For HSDPA in WCDMA/UMTs, UE will be assigned a number of SF16 channelization codes by the base station during fast scheduling.
Assuming a theoretical effective code rate of 1.0 and 16QAM modulation, the data rate achieved by each SF16 channelization code is given by
Chip rate / SF * (bits per modulation symbol) * (code rate) = 3.84M / 16 * 4 * 1.0 = 0.96 Mb/s.
while the peak data rate is 0.96 * N Mb/s, where N is the number of assigned SF16 code blocks.
If the UE is assigned 15 codes, which is the maximum possible number of codes since the 1 remaining SF16 or 16*SF256 channelization code resource has to be reserved for HS-SCCH, associated DPCH and downlink common control channels, the peak rate is 0.96*15=14.4 Mb/s.
So, why is the peak rate only 7.2 Mb/s for a UE, as alleged in commercials presented by China Unicom, the only WCDMA carrier in the Heaven Kingdom?
The answer lies in the HS-DSCH physical layer category. Only UEs with a capability to use up to 15 codes, such as Category 9 or 10 UEs, can reach the maximum data rate of 14.4Mb/s. Best UEs available on the market are usually Category-8 UEs that can only use up to 10 codes, resulting in a peak rate of 0.96*10=9.6 Mb/s. So why is it 7.2Mb/s?
By consulting 3GPP specification 25.306 (Table 5.1a), you'll find that for a UE that can use up to 10 codes, the possible HS-DSCH transport block is 14,411 bits. For a TTI of 2ms, the peak rate is 14411bits/2ms ~= 7.2 Mb/s, whose corresponding physical layer data rate is 9.6Mb/s, where the 2.4 Mb/s in difference is consumed by channel coding and other physical layer overhead.
Now we have found the answer. The 7.2Mb/s peak rate alleged by China Unicom is actually the MAC-hs throughput for Category-8 terminals.
A mathemastic explation for elastic traffic.
reciprocal
mutual
- CPCH is intended to carry packet-based user data
in the uplink. The reciprocal channel providing the data in the
downlink direction is the FACH. ([Holma 2007] P. 95)
crop up
to occur or appear, esp unexpectedly
- The basic reason behind a handover crops up when the air
interface connection no loger fulfills the desired criteria set for it.
([Kaaranen 2005] P.113)
Literature