5.5 Physical layer Procedures


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.
If the UE has received some related information about the scrambles, step 2 and 3 can be simplified.

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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