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Fast packet scheduling

The HS-DSCH downlink channel is shared between users using channel-dependent scheduling to make the best use of available radio conditions. Each user device continually transmits an indication of the downlink signal quality, as often as 500times per second. Using this information from all devices, the base station decides which users will be sent data in the next 2 ms frame and how much data should be sent for each user. More data can be sent to users which report high downlink signal quality.

The amount of the channelisation code tree, and thus network bandwidth, allocated to HSDPA users is determined by the network. The allocation is "semi-static" in that it can be modified while the network is operating, but not on a frame-by-frame basis. This allocation represents a trade-off between bandwidth allocatedfor HSDPA users, versus that for voice and non-HSDPA data users. The allocationis in units of channelisation codes for Spreading Factor 16, of which 16 exist and up to 15 can be allocated to the HS-DSCH. When the base station decides whichusers will receive data in the next frame, it also decides which channelisationcodes will be used for each user. This information is sent to the user on one of up to 4 HS-SCCHs, which are not part of the HS-DSCH allocation previously mentioned, but are allocated separately. Thus, for a given 2 ms frame, data may be sent to a number of users simultaneously, using different channelisation codes

High-Speed Downlink Shared Channel

For HSDPA, a new transport layer channel, High-Speed Downlink Shared Channel (HS-DSCH), has been added to UMTS release 5 and further specification. It is implemented by introducing three new physical layer channels: HS-SCCH, HS-DPCCH and HS-PDSCH. The High Speed-Shared Control Channel (HS-SCCH) informs the user that data will be sent on the HS-DSCH, 2 slots ahead. The Uplink High Speed-Dedicated Physical Control Channel (HS-DPCCH) carries acknowledgment information and current channel quality indicator (CQI) of the user. This value is then used by the base station to calculate how much data to send to the user devices on the next transmission. The High Speed-Physical Downlink Shared Channel (HS-PDSCH) is the channel to which the above HS-DSCH transport channel is mapped that carries actualuser data.

Hybrid automatic repeat-request (HARQ)

Data is transmitted together with error correction bits. Minor errors can thus be corrected without retransmission; see forward error correction.

If retransmission is needed, the user device saves the packet and later combinesit with retransmitted packet to recover the error-free packet as efficiently aspossible. Even if the retransmitted packets are corrupted, their combination can yield an error-free packet. Retransmitted packet may be either identical (chase combining) or different from the first transmission (incremental redundancy).

Since HARQ retransmissions are processed at the physical layer, their 12 ms round-trip time is much lower compared to higher layer retransmissions.

Adaptive modulation and coding

The modulation scheme and coding are changed on a per-user basis, depending on signal quality and cell usage. The initial scheme is quadrature phase-shift keying (QPSK), but in good radio conditions 16QAM and 64QAM can significantly increase data throughput rates. With 5 Code allocation, QPSK typically offers up to 1.8Mbit/s peak data rates, while 16QAM offers up to 3.6 Mbit/s. Additional codes (e.g. 10, 15) can also be used to improve these data rates or extend the network

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capacity throughput significantly.

Dual-Cell

Dual Cell (DC-)HSDPA, known also as Dual Carrier, is the natural evolution of HSPA by means of carrier aggregation in the downlink.[2] UMTS licenses are often issued as 10 or 15 MHz paired spectrum allocations. The basic idea of the multicarrier feature is to achieve better resource utilization and spectrum efficiencyby means of joint resource allocation and load balancing across the downlink carriers.

An advanced HSPA network can theoretically support up to 28 Mbit/s and 42 Mbit/swith a single 5 MHz carrier for Rel7 (MIMO with 16QAM) and Rel8 (64-QAM + MIMO), in good channel conditions with low correlation between transmit antennas. Analternative method to double the data rates is to double the bandwidth to 10 MHz(i.e. 25 MHz) by using DC-HSDPA. Additionally, some diversity and joint scheduling gains can also be expected[3] with improved QoS for end users in poor environment conditions where existing techniques such as MIMO spatial multiplexing cannot be used to increase data rates. In 3GPP a study item was completed in June 2008. The outcome can be found in technical report 25.825.[4] New HSDPA User Equipment categories 21-24 have been introduced that support DC-HSDPA. DC-HSDPA can support up to 42 Mbit/s, but unlike HSPA, it does not need to rely on MIMO transmission.

From Release 9 onwards it will be possible to use DC-HSDPA in combination with MIMO used on both carriers.[5] This will allow theoretical speed of up to 84 Mbit/s.

The support of MIMO in combination with DC-HSDPA will allow operators deployingRelease 7 MIMO to benefit from the DC-HSDPA functionality as defined in Release8. While in Release 8 DC-HSDPA can only operate on adjacent carriers, Release 9also allows that the paired cells can operate on two different frequency bands.Future releases will allow the use of up to four carriers simultaneously.