RNC informs the WTRU of channel configurations by means of RRC signaling and the Node B by means of NBAP (Node B application part) signaling over the Iub/Iur network interfaces. 
The high number of channels (i.e., up to four DL HS-SCCHs, one DL associated DCH, one UL associated DCH, and eventually an UL HS-SICH) which a WTRU must regularly receive or send while in HSDPA service constitute a significant burden on its power consumption and therefore its battery efficiency.  This is especially relevant during sleep mode, i.e., the short (one to several tens or more idle frames) but frequent HSDPA transmission pauses when other WTRUs are being serviced by the Node B packet scheduler. 
The determining factor of overall WTRU power consumption is the length of time that its radio frequency (RF) part (power amplifiers, frequency oscillation stage, intermediate frequency converters, and filters) is powered on, such as necessary for receiving in a timeslot containing data or for sending in a timeslot containing data.  A WTRU's baseband operation parts (digital DSPs, ASICs, etc.)  are less demanding in terms of power consumption, and baseband parts are often operating on background tasks or decoding while the RF part is powered off.  Typically, approximately 80% of WTRU battery power consumption goes into the WTRU's RF part when both the baseband part and the RF part are simultaneously active. 
The high number of simultaneous channels necessary to support HSDPA service therefore poses a challenge in how to allocate the channels in a way to minimize a WTRU's power consumption.