Page 193 - From GMS to LTE

Universal Mobile Telecommunications System (UMTS) and High-Speed Packet Access (HSPA) 179

usage so that voice calls have less impact on the capacity available for high‐speed
Internet access. Parameters like bandwidth, delay and lossless handovers are not guar-
anteed for an HSDPA connection as the bandwidth available to a user depends, among
other factors, on the current signal quality and the number of simultaneous users of the
current cell. HSDPA thus sacrifices the concept of a DCH with a guaranteed bandwidth
for a significantly increased bandwidth. For many applications like web surfing or the
transfer of big files or e‐mails with file attachments, this is very beneficial.
The assignment of timeslots on HS‐DSCH channels to a user is done via several simul-
taneous broadcast High‐Speed Shared Control Channels (HS‐SCCHs), which use a
spreading factor length of 128. A mobile device has to be able to receive and decode at
least four of those channels simultaneously. Thus, it is possible to inform many users at
the same time on which HS‐PDSCH channel data will be sent to them in the next timeslot.
In addition to the shared channels, an HSDPA connection requires a number of dedi-
cated channels per subscriber:
A High Speed Dedicated Physical Control Channel (HS‐DPCCH) in the uplink direction
●
with a spreading factor of 256 for HSDPA controls information like acknowledgments
and retransmission requests for bad frames as well as transmitting signal quality
information. This channel uses its own channelization code and is not transmitted
with other channels by using time‐ or IQ‐multiplexing.
A Dedicated Control Channel (DCCH) for RRC messages in the uplink and downlink
●
directions between the RNC and the mobile device, used for tasks like mobility man-
agement, which is necessary for cell changes, for example.
A Dedicated Physical Control Channel (DPCCH) for transmit power control
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information.
A DTCH for IP user data in the uplink direction, as HSDPA only uses shared channels
●
in the downlink direction. The uplink bearer can have a bandwidth of 64, 128 or 384
kbit/s if a Release 99 DCH is used or more if HSUPA is supported by the network and
the mobile device.
Optionally, an additional DTCH is used in the uplink and downlink directions in case
●
a circuit‐switched connection (e.g. for a voice call) is established during a HSDPA
connection. The channel can have a bandwidth of up to 64 kbit/s.

3.10.2 Shorter Delay Times and Hybrid ARQ (HARQ)
Apart from offering increased bandwidth to individual users and increasing the capac-
ity of a cell in general, another goal of HSDPA was to reduce the round‐trip delay (RTD)
time for both stationary and mobile users. HSDPA further reduces the RTD times expe-
rienced with Release 99 dedicated channels of 160–200 milliseconds to about 100 mil-
liseconds. This is important for applications like web browsing, as described in
Section 3.13, and also for EDGE, as described in Section 2.12.1, which require several
frame round trips for the DNS query and establishment of the TCP connections before
the content of the web page is sent to the user. To reduce the round‐trip time, the air
interface block size has been reduced to 2 milliseconds. This is quite small compared to
the block sizes of dedicated channels of at least 10 milliseconds.
Owing to the frequently changing signal conditions experienced when the user is, for
example, in a car or train, or even walking in the street, transmission errors will fre-
quently occur. Owing to error detection mechanisms and retransmission of faulty