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1 © NOKIA 3gppl1.PPT/ 27-Sep-2000 / [email protected]
Lauri Pirttiaho, NMP/Oulu Contents:
• General about radio communications systems • 3GPP WCDMA L1, the physical layer structure • Transmitting and receiving • Channels • Codings • Procedures Not included: • diversity techniques, ARQ, power control, device classes etc.
2 © NOKIA 3gppl1.PPT/ 27-Sep-2000 / [email protected]
Radio communications system
• The Purpose of physical radio link is to establish a reasonably reliable radio communication path between the communicating system.
• In commercial systems some drivers for further development are:
• The capacity, or number of users serviced by the system. • The data rate, or the qualities perceived by one user.
• Physical radio link carries information from one system to another in a form of electromagnetic waves.
• The receiver is interfered by • signals from other systems, and • ever present physical noises.
3 © NOKIA 3gppl1.PPT/ 27-Sep-2000 / [email protected]
Multiple Access Methods • Multiple access refers to how different users access the
system simultaneously.
time
frequ
ency
FDMA
time
frequ
ency
TDMA
time
frequ
ency
Code
CDMA
4 © NOKIA 3gppl1.PPT/ 27-Sep-2000 / [email protected]
Duplexing Methods • Duplexing refers to how one user can establish a two way
communications link with the system.
time
frequ
ency
• Frequency division duplex (FDD)
• (Full duplex in analog systems)
time
frequ
ency
• Time division duplex (TDD)
• (Half duplex in analog systems)
5 © NOKIA 3gppl1.PPT/ 27-Sep-2000 / [email protected]
Structure of the Physical Layer
RF
L1 Protocol
BB
RRC
MAC
6 © NOKIA 3gppl1.PPT/ 27-Sep-2000 / [email protected]
The UMTS FDD Frequencies
The first allocation of the UMTS FDD system (ITU regions 1 & 3) is:
• 1920 — 1980 MHz up-link • 2110 — 2170 MHz down link • duplex distance 190 MHz The radio channel structure: • 5 MHz channel spacing • 200 kHz channel raster • UTRA absolute radio frequency channel number (UARFCN):
(channel number = 5 x frequency in MHz)
7 © NOKIA 3gppl1.PPT/ 27-Sep-2000 / [email protected]
Modulation and Filtering
Modulation method: • Chip frequency 3.84 MHz • QPSK
Filter
Filter
π/2
I
Q
Filtering: • Pulse shaping before modulation • RRC with frequency domain roll-off
0.22
8 © NOKIA 3gppl1.PPT/ 27-Sep-2000 / [email protected]
WCDMA TX: Spreading
• Channelization within one frequency is achieved by representing bits as sequences of chips.
0 1 0 1 1 0
9 © NOKIA 3gppl1.PPT/ 27-Sep-2000 / [email protected]
WCDMA TX: Spreading
• The sequences are quasi-orthogonal between the channels. • Combination of:
• Bit long orthogonal sequences (Walsh sequences), and • Frame long pseudo noise sequences (Gold sequences)
I
Q
10 © NOKIA 3gppl1.PPT/ 27-Sep-2000 / [email protected]
WCDMA TX: Spreading
• The complex symbols are scrambled by complex sequences formed of truncated Gold sequences.
Bits Pairing
Odd Bits
Walsh code
Even Bits
Q
I
(I,Q) Pair to
Complex (I + jQ)
Complex Gold code
Chips
• The transmitted bits are paired, spread to a chip sequences using Walsh sequences and mapped to complex symbols.
11 © NOKIA 3gppl1.PPT/ 27-Sep-2000 / [email protected]
WCDMA RX
The receiver also needs to remove: • Noise • Interference from other channels and frequencies • Multi path propagation interference (fading)
Receiving a CDMA transmission requires: • Demodulation and filtering of the chip sequence. • Correlating with the scrambling and spreading codes to detect
the channel contents (chip sequence to bit sequence).
12 © NOKIA 3gppl1.PPT/ 27-Sep-2000 / [email protected]
WCDMA RX: Rake
• Inter symbol interference due to multipath propagation.
13 © NOKIA 3gppl1.PPT/ 27-Sep-2000 / [email protected]
Chips Delay Chips
WCDMA RX: Rake
• Correlating the received chip stream with sequences delayed by the propagation time differences.
Scrambling sequence
Spreading Sequence
Integrate & dump
Chips Delay Chips
Scrambling sequence
Spreading Sequence
Integrate & dump
Chips Delay Chips
Scrambling sequence
Spreading Sequence
Integrate & dump
Com
bining
Bits
14 © NOKIA 3gppl1.PPT/ 27-Sep-2000 / [email protected]
Error Correction and Detection
Convolutional codes, K = 9
G0 = 561 G1 = 753
G0 = 557 G1 = 663 G2 = 711
Turbo code: PCCC, 2 x K=3 RCC's
Interleaver
CRC check sum: gCRC24(D) = D24 + D23 + D6 + D5 + D + 1 gCRC16(D) = D16 + D12 + D5 + 1 gCRC12(D) = D12 + D11 + D3 + D2 + D + 1 gCRC8 (D) = D8 + D7 + D4 + D3 + D + 1
15 © NOKIA 3gppl1.PPT/ 27-Sep-2000 / [email protected]
WCDMA FDD Transport Channels L1 provides the upper layers a variety of transport channels: • Common channels
• BCH, the broadcast channel for system and cell information • PCH, the paging channel • FACH, the forward access channel for down-link
connectionless random access communication. • RACH, the random access channel for up-link random
access communication. • CPCH, the common packet channel for higher volume up-
link packet communication. • DSCH, the down-link shared channel for improving down
link peak communication throughput. • Dedicated channels
• DCH, the dedicated channel, for CS communication.
16 © NOKIA 3gppl1.PPT/ 27-Sep-2000 / [email protected]
Dedicated channels
Dedicated channels
Common channels
Common channels WCDMA FDD Physical Channels
CPICH
PCCPCH PSCH
SSCH
SCCPCH
AP-AICH
AICH
PICH
CD/CA-ICH
CSICH
PDSCH
DPCCH DPDCH
DPDCH DPCCH
PRACH
PCPCH
BCH
PCH, FACH
DCH
DCH
RACH
CPCH
DSCH
17 © NOKIA 3gppl1.PPT/ 27-Sep-2000 / [email protected]
WCDMA FDD Frame and Slot Structure
Frame: 10 ms, 38400 chips
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14
Slot: 2/3 ms, 2560 chips (example of DL DPCH)
Data TPC Data TFCI Pilots
18 © NOKIA 3gppl1.PPT/ 27-Sep-2000 / [email protected]
Synchronization
...Established
Identify scrambling code from CPICH
Identify code group & frame sync from SSCH
Acquire slot sync from PSCH
Synchronization...
PSCH
PCCPCH
CPICH
SSCH
PCCPCH (carries BCH)
19 © NOKIA 3gppl1.PPT/ 27-Sep-2000 / [email protected]
Establishing Radio Link
• Cell information • From BCH, tells parameters needed for establishing
communication
• Paging • From PCH when PICH indicates there is paging coming
• RACH/FACH • RACH/AICH up-link using slotted aloha protocol • reply on FACH
• DCH • Will carry both communication and control data • Paging during DCH comes on the DCCH
20 © NOKIA 3gppl1.PPT/ 27-Sep-2000 / [email protected]
Soft & Hard Hand-over
Frequency A Frequency B
21 © NOKIA 3gppl1.PPT/ 27-Sep-2000 / [email protected]
Soft & Hard Hand-over
Node B 1 Node B 2 Node B 3
29 © NOKIA 3gppl1.PPT/ 27-Sep-2000 / [email protected]
WCDMA/GSM Hand-over
GSM coverage area
WCDMA coverage area