3G/UMTS

 

 

 

HSDPA Category Table - TDD

 

Following Table comes from TS 25.306. It lists the HS-DSCH categories of 1.28 Mcps TDD, the TDD option that TD-SCDMA networks use. The idea is the same as for FDD: a category tells the Node B how much the UE receiver can take in one TTI. But the unit of resource is different. A TDD UE counts timeslots as well as codes, and its TTI is 5 ms instead of 2 ms. The table on this page is checked against 25.306 v19.0.0.

The topics on this page are listed below.

What does the 1.28 Mcps TDD category table say?

Thirty categories fit into a simple pattern once you see the three groups. Categories 1 to 15 use QPSK and 16QAM, Categories 16 to 24 add 64QAM, and Categories 25 to 30 add MIMO. Inside each group, the categories step through the number of timeslots and the soft buffer size.

The table below is 25.306 Table 5.1c, split over two images. Each row gives the codes per timeslot, the timeslots per TTI, the bits per TTI, the soft channel bits and the supported modulations. The lower image also carries the rule for HS-DSCH reception outside CELL_DCH and the three notes on Categories 25 to 27.

< Table 5.1c: 1.28 Mcps TDD HS-DSCH physical layer categories >

25.306 Table 5.1c 1.28 Mcps TDD HS-DSCH physical layer categories 1 to 26

25.306 Table 5.1c categories 27 to 30 with the CELL_FACH rule and notes 1 to 3

25.306 Table 5.1c, Categories 1 to 30. The values match 25.306 v19.0.0. Every category allows 16 codes per timeslot, so the timeslot count and the soft buffer are what separate them.

  • 16 codes per timeslot in every row : a 1.28 Mcps TDD timeslot holds 16 codes at spreading factor 16, and every category can receive all of them.
  • 2, 3, 4 or 5 timeslots per TTI : this is the column that sets the rate. The transport channel bits per TTI grow with it, for example 2788, 8416, 11226 and 14043 bits in Categories 3, 9, 12 and 15.
  • Three soft buffer sizes per timeslot count : Categories 1, 2 and 3 share 2788 bits per TTI, but have 11264, 22528 and 33792 soft channel bits. A larger buffer lets the UE keep more HARQ processes going at full size.
  • 64QAM from Category 16 : Categories 16 to 24 repeat the 3, 4 and 5 timeslot steps with a larger transport block.
  • MIMO from Category 25 : NOTE 1 to NOTE 3 say that Categories 25, 26 and 27 behave as Categories 18, 21 and 24 when MIMO is not configured. The second row of each gives the MIMO capability, with QPSK and 16QAM only. Categories 28 to 30 support 64QAM with MIMO as well.
  • Category 9 outside CELL_DCH : a UE with HS-DSCH reception in CELL_FACH, CELL_PCH or URA_PCH supports Category 9. In those states it uses the octet aligned transport block size table of Category 9.

The TTI of 1.28 Mcps TDD is 5 ms, as 25.308 states. So the peak rate at the physical layer is the bits per TTI divided by 5 ms. Category 3 gives 2788 bits / 5 ms = 0.56 Mbps. Category 15 gives 14043 bits / 5 ms = 2.8 Mbps, and Category 24 gives 21076 bits / 5 ms = 4.2 Mbps. As in FDD, the CQI and the protocol headers keep a real test below these numbers.

  • Timeslots, not codes, separate the categories : every category takes 16 codes per timeslot.
  • Peak rate = bits per TTI / 5 ms : Category 24 reaches about 4.2 Mbps on one carrier.
  • Category 9 is the common ground : every UE with HS-DSCH reception outside CELL_DCH falls back to it there.

How does a TDD category differ from an FDD category?

If you already know the FDD table, you may try to read this one the same way. Three columns work differently, and each difference comes from the TDD frame. So let's compare the two tables column by column before you use any number from one on the other.

The first difference is the time dimension. An FDD UE receives continuously, so its category only limits codes. A 1.28 Mcps TDD subframe of 5 ms has 7 traffic timeslots, and 25.221 fixes TS0 to downlink and TS1 to uplink. The switching point decides how many of the rest go to downlink. So a TDD category limits the timeslots per TTI as well, and 5 is the largest value in Table 5.1c.

The second difference is the bit column. The FDD table gives the maximum bits of one transport block. The TDD table gives the maximum transport channel bits that can be received within one HS-DSCH TTI. The third difference is the TTI itself: 2 ms in FDD and 5 ms in 1.28 Mcps TDD. So 14043 bits in a TDD row and 14411 bits in an FDD row are close in size, but they give very different rates.

25.306 also has separate tables for the other TDD options. Table 5.1e lists 9 categories for 3.84 Mcps TDD, and Table 5.1f-a covers 7.68 Mcps TDD. The table on this page covers 1.28 Mcps TDD only.

The UE reports its TDD category in its own field. In 25.331 v19.0.1, PhysicalChannelCapability-hspdsch-r5 has a tdd128-hspdsch choice next to fdd-hspdsch, and its supported branch carries HSDSCH-physical-layer-category. 25.331 asks every UE to signal a value between 1 and 15 in that field, even when the real category is higher, and uses it when MAC-ehs is not configured. A Rel-8 UE adds hSDSCH-physical-layer-category-extension in DL-PhysChCapabilityTDD-128-v860ext, which is the category for MAC-ehs.

  • TDD limits timeslots and codes : FDD limits codes only.
  • The TDD bit column counts one whole TTI : the FDD column counts one transport block.
  • 5 ms versus 2 ms : never compare TDD and FDD rows without dividing by the right TTI.
  • The base field stops at Category 15 : the MAC-ehs category goes in the Rel-8 extension.

Which categories apply to multi-frequency HS-DSCH?

A single 1.28 Mcps carrier is only 1.6 MHz wide, so TD-SCDMA networks raise the rate by putting more carriers together. That needs a second category list, because one carrier can never carry 30 timeslots. 25.306 gives it in Table 5.1d-a, for multi-frequency HS-DSCH operation mode only.

Table 5.1d-a has 44 categories and three columns. They give the total HS-DSCH timeslots on all assigned carriers per TTI, the bits per TTI and the soft channel bits. The timeslot counts are 30, 24, 18, 15, 12, 9, 8 and 6. Categories 1 to 18 go from 30 down to 9 timeslots with QPSK and 16QAM. Categories 19 to 36 repeat the same steps with 64QAM, and Categories 37 to 44 fill in 8 and 6 timeslots. For example, Category 19 takes 30 timeslots and 126456 bits per TTI, which is about 25.3 Mbps over 5 ms. A note under the table sets the modulation: Categories 1 to 18, 39, 40, 43 and 44 support QPSK and 16QAM, and the others add 64QAM.

The UE signals these in separate IEs. DL-PhysChCapabilityTDD-128-v770ext carries multiCarrier-physical-layer-category, and 25.331 restricts it to 1 to 18 or 39, 40, 43 and 44, for use without MAC-ehs. The v860 extension adds multiCarrier-physical-layer-category-extension for 1 to 36 with MAC-ehs, and DL-PhysChCapabilityInfoTDD-128-va40ext adds extension2 for 37 to 64. In NBAP, 25.433 gives the Node B a Multi-carrier HS-DSCH Physical Layer Category IE next to the single carrier one. The Node B uses the two together to allocate HSDPA resources over several carriers.

  • Two category lists for one UE : Table 5.1c for one carrier and Table 5.1d-a for multi-frequency operation.
  • Timeslots add up across carriers : 30 timeslots per TTI only fit on several carriers.
  • Separate IEs in RRC and NBAP : the multi-carrier category never replaces the single carrier category.

Reference

  • 25.306 UE Radio Access capabilities - v19.0.0, Tables 5.1c, 5.1d-a, 5.1e and 5.1f-a
  • 25.308 High Speed Downlink Packet Access HSDPA overall description - v19.0.0, TTI of 1.28 Mcps TDD
  • 25.221 Physical channels and mapping of transport channels onto physical channels TDD - v19.0.0, frame structure of 1.28 Mcps TDD
  • 25.331 Radio Resource Control - v19.0.1, clause 10.3.3.25 notes and the ASN.1 of PhysicalChannelCapability-hspdsch-r5 and DL-PhysChCapabilityTDD-128 extensions
  • 25.433 UTRAN Iub interface NBAP signalling - v19.0.0, UE Capabilities Information in HS-DSCH Information