FAQ on WCDMA Physical Layer (Layer 1)
By Zahid Ghadialy (zahidtg@yahoo.com)
What happens when the Mobile is switched on?
How does it find the Scrambling code to camp on?
When the mobile The
synchronization procedure starts with downlink SCH synchronization. The UE
knows the SCH primary synchronization code, which is common to all cells. The
slot timing of the cell can be obtained by receiving the primary
synchronization channel (P-SCH) and detecting peaks in the output of a filter
that is matched to this universal synchronization code. The slot
synchronization takes advantage of the fact that the P-SCH is only sent
during the first 256 chips of each slot. The whole slot is 2,560 chips long.
This is depicted in Figure above. Thus the UE can determine when a slot
starts, but it does not know the slot number yet (there are 15 slots in each
frame), and thus it does not know where the radio frame boundary may be.
Thereafter the UE correlates the
received signal from the secondary synchronization channel (S-SCH) with all
secondary synchronization codes (SSC), and identifies the maximum correlation
value. The S-SCH is also only sent during the first 256 chips of every slot.
One SSC is sent in every time slot. There are 16 different SSCs, and they can
form 64 unique secondary SCH sequences. One sequence consists of 15 SSCs, and
these sequences are arranged in such a way that in any nonzero cyclic shift
less than 15 of any of the 64 sequences is not equivalent to some other sequence.
This means that once the UE has identified 15 successive SSCs, it can
determine the code group used as well as the frame boundaries (i.e., frame
synchronization).
What is RACH and how does it work?
The Random Access Channel (RACH)
is an uplink transport channel. The RACH is always received from the entire
cell. The RACH is characterized by a collision risk and by being transmitted
using open loop power control. The Random Access Channel (RACH) is typically
used for signalling purposes, to register the terminal after power-on to the
network or to perform location update after moving from one location area to
another or to initiate a call. The structure of the physical RACH for
signalling purposes is the same as when using the RACH for user data
transmission.
In UTRA the RACH procedure has the
following phases:
·
The
terminal decodes the BCH to find out the available RACH sub-channels and
their scrambling codes and signatures.
·
The
terminal selects randomly one of the RACH sub-channels from the group its
access class allows it to use. Furthermore, the signature is also selected
randomly from among the available signatures.
·
The downlink
power level is measured and the initial RACH power level is set with the
proper margin due to the open loop inaccuracy.
·
A 1 ms
RACH preamble is sent with the selected signature.
·
The
terminal decodes AICH to see whether the base station has detected the
preamble.
·
In case
no AICH is detected, the terminal increases the preamble transmission power
by a step given by the base station, as multiples of 1 dB. The preamble is
retransmitted in the next available access slot.
·
When an
AICH transmission is detected from the base station, the terminal transmits
the 10 ms or 20 ms message part of the RACH transmission.
The RACH procedure is illustrated
in Figure above, where the terminal transmits the preamble until
acknowledgement is received on AICH, and then the message part follows. In
the case of data transmission on RACH, the spreading factor and thus the data
rate may vary; this is indicated with the TFCI on the DPCCH on PRACH.
Spreading factors from 256 to 32 have been defined to be possible, thus a
single frame on RACH may contain up to 1200 channel symbols which, depending
on the channel coding, maps to around 600 or 400 bits. For the maximum number
of bits the achievable range is naturally less than what can be achieved with
the lowest rates, especially as RACH messages do not use methods such as
macro-diversity as in the dedicated channel.
Reference: WCDMA for UMTS: Radio Access
What is the significance of SFN and CFN?
SFN is the frame number used by
the physical layer. CFN is the frame number used by the MAC layer. SFN is
independent of the UE contexts, but associated with the Radio Link. CFN is
associated with a UE context. The RRC layer maintains the mapping between CFN
and various (for each RL) SFNs.
What is compressed mode and is it necessary
for the UE to support compressed mode?
Compressed mode is needed if the
UE needs to perform Inter-Frequency or Inter-RAT measurements. More details
on what compressed mode is and how its performed can be seen in Compressed Mode Tutorial.
Compressed Mode is performed in
Uplink(UL) as well as in Downlink(DL). Uplink compressed mode must be used if
the frequency to be measured is close to the uplink frequency used by the
UTRAN air interface (i.e., frequencies in TDD mode/GSM 1800/1900 band).
Otherwise inter-frequency interference may affect the results. Downlink
compressed mode is not necessary if the UE has dual receivers. In that case
one receiver can perform inter-frequency measurements while the other handles
the normal reception. Note however, that double receivers in the UE do not
remove the need for uplink compressed mode. If the uplink frequency is close
enough to the downlink frequency to be measured, then compressed mode must be
employed in the uplink to prevent inter-frequency interference.
Why is secondary scrambling code needed?
For each primary scrambling code
there is a set of 16 secondary scrambling codes. They can be employed while
transmitting channels that do not need to be received by everyone in the
cell. They should be used sparingly because channels transmitted with
secondary scrambling codes are not orthogonal to channels that use the
primary scrambling code. One possible application could be in sectored cells,
where separate sectors do not have to be orthogonal to each other.
The secondary downlink scrambling
codes can be applied with the exception of those common channels that need to
be heard in the whole cell and/or prior to the initial registration. Only one
scrambling code should be generally used per cell or sector to maintain the
orthogonality between different downlink code channels. With adaptive
antennas the beams provide additional spatial isolation and the orthogonality
between different code channels is less important. However, in all cases the
best strategy is still to keep as many users as possible under a single
scrambling code to minimise downlink interference. If a secondary scrambling
code needs to be introduced in the cell, then only those users not fitting
under the primary scrambling code should use the secondary code. The biggest
loss in orthogonality occurs when the users are shared evenly between two
different scrambling codes.
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