Physical Channel Structure
Within the physical layer itself, the composite coded transport channel (CCTrCH) is mapped onto the data part of physical channel.
Within the physical channel, in addition to the data part, there are channels part and physical signals.
Physical signals are similar to physical channels except that they do not have transport channel mapped onto them. However physical signals, maybe associated with physical channels in order to support the function of physical channels.
In addition, there different physical channels associated with FDD and TDD mode of operation.
The uplink physical channels are divided into dedicated and common channel. There are tow types of dedicated physical channel: Uplink Dedecated Physical Data Channel (DPDCCH) and Uplink Dedicated Physical Control Channel (DPCCH).
The uplink common physical channels are the physical random access channel (PRACH) and Physical Common Package Channel (PCPCH).
In the FDD mode there are two uplink common physical channel PRACH and PCPCH.
In the TDD mode there is only one common chysical channel, PRACH.
The Uplink DPDCH occupies one timeslot within a 10ms radio frame, and contains 2560 chips. The bit rate depends on the spreading factor use. The spreading factor for the uplink DPDCH may varies from 256 down to 4. corresponding to a bit rate of between 15kbps and 960kbps. Consequently the number of bit per time slot may vary between 10 and 640. To achieve even higher bit rates, multiple uplink DPDCHs and be transmitted in parallel on one connection.
L1 Control Information:
Uplink DPCCH: - Pilot bits
- Transmit Power Commands (TPC)
- Feed Back Information (FBI)
- Transport Format Combination Indicators (TFCI)
The Uplink DPCCH is used to carry control information generated at the physical layer.
The physical layer control information consists on known pilot bits (use to support channel estimation for coherent detect), Transmit power commands (TPC), Feed back information (FBI), Transport format combination indicators (TFCI).
The FBI data is used to support such performance enhancement techniques as close loop mode transmit diversity transmit (SSDT).
The TFCI is used to inform the receiver about transport format combination of transport channel being mapped onto the transmit uplink DPDCH radio frame.
Unlike the uplink DPDCH there is only one DPDCH per radio link.
The Uplink physical random access channel (PRACH) is used to carry RACH, the PRACH is base upon a slotted ALOHA scheme. This mean that mobile station can only transmit its random burst at a beginning of series of well-define access slot, space 5120 chips apart. Consequently there are 15 access slot every 2 radio frame.
The random access burst consist of one or several preambles of length 4096 chips, and a message of length 10 or 20 ms.
The data part users a spreading factor of between 256 and 32, corresponding to a bit rate of btween 15 kbps and 120 kbps. Consequently, the time slot for the data part will contain between 10 and 80 bits.
The control part uses a fixed spreading factor of 256, which corresponds to a bit rate of 15 kbps. Consequently, eahc time slot contains 10 bits, which are divided between the pilot, TFCI, FBI, TPC fields. THe control field consists of 8 pilot bits, for channel estimation, and 2 TFCI bits.
The mobile station can start transmission at the beginning of a number of well-defined time intervals, relative to the fram boundary of the BCH of the current cell. The PCPCH access transmission consists of one or several access preambles (AP) 4096 chips long, one collision detection preamble (CDP) 4096 chips long, a power control preamble (PCP) eigher 0 or 8 slots long, and finally a message part.
The PCPCH access preambles (AP) and PCPCH collision detection preambles (CDP) are similar to the preambles used for the PRACH. The CPCH message part may contain a number of 10 ms radio frames, each of which is split into 10 time slots containing 2560 chips each, Each slot consists of 2 parts: a data part, which carries higher layer information, and a control part, which carries layer 1 control information. The data and control parts are transmitted in parallel.
The data part uses a spreading factors of between 256 and 4, corresponding to a bit rate of between 15 and 960 bit. Consequently the time slot for the data part will contain between 10 and 640 bit. The control part user a fixed spreading factor of 256, which are divided between the pilot, TFCI, FBI, and TPC fields. The PCPCH power control preamble (CDP) is used to carry information from higher layers, for example the radio resource controller (RRC), and has the same form as the message part.
The Downlink physical channels are also divided into dedicated and common channels.
There is only one type of downlink dedicated physical channel, the downlink Dedicated Physical Channel (downlink DPCH).
The downlink common physical channels are the Common Pilot Channel (CPICH), the Primary Common Control Physical Channel (P-CCPCH), the Secondary Common Control Physical Channel (S-CCPCH), the Synchronization Channel (SCH), and the Physical Downlink Share Channel (PDSCH).
The downlink DPCH occupies one time slot within a 10 ms time frame, and contains 2560 chips. The spreading factor may be between 512 and 4, corresponding to a channel bit rate of between 15 kbps and 1920 kpbs. Consequently, the number of bits per time slot may be between 10 bits and 1280 bits. These are divided between the DPDCH and DPCCH fields.
The downlink CPICH is a fixed rate (30 kbps) physical signal channel containing a predefined symbol sequence, which is used as a phase reference for the downlink common physical channels.
There are tow types of CPICH: a primary CPICH (P-CPICH), used as phase reference for the SCH, primary CCPCH, AICH, and PICH and a secondary CPICH (S-CPICH), used as a phase reference for the secondary CCPCH and downlink DPCH.
The CPICH has a fixed spreading factor of 256, corresponding to channel bit rate of 30 kbps, Consequently, there are 20 bits per time slot.
The frame structure of the P-CCPCH contains no pilot bits, TPC commands, or TFCI. In addition , the P-CCPCH is not transmitted during the first 256 chips of each slot. Instead, the primary SCH and secondary SCH are transmitted during this period. The data field contains 18 bits.
The S-CCPCH is used to crry the FACH and PCH. The main difference between the Primary and Secondary CCPCH is that the transport channel mapped to the Primary CCPCH (BCH) can only have a fixed predefined transport format combination, while the S-CCPCH supports multiple transport format combination using TFCI. Furthermore, a P-CCPCH is tranmitted over the entire cell, while a S-CCPCH may be only transmitted over part of a cell.
The S-CCPCH may have a spreading factor of between 256 and 4, corresponding to a channel bit rate of between 30 kbps and 1920 kbps. Consequently, the number of bits per time slot will vary between 20 and 1280 bits. These are divided between the TFCI, data, and pilot bits fields.
The SCH is a downlink signal used to support efficient cell searching. This function is a requirement for asynchronous base station operation.
THe SCH consists of two sub-channels, the primary and secondary SCH. The primary SCH consists of a modulated code of length 256 chips and is transmitted once every slot. The Primary Synchronization code (PSC), denoted cp, is the same for every cell in the system.
The Secondary SCH consists of repeatedly transmitting a sequence of 15 modulated codes of length in parallel with the Primary SCH.
The PDSCH is used to carry the DSCH transport channel. A PDSCH is allocated on a radio frame basis to a base station. Within the same radio frame, multiple parallel PDSCHs, with the same spreading factor, may be allocated to the same base station by using different channelization codes. However, PDSCH allocated to the same base station on different radio frames may have different spreading factors.
The PDSCH may have a spreading factor between 256 and 4, corresponding to a channel bit rate between 30 kbps and 1920 kbps. Consequently, the PDSCH data field may contain between 20 and 1280 bit.
The Acquisition Indicator channel (AICH) is a fixed rate (30 kbps) physical channel used to carry acquisition Indicator (AI), which correspond to the signature on the PRACH.
The AICH consists of a repeated sequence of 15 consecutive access slots (AS), each of length 5120 chips. Each access slot consists of two parts: An Acquisition-Indicator (AI) of duration 4096 chips, containing 32 symbols which themselves contain the AI; and a part of duration 1024 chips, where is no symbol transmission. The phase reference ofr the AICH is the Primary CPICH.
The AP-AICH is a fixed rate (30 kbps) physical channel, used to carry AP acquisition Indicator (API) of the CPCH.
THe AP-AICH and AICH may use the same or different channelization codes. The AP-AICH has two parts: apart 4096 chips long, where the AP acquisition indicator (API) is transmitted, followed by a part 1024 chips long, where there is no symbol transmission.
The phase reference for the AP-AICH is the Primary CPICH.
The CD/CA-ICH is a fixed rate (30 kbps) physical channel used to carry the CD Indicator (CDI) where hte CA is not active, or the CD Indicator (CDI/CAI) where the CA is active. The CD/CA-ICH and AP-AICH may or may not use the same channelization codes.
The CD/CA-ICH has two parts: a part 4096 chips long, where the CDI/CAI is transmitted, foolowed by apart 1024 chips long, where there is no transmission.
The Paging Indicator Channel (PICH) is a fixed tare (30 kbps) physical channel used to carry the paging indicators, The PICH is always associated with an S-CCPCH onto which a PCH transport channel is mapped.
One PICH radio frame 10 ms long consists of 300bits (b0,...... b299), of these bits, 288 (b0,b1, .... b287) are used to carry paging indicators. The remaining 12 bits are not transmitted.
The CPCH Status Indicator Channel (CSICH) is a fixed rate (30 kbps) physical channel, used to carry CPCH status information. A CSICH is always associated with a physical channel used for transmission of the CPCH AP-AICH, and uses the same channelization and scrambling codes.
The CSICH frame consists of 15 consecutive access slots (AS) each 40 bits long. Each access slot consists of two parts, a par 4096 chips long, where there is no transmission, and a status indicator (SI).
The phase reference for the CSICH is the Primary CPICH.
The above diagram illustrates the mapping of the various transport channels onto the physical channels.
The association between the physical channels and signal channel is also shown above.