5.4 Physical Channel

5.4.1  Relevant Concepts of the Physical Channel

The physical channel is defined by a specific carrier frequency, scramble, channelization code (optional), time bucket from beginning to end (there is time duration) and the corresponding phase of the uplink. The duration is defined by the beginning and end time, measured by using the integer times of chip.

Radio frame: It is a processing frame containing 15 timeslots. The length of a radio frame is 38400 chips.

Timeslot: It is a unit composed of some bit fields, 2560 chips in length.

The default duration of a physical channel lasts from the beginning hour to end hour. The non-consecutive physical channels will be noted clearly.

The transport channel (more abstract high-layer than the physical layer) can be mapped to the physical layer. For the physical layer, it maps the data from a Coded Composite Transport Channel (CCTrCH) to the physical channel. Besides the data, it also maps channel control commands and physical signaling.

Like the physical channel, the physical signaling is also an entity based on air features, but no transport channel or indicator is mapped to physical signaling. The physical signaling can support the functions of the physical channel.

5.4.2  Architecture of the Uplink Physical Channel

The uplink physical channel is divided into two types: Uplink dedicated physical channel and uplink common physical channel.

The uplink dedicated physical channel is divided into uplink Dedicated Physical Data Channel (uplink DPDCH) and uplink common physical channel.

The uplink common physical channel is divided into Physical Random Access Channel (PRACH) and Physical Common Packet Channel (PCPCH).

1. DPDCH/DPCCH

Figure 5-6 shows the frame structure of the uplink dedicated physical channel. Each frame is 10 ms long, divided into 15 timeslots. And each timeslot is 2560 chips long, and corresponds to a power control period.

The DPDCH is a dedicated transport channel. There may be zero, one or more DPDCHs in a radio link.

The Dedicated Physical Control Channel (DPCCH) includes the following: The known pilot bits used to support channel estimation for the relevant detection, Transmit Power Control (TPC), FeedBack Information (FBI) and an optional Transport Format Combination Indicator (TFCI). There is only one DPCCH in each radio link.

The parameter “k” in Figure 5-6 determines the number of bits of each uplink DPDCH/DPCCH. It relates to the Spreading Factor (SF) of the physical channel. 1 SF=256/2^k The SF of the DPDCH ranges from 256 to 4. The SF of the uplink DPCCH is always 256, that is, there are 10 bits for each uplink DPCCH timeslot.

2. PRACH

The physical random access channel is used to transport RACH.

The transmission of the RACH is based on the timeslot ALOHA mode with fast acquisition indication. The UE may begin to transport in a time offset defined in advance, indicated as access timeslots. Every two frames have 15 access timeslots, at an interval of 5120 chips. 
The information about the availability of the access timeslot in the current cell is provided by the high-layer information.

The structure of the random access transmission includes one or more 4096-chip preambles and a 10-ms or 20-ms message part.

l   Preambles for the RACH

The preamble of the random access transmission is 4096 chips long, and it is 256 times repetition of a signature with the length of 16 chips. Totally, there are 16 different signatures.

l   RACH message part

The 10-ms message is divided into 15 timeslots, with T_slot=2560 chips. Each timeslot contains two parts: One is the data part, where the RACH transport channel is mapped; the other is control part, which is used to transport the control information of layer 1.The data part and control part are transmitted concurrently. A 10-ms message is composed of one radio frame, while a 20-ms message is of two 10-ms radio frames. The length of the message part is determined by the signatures and/or timeslot, which is configured by the high layer.

The data part contains 10*2^k bits, where k = 0, 1, 2, 3.For the message data part, they correspond respectively to 256, 128, 64 and 32 spreading factors.

The control part includes eight known pilot bits used to support the channel estimation for the relevant detection and two TFCI bits, which corresponds to 256 spreading factors for the message control part. In the random access message, the total of TFCI bits is 15*2=30 bits. The value of TFCI corresponds to a specific transport format of the current random access information. In case that the PRACH is 20 ms long, the TFCI will repeat in the second radio frame.

3. PCPCH

The Physical Common Packet Channel (PCPCH) is used to transport CPCH.

l   The transmission of the CPCH

The transmission of the CPCH is based on the DSMA-CD (Digital Sense Multiple Access-Collision Detection) mode with fast acquisition indication. The UE may begin to transport at the predefined time offset corresponding to the frame border of the BCH received by the current cell. The timing and structure of the access timeslot is the same as that of the RACH. The structure of the CPCH random access transmission is shown in Figure 5-10.The CPCH random access transmission includes the following parts: One or more 4096-chip-long Access Preambles (APs), one 4096-chip-long Collision Detection Preamble (CD-P), one 0-timeslot-long or 8-timeslot-long DPCCH Power Control Preamble (PC-P) and one variable Nx10 ms message.

l   Access preambles of the CPCH

Like the RACH preamble part, the feature sequence of the RACH are used here, but fewer than the RACH preambles. You can select a scramble from different code segments of the Gold code which consists of the RACH preamble scrambles, and you can also use the same scramble if the signature is shared.

l   Collision detection preamble part of the CPCH

Like the RACH preamble part, the CPCH collision detection preamble part uses the feature sequence of the RACH. You can select a scramble from different code segments of the Gold code which consists of RACH and CPCH preamble scrambles.

l   Power control preambles of the CPCH

The Preamble part of power control is called Power Control Preamble (PC-P) of the CPCH. The length of the power control preamble (L_pc-preamble) is a high-layer parameter, and can have zero or eight timeslots.

l   CPCH messages

Figure 5-11 shows the frame structure of the uplink common physical channel. Each frame is 10 ms long, divided into 15 timeslots. And T _slot = 2560 chips, equal to a power control period.

The data part contains 10*2^k bits. Here, k = 0, 1, 2, 3, 4, 5, 6, which corresponds respectively to 256, 128, 64, 32, 16, 8 and 4.

Each message contains N_Max_frames 10 ms frames at most.N_Max_frames is a high layer parameter. Each 10 ms frame is divided into 15 timeslots, with T_slot=2560 chips. And each timeslot contains two parts: The data part used to transport high-layer information and the control part of the layer 1 control information. The data part and control part are transmitted concurrently.

The spreading factor of the CPCH message control part is 256.

The Dedicated Physical Control Channel (DPCCH) includes the following: The known pilot bits used to support channel estimate for the relevant detection, Transmit Power Control (TPC), FeedBack Information (FBI) and an optional Transport Format Combination Indicator (TFCI).

5.4.3  Structure of the Downlink Physical Channel

1. DPCH

Only one kind of downlink dedicated physical channel exists, that is, downlink Dedicated Physical Channel (downlink DPCH).

Within a downlink DPCH, the dedicated data is generated at layer 2 or higher layer (i.e. DCH) and is transmitted by time multiplexing together with the control information (including the known pilot bits, TPC instruction and an optional TFCI) generated by layer 1.Therefore, the downlink DPCH can be regarded as a time multiplexer between the downlink DPDCH and DPCCH.

Figure 5-12 shows the frame structure of the downlink DPCH. Each frame is 10 ms long, divided into 15 timeslots. And Tslot=2560 chips, corresponding to a power control period.

The parameter “k” in Figure 5-12 determines the total number of bits in each uplink DPCH timeslot. It relates to the Spreading Factor (SF) of the physical channel, that is, 1 SF = 512/2^k So, the SF ranges from 512 to 4.

There are the following two kinds of downlink DPCHs: One is TCFI included (i.e. services occurred simultaneously), and the other one is TFCI excluded (i.e. fixed rate services).

2. The DL-DCCH of CPCH

The spreading factor of the DL-DPCCH (message control part) is 512. Figure 5-13 shows the frame structure of the DL-DPCCH of the CPCH.

The DL-DPCCH of the CPCH is composed of known pilot bits, TFCI, TPC commands and CPCH Control Command (CCC). The CPCH control command is used to support CPCH signaling. There are two types of CPCH control commands: Layer 1 control command (message start indication) and high-layer control command (i.e. emergency abort command).

3. CPICH

The CPICH is a downlink physical channel with fixed rate (30 kbps, SF=256), used to transport the predefined bit/symbol sequence. 

When the transmit diversity () is used in any downlink channel of a cell, the two antennas will use the same channelization code and scramble to transmit CPICH. In this case, for antenna 1 and 2, the predefined symbol sequences are different, as shown in Figure 5-15. Without the transmit diversity, the symbol sequence of antenna 1 in the figure will be used.

There are two types of CPICH: Primary and secondary CPICH. They have different purposes in the physical characteristics.

l   Primary Common Pilot Channel (P-CPICH)

The Primary Common Pilot Channel (P-CPICH) has the following characteristics:

-          The same channelization code always used

-          Primary scrambles used

-          Only one CPICH in each cell

-          Broadcasting in the whole cell

The primary CPICH is the phase reference for the following downlink channels: SCH, primary CCPCH, AICH and PICH. It is also the default phase reference for other downlink physical channels.

l   Secondary Common Pilot Channel (S-CPICH)

The Secondary Common Pilot Channel (S-CPICH) has the following characteristics:

-          Any channelization code with SF = 256 can be used

-          Primary or auxiliary scrambles may be used

-          Zero, one or more secondary CPICHs may exist in every cell

-          Transmission allowed in the whole or part of the cell

-          The secondary CPICH may be the reference for the secondary CCPCH and downlink DPCH. In this case, the high-layer signaling will notify the UE.

4. P-CCPCH

The CCPCH is a downlink physical channel with fixed rate (30 kbps, SF=256), used for BCH transportation.

Figure 5-16 shows the frame structure of the P-CCPCH. Compared with the downlink DPCH, it does not have TPC commands, TFCI or pilot bits. Within the first 156 chips of each timeslot, the P-CCPCH does not transmit. However, in this period, the primary and secondary SCHs transmit.

When diversity antennas are used in the UTRAN and open loop diversities are used to transmit P-CCPCH, the data part of the P-CCPCH is encoded via STTD. Except #14, the last two bits of an even timeslot are performed STTD coding together with the first data bits of the next timeslot. The last two bits of the timeslot #14 are not performed STTD coding, but are transmitted from the two antennas with the same power, as shown in Figure 5-17. The high-layer signaling determines whether the P-CCPCH is performed STTD coding. Furthermore, by modulating the SCH, the high-layer signaling also points out whether the STTD codes exist on the P-CCPCH. During the period of power-on and handoff between cells, the UE can ensure whether the STTD codes exist on the P-CCPCH by receiving the high-layer message, demodulating the SCH or through the combination of the above two methods.

5. S-CCPCH

Secondary CCPCH is used to transmit FACH and PCH. There are two kinds of S-CCPCHs: TECI included and TFCI excluded. Whether the TFCI is transmitted is determined by the UTRAN. Therefore, it is mandatory for all UEs to support the TFCI using. The possible rate set is the same as that of the downlink DPCH. 

The parameter “k” in Figure 5-18 determines the total number of bits in each downlink CCPCH timeslot. It relates to the Spreading Factor (SF) of the physical channel, SF=256/2^k.So, the SF ranges from 256 to 4.

The FACH and PCH can map the same or different S-CCPCHs. If they map the same S-CCPCH, they can map the same frame. The main difference between the CCPCH and a downlink dedicated physical channel is that the CCPCH is not controlled by inner loop power. And the main difference between the P-CCPCH and the S-CCPCH is: The P-CCPCH has a pre-defined fixed rate, while the S-CCPCH can support a variable rate by containing TFCI. Furthermore, the P-CCPCH can transmit consecutively in the whole cell, however, the S-CCPCH can use the same method as that of the DCH to transmit in narrow beams (only valid for the S-CCPCH of the transport FACH).

6. SCH

The Supplemental Channel (SCH) is a downlink channel used for cell search. It contains two channels: Primary SCH and Secondary SCH. The 10 ms radio frame of the primary and secondary SCHs is divided into 15 timeslots, each 2560 chips in length. 

The primary SCH includes a 256-chip-long modulation code and a Primary Synchronization Code (PSC), which is indicated with Cp in Figure 5-19, and transmitted once every timeslot. The PSCs of each cell in the system are the same.

The secondary SCH transmits repeatedly one 256-chip-long modulation code with 15 series and one SSC together with the Primary SCH. In Figure 18, the SSC is indicated with c_s^i,k, of which i = 0, 1,…, 63 is the serial number of the scramble group, and k = 0, 1, 2,…,14 is the timeslot number. Each SSC is a code selected from 16 different 256-chip-long codes. The sequence of the secondary SCH indicates which code group the uplink scramble belongs to.

When the transmit diversity is used, the TSTD method will be adopted.

7. PDSCH

The Physical Downlink Shared Channel (PDSCH) is used to transport the Downlink Shared Channel (DSCH).

One PDSCH corresponds to a root PDSCH channelization code or a channelization code under it. The PDSCH is allocated within a radio frame, on the basis of a signal UE. Within a radio frame, the UTRAN can allocate different PDSCHs to different UEs under the same PDSCH root channelization code, on the basis of code multiplexing. Within the same radio frame, multiple parallel PDSCHs with the same spreading factor can be allocated to a signal UE. This is a special example of multi-code transport. The frames of all PDSCHs under the same PDSCH root channelization code are synchronous.

Within different radio frames, the PDSCHs allocated to the same UE can have different spreading factors.

For radio frames, each PDSCH always associates with a downlink DPCH. The PDSCH need have neither the same spreading factor as the associated DPCH, nor frame alignment.

The DPCCH part of the associated DPCH transmits all information related to layer 1, that is, the PDSCH does not carry any layer 1 information. To notify the UE that there are data to be decoded on the DSCH, two possible signaling methods, or the TFCI field or the high-layer signaling carried by the associated DPCH will be used.

If the signaling method based on the TFCI is used, the TFCI will inform the UE of the PDSCH channelization code as well as of the transient PDSCH-related transport format parameters.

In other cases, information will be given by the high-layer signaling.

8. PICH

The Paging Indicator Channel (PICH) is a physical channel with fixed rate (SF=256), used to transport paging indicators (PIs). The PICH always associates with a S-CCPCH which is the mapping of a PCH transport channel.

Figure 5-21 shows the frame structure of the PICH.A PICH frame is 10 ms long, including 300 bits (b₀, b₁, …, b₂₉₉).Of which, there are 288 bits (b₀, b₁, …, b₂₈₇) used to transport the paging indicator. The left 12 bits are not used, which are reserved for the future.

9. AICH

The Acquisition Indication Channel (AICH) is a physical channel used to transport the acquisition indicators (AIs). The AIs correspond to the signatures on the PRACH.

Figure 5-22 shows the structure of the AICH. The AICH consists of 15 consecutive AS sequences, each 5120 chips in length. Each timeslot is composed of two parts: One is Access Indication (AI) consisting of 32 real value symbols (a₀, …, a₃₁); the other is the free part of the consecutive 1024 bits, which is not the formal composition of the AICH. The non-transmit part of the timeslot is reserved for the future CSICH or other physical channels.

The spreading factor of the AICH channelization code is 256.

The phase reference of the AICH is the P-CPICH. 

10. AP-AICH

The Access Preamble Acquisition Indication Channel (AP-AICH) is a physical channel with a fixed rate (SF = 256) used to transport the API of the CPCH. The API corresponds to the AP signature transmitted by the UE.

The AP-AICH and the AICH can use the same or different channelization codes. The phase reference of the AP-AICH is the P-CPICH. Figure 5-23 shows the structure of the AP-AICH. The AP-AICH uses a 4096-chip-long part to transmit the API, followed by a 1024-chip-long free part which is not the formal composition of the AP-AICH. And this free part of the timeslot is reserved for the future CSICH or other physical channels.

The spreading factor of the AP-AICH channelization code is 256.

11. CD/CA-ICH

Collision Detection/Channel Assignment Indication Channel (CD/CA-ICH) is a physical channel with a fixed rate (SF = 256). When the CA is inactive, it is used to transport the CDI, or when the CA is active, it is used to transport the CDI/CAI. Figure 5-24 shows the structure of the CD/CA-ICH. The CD/CA-ICH and the AP-AICH can use the same or different channelization codes.

The CD/CA-ICH uses a 4096-chip-long part to transmit the CDI/CAI, followed by a 1024-chip-long free part which is reserved for the CSICH or other physical channels.

The spreading factor of the CD/CA-ICH channelization code is 256.

12. CSICH

The CPCH Status Indication Channel (CSICH) is a physical channel with a fixed rate (SF=256), used to transport the CPCH status information.

The CSICH always associates with a physical channel used to transmit the AP-AICH of the CPCH, and uses the same channelization code and scrambling code as this channel. Figure 5-25 shows the frame structure of the CSICH. The CSICH consists of 15 consecutive ASs, each 40 bits in length. Each timeslot is composed of two parts: One is a 4096-chip-long free point, the other is Status Indication (SI) consisting of 8 bits (b_8i,….b_8i+7), of which i is the access timeslot number. The CSICH uses the same modulation as the PICH. And its phase reference is also the P-CPICH.

5.4.4  Mapping from the Transport Channels and Physical Channels

In the UTRAN, the data generated by the high layer is air transmitted by a transport channel of the different physical channels in the mapping physical layer. Therefore, the physical layer is required to have a variable rate-supported transport channel for providing broadband services, and at the same time, several services can be multiplexed to the same connection in it.

A physical channel together with one or more physical data channels constitutes a Coded Composite Transport Channel (CCTrCH). In a given connection, there may be more CCTrCHs, but only one physical control channel.

5.4.5  Spreading and Modulation of Physical Channels

1. Spreading of the uplink channels

When spreading spectrum is applied in the physical channel, it includes the following two operations: The first is channelization operation. Each data symbol is converted into some chips, so the bandwidth of the signaling is enhanced. The number of chips converted from the data symbols is called spreading factor. The second is scramble operation. In this operation, scrambles are added to the spreading signals. In channelization operation, the data symbol of I and Q channels respectively multiply by the orthogonal spreading factor. And in scramble operation, the signals of I and Q channels multiply by the scrambles of the complex value. Here, I and Q represent the real part and imaginary part.

1)      DPCH

The binary DPCCH and DPDCH used for spreading spectrum are indicated via real sequence, that is, the binary 0 is mapped as the real +1, and the binary 1 is mapped as the real -1.The DPCCH spreads to the dedicated chip rate via the channelization code c_c, and the n^th DPCCH (DPDCH_n) spreads to the dedicated chip rate via the channelization code c_d,n. One DPCCH can transmit simultaneously together with six parallel DPDCHs.

After channelization, the spreading signals of the real value are performed emphasized processing. The DPCCH is processed via the gain factor _c, and DPDCH via _d. After emphasized processing, the power distribution rate of the DPCCH and DPDCH can be adjusted.

2)      PRACH

l   Preambles for the PRACH

The PRACH preambles include the code of the multiplex value.

l    PRACH messages

Figure 5-28 describes the principle of spreading spectrum and scrambling of the PRACH messages, which include data part and control part. The binary data and control part used for spreading spectrum are indicated via real sequence, that is, the binary 0 is mapped as the real +1, and the binary 1 is mapped as the real -1.The control part spreads to the dedicated chip rate via the channelization code c_c, while the data part spreads to the dedicated chip rate via the channelization code c_d.

After channelization, the spreading signals of the real value are performed emphasized processing. The data part is processed via the gain factor _d, and the control part via _c.

After emphasized processing, the code streams of the I and Q channels become those of the complex value. And then, the signals of this complex are scrambled via S_r-msg,n code. The 10 ms scrambles correspond to the messages part of the 10 ms radio frame, e.g. the first scramble corresponds to the starting part of radio frame messages.

3)      PCPCH

l   Preambles for the PCPCH

The PCPCH preambles include the code of the multiplex value.

l   PCPCH messages

2. Modulation of the uplink channel

The chip rate of the modulation code is Mcps.

In the uplink, the chip sequences of the complex value generated by spreading are modulated in QPSK mode.

3. Spreading spectrum of the downlink channels

The spreading spectrum for all downlink physical channels except the SCH, e.g. the P-CCPCH, S-CCPCH, CPICH, AICH, PICH, PDSCH and downlink DPCH. The physical channel without spreading spectrum includes a sequence of a real value symbol. The symbols of all the channels except the AICH may have the following values: +1, -1 and 0, of which 0 represents DTX (Discontinuous Transmission).

Each pair of symbols are divided into I channel and Q channel after the serial-parallel conversion. The rule for division is that the symbols with even numbers are allocated to the I channel and those with odd numbers are allocated to the Q channel. After spreading spectrum, phase justification and additional aggregation, the I and Q channels of a real value become the sequence of a complex value. This sequence is scrambled by the scrambling code Sdl,n of the complex value.

 The signals of the complex value after spreading spectrum (the arrow S in Figure 5-32) are emphasized via the emphasized factor G. The complex P-SCH and S-SCH are emphasized respectively by G_p and G_s. All the downlink physical channels are combined together through complex emphasis.

4. Modulation of the downlink channels

The chip rate of the modulation code is 3.84 Mcps.

In the downlink, the chips of the complex value generated by spreading are modulated in QPSK mode,