Page 197 - From GMS to LTE

Universal Mobile Telecommunications System (UMTS) and High-Speed Packet Access (HSPA) 183

In addition to changing the modulation scheme, the network can also alter the coding
scheme and the number of simultaneously used HS‐DSCH channels for a mobile device
on a per‐frame basis. This behavior is influenced by the Channel Quality Index (CQI),
which is frequently reported by the mobile device. The CQI has a range from 1 (very
bad) to 31 (very good) and tells the network how many redundancy bits are required to
keep the Block Error Rate (BLER) below 10%. For a real network, this means that under
less favorable conditions more bits per frame are used for error detection and correc-
tion. This reduces transmission speed but ensures that a stable connection between
network and mobile device is maintained. As modulation and coding is controlled on a
per‐user basis, bad radio conditions for one user have no negative effects for other users
in the cell to which the same HS‐DSCHs are assigned for data transmission.
By adapting the modulation and coding schemes, it is also possible to keep the power
needed for the HSDPA channels at a constant level or to only need to vary it when the
DCH load of the cell changes. This is different from the strategy of Release 99 dedicated
channels. Here, the bandwidth of a connection is stable while the transmission power is
adapted depending on the user’s changing signal quality. Only if the power level cannot
be increased any further, to ensure a stable connection, does the network take action
and increase the spreading factor to reduce the bandwidth of the connection.
The capabilities of the mobile device and of the network limit the theoretical maxi-
mum datarate. The standard defines a number of different device categories, which are
listed in 3GPP TS 25.306 [17]. Table 3.6 shows some of these categories and their prop-
erties. Not listed in the table is category 12, which was used by early HSDPA devices
that are no longer available. Such devices could support five simultaneous high‐speed
channels and QPSK modulation only. The resulting datarate was 1.8 Mbit/s.
With a category 24 mobile device, found in practice today, that supports QPSK, 16‐
QAM, 64‐QAM and dual‐carrier operation, the following maximum transmission
speed can be reached: 42, 192 bits per TTI (which is distributed over 2 × 15 HS‐PDSCH
channels) every 2 milliseconds = (1/0.002) × 42, 192 = 42.2 Mbit/s. This corresponds to a
speed of 1.4 Mbit/s per channel with a spreading factor of 16. Compared to a Release 99

Table 3.6 A selection of HSDPA mobile device categories.

Maximum number
HS‐DSCH of simultaneous MIMO/Dual‐ Maximum
category HS‐PDSCH Best modulation carrier Code rate datarate (Mbit/s)

6 5 16‐QAM – 0.76 3.6
8 10 16‐QAM – 0.76 7.2
9 15 16‐QAM – 0.7 10.1
10 15 16‐QAM – 0.97 14.4
14 15 64‐QAM – 0.98 21.1
16 15 16‐QAM MIMO 0.97 27.9
20 15 64‐QAM MIMO 0.98 42.2
24 15 64‐QAM DC 0.98 42.2
28 15 64‐QAM DC + MIMO 0.98 84.4