5.5.1 Synchronous Procedure
1. Cell search
During the cell search, the UE searches a
cell and determines the downlink scrambles are synchronous with the frames of
the common channel. Generally, it has the following three steps:
Step 1: Timeslot synchronization
At this step, the
UE uses the primary synchronization code of the SCH to get the timeslot
synchronization of this cell. Typically, it uses a matched filter to match the
primary synchronization code which is common for all cells. The timeslot timing
of this cell can be got from the wave peak value output by the detection
matched filter.
Step 2: Frame synchronization and code set
identification
At this step, the
UE uses the secondary synchronization code of the SCH to get the timeslot
synchronization, and identifies the cell set found at the first step, which is
implemented by correlating the signals received and all possible secondary
synchronization code sequences, and identifying the maximum correlation value. The
periodical shift of the sequence is unique, so if the code set is the same as
the frame synchronization, it can be confirmed.
Step 3: Scramble identification
At this step, the UE finds the definite
primary scramble used by the cell. The primary scramble is got by correlating
all codes in the identified code set on the CPICH through symbols. After
identifying the primary scramble, the primary CCPCH can be detected, so the
specific BCH information of the system and cell can be read.
2. Common channel synchronization
The radio frame timing of all common
physical channels can be confirmed after the cell search. During the cell
search, the radio frame timing of the P-CCPCH can be got, and then the timing
for these channels can be confirmed, according to the related timing relation
between the P-CCPCH and other common physical channels.
3. Dedicated channel synchronization
After synchronizing the common channels, during the establishment of
services and other correlations, the UE can synchronize uplink and downlink
dedicated channels, according to the corresponding protocol specification.
5.5.2 Paging
Procedure
After registering a network, the UE is
allocated to a paging group. If there is paging information sent to any UE
belonging to this paging group, the Paging Indicator (PI) will appear
periodically in the Paging Indicator Channel (PICH).
After
detecting the PI, the UE will decode the next PCH frame transmitted in the
S-CCPCH to check whether there is paging information sent to it. When the PI
receiving indication judgment is less reliable, the UE is also required to
decode the PCH. The paging intervals are shown in Figure 5-34.
The less the PI appears, the less the UE
is waken from the hibernating mode, and the longer the battery lives. Obviously, the compromising solution lies on the time corresponding
to the call from the network. However, increased PI intervals can not infinitely
extend the service life of batteries, so in free mode, the UE still has other
tasks for processing.
5.5.3 Random
Access Procedure
During the random access procedure of the
CDMA system, the near-far effect should be suppressed, because the power
required for sending is unknown in the initialization transmission. The
transmit power, set by using the principle of open loop power control and
according to the absolute power got by measuring the received power, fluctuates
greatly. The RACH of UTRAN has the following operation procedures:
l
The
UE decodes the BCH so as to find the usable RACH sub-channels and scrambles as
well as signatures.
l
The
UE selects a RACH sub-channel randomly from the usable access group as well as
a signature from the usable signatures.
l
The
UE measures the downlink power level, and sets initial power level for the
uplink RACH.Sending the selected signature in the access preamble
l
The
UE decodes the AICH, views the transmit power of the enhanced preamble for the
1dB times step length provided by the base station. The
preamble will be resent in the next access timeslot.
l
After detecting the AICH of the
base station, the UE begins to send 10 ms or 20 ms messages transmitted by the
RACH.
The
UE keeps sending the preamble until it receives the confirmation of the AICH,
then sends the messages.
When the RACH transmits data, spreading
factors and data rates are variable between frames, which is indicated by the
TFCI of the PRACH control part.Usable spreading factors range from 256 to 32,
therefore, a signal frame in the RACH may have 1200 signaling symbols, which
can be mapped as 600 or 400 bits according to the channel codes.For the maximum
bit, its reachable coverage area is smaller than that transmitted with the
minimum rate, especially when the RACH does not use macro diversity in the
dedicated channel.
5.5.4 Access
Procedure of the CPCH
The operation of the uplink Common Packet
Channel (CPCH) is similar to that of the RACH, but they differ in the CPCH and
the first layer Collision Detection (CD) which is similar to the preamble
symbol structure of the PRACH.
To reduce collision and interference, the
CPCH Status Indication Channel (CSICH) is added in the new protocol version for
the CPCH. The CSICH is an independent channel of the BS for transmission, which
has indicator bits to indicate different CPCH status. When all the CSICH are
occupied, it avoids unnecessary access attempt so that the throughput of the
CPCH is enhanced. Only when the CSICH indicates that free CPCH part is
available, can the UE send random access preambles on the uplink CPCH.
l
Before the UE detects the AICH,
operations of the CPCH are the same as those of the RACH.
l
After
that, the UR sends another feature sequence Collision Detection (CD) preamble
with the same power level. This feature sequence is
chosen randomly from the given feature sequence set.
l
Then, the BS will send the same
feature sequence in the CD Indication Channel (CD-ICH) to respond to the UE. In
this way, the collision probability of the first layer can be reduced.
l
After receiving the correct
response from the BS on the CD Indication Channel (CD-ICH), the UE will
transmit the CPCH messages, which may last several frames.
l
The CPCH Channel Assignment
Indication channel (CPCH-CAI) is an option of the system. It indicates the UE to
use the unoccupied CPCHs in the form of channel assignment. CA messages and
collision detection messages are sent in parallel.
Why is the CPCH required to use the
collision detection mechanism, while not required in the RACH?
First, the long-time transmission requires
the collision detection mechanism of the physical layer. During the RACH
procedure, only one RACH message may be lost due to collision, however, during
the CPCH procedure, an undetected collision may cause several frames lost as
well as extra interference.
Secondly,
the fast power control of the CPCH helps reduce the interference caused by data
transfer, at the same time, it also emphasizes the importance of adding
collision detection mechanism into the CPCH. If a UE adjusts the power via the
power control command used for other UEs, and sends the data within several
frames, it will cause serious interference in the cell, and more serious during
the high rate data transmission.
Before the transmission of CPCH messages,
there is a segment of length-optional power control preamble for choosing. To
quicken the convergence speed of power control, the power control preambles of
eight timeslots use 2dB step length.
The CPCH must restrict the maximum duration for transmission,
because the CPCH does not support soft handoff and compressed mode for
measuring within the frequency and the system. Too long transmission may cause
call dropping and strong interference. The UTRAN set maximum CPCH transmission
during service negotiation.
5.5.5 Downlink
Transmit Diversity
1. Space Time Transmit Diversity (STTD) based on space and time block
codes
The downlink open loop transmit diversity
adopts the Space Time Transmit Diversity (STTD) based on the space and time
block codes. In the UTRAN, STTD codes are optional, but the supporting for the
STTD on the UE is mandatory.
STTD codes are used in four consecutive
channel bit blocks. The diagram of the general STTD encoder for the channel
bits b0, b1, b2, b3. Channel
encoding, rate matching and interleaving are performed in the non-diversity
mode.
2. Time Switched Transmit Diversity (TSTD) used for the SCH
The TSTD can be used for the SCH. In the
UTRAN, the TSTD used for the SCH is optional. But on the UE, supporting for the
TSTD is mandatory.
The structure of the SCH by using TSTD. In even-number timeslots, both the PSC
and the SSC transmit on antenna 1, while in odd-number timeslots, both of them
transmit on antenna 2.
3. Closed loop transmit diversity
Channel
encoding, interleaving and spreading spectrum are the same as those of
non-diversity mode. The compound signals after spreading are sent to two
transmitter antennas, and are weighted by the specific weighting factors w1 and w2 of the antennas. Weighting
factors are determined by the UE, and are notified of the transceiver in the
UTRAN cell by using D bits of the FBI field in the downlink DPCCH.
The key of a closed loop transmit diversity
is calculation of the weighting factors, and it is divided into the following two
modes, according to different calculation methods of weighting factors:
l
Mode 1 uses phase
justification: The dedicated pilot symbols (orthogonal) used by the two
antennas to transmit DPCCH are different.
l
Mode 2 uses phase/amplitude
justification: The dedicated pilot symbol used by the two antennas to transmit
DPCCH are the same.
Table 5-2
summarizes possible modes of open and closed loop transmit diversity applied in
different types of downlink physical channels. The STTD and closed loop mode
are forbidden to be used in the same physical channel at the same time. What is
more, if any downlink physical channel uses a transmit diversity, the P-CCPCH
and the SCH will also use it.
Furthermore, the
mode of a transmit diversity used on the PDSCH frame must be the same as that used
on the associated DPCH. Within the duration of the PDSCH frame and a timeslot
before this PDSCH frame, the mode (open or closed loop) of a transmit diversity
used the associated DPCH cannot be changed. However, it is allowed to convert
closed loop mode 1 into closed loop mode 2, or vice versa.
The “√” under the modes of
downlink physical channels indicates the mode can be used, and the “×”
indicates it cannot.
Table 5-2 Types of physical channels and modes of transmit diversities
Types
of physical channels
|
Open
loop transmit diversities
|
Closed
loop transmit diversity
|
|
TSTD
|
STTD
|
||
P-CCPCH
|
X
|
√
|
X
|
SCH
|
√
|
X
|
X
|
S-CCPCH
|
X
|
√
|
X
|
DPCH
|
X
|
√
|
√
|
PICH
|
X
|
√
|
X
|
PDSCH
|
X
|
√
|
√
|
AICH
|
X
|
√
|
X
|
CSICH
|
X
|
√
|
X
|
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