Synchronization

The SCH consists of two channels: the primary SCH and the secondary SCH (see Figure
3.4). These are used to enable the mobile to synchronize to the network in order for
the mobile to identify the base station-specific scrambling code.

The primary SCH is transmitted once every slot�"that is, 15 times in a 10-ms frame.
It is a 256-chip unmodulated spreading sequence that is common for the whole network�"
that is, identical in every cell. It is sent at the front of the 0.625-ms burst and
defines the cell start boundary. Its primary function is to provide the handset with a
timing reference for the secondary SCH. The secondary SCH, also 256 chips in length,
is transmitted in every slot, but the code sequence, which is repeated for each frame,
informs the handset of the long code (scrambling) group used by its current Node B.
As the primary SCH is the initial timing reference; in other words, it has no prior
time indicator or marker, the receiver must be capable of detecting it at all times. For
this reason, a matched filter is usually employed. The IF, produced by mixing the
incoming RF with the LO, is applied to the matched filter. This is matching against the
256-bit primary SCH on the CCPCH. When a match is found, a pulse of output energy
is produced. This pulse denotes the start of the slot and so is used to synchronize slotrecovery
functions.
A256 tap matched filter at a chip rate of 3.84 Mcps requires a billion calculations per
second. However, as the filter coefficients are simply +1 -1 the implementation is reasonably
straightforward. The remaining 2304 chips of the P-CCPCH slot form the
BCH. As the BCH must be demodulated by all handsets, it is a fixed format. The channel
rate is 30 kbps with a spreading ratio of 256, that is, producing a high process gain
and consequently a robust signal. As the 256-bit SCH is taken out of the slot, the true
bit rate is 27 kbps.

The Common Channels also include the Common PIlot CHannel (CPICH). This is
an unmodulated channel that is sent as a continuous loop and is scrambled with the
Node B primary scrambling code for the local cell. The CPICH assists the handset to
estimate the channel propagation characteristic when it is in idle mode�"that is, not in
dedicated connection mode (making a call). In dedicated connection mode the handset
will still use CPICH information (signal strength) to measure for cell handover and reselection.
In connection mode the handset will use the pilot symbols carried in the dedicated
channels to assess accurately the signal path characteristics (phase and
amplitude) rather than the CPICH. The CPICH uses a spreading factor of 256�"that is,
high process gain for a robust signal.
Because the mobile only communicates to a Node B and not to any other handset,
uplink common physical channels are not necessary. All uplink (handset to Node B)
information�"that is, data and reporting�"is processed through dedicated channels. 66