Tài liệu Tài liệu về mạng di động 3G - AIRCOM

CDMA Spreading 1 Copyright 2010 AIRCOM International Release 99 Iub Node B Node B Iu-CS RNC MSC/VLR D Node B Iur Gs HLR Node B Node B RNC Uu Iu-PS SGSN Gr Gn Node B GGSN UE 2 Iu-PS Iu-CS Iur Iub Control User Control User Control User Control User AAL5 AAL2 AAL5 AAL2 AAL5 AAL2 AAL5 AAL5 ATM Layer ATM Layer ATM Layer ATM Layer Physical Layer Physical Layer Physical Layer Physical Layer Copyright 2010 AIRCOM International UMTS & GSM Network Planning GSM900/1800: 3 3G (WCDMA): Copyright 2010 AIRCOM International UMTS Air Interface technologies TDD FDD 5MHz 4 5MHz 5MHz Copyright 2010 AIRCOM International 5 Copyright 2010 AIRCOM International UK Spectrum Allocations Example D E C A A A A C C B B B D D E E TDD Vodafone Hutchison mmO2 TMobile 20MHz 6 0.3MHz 14.6MHz 10MHz Orange 14.8MHz 0.3MHz 10MHz 10MHz Copyright 2010 AIRCOM International Carrier Spacing and Carrier Spacing Raster 5MHz ▪ The nominal carrier spacing for UMTS is 5 MHz ▪ It is possible to move the centre frequency of the carrier on a 200 kHz raster ▫ We can have carrier spacings between 4.2MHz and 5.8MHz 200kHz ▫ This may be set within the license conditions, or to the operators discretion 7 Copyright 2010 AIRCOM International Visualising the Processing Gain w/o W/Hz W/Hz W/Hz Ec Before Spreading After Spreading Io With Noise f f W/Hz W/Hz After Despreading/ Correlation Post Filtering Orthog = 0 f Eb dBW/Hz Eb Io f Eb/Io Io f f Signal Intra-cell Noise Inter-cell Noise W/Hz Post Filtering Orthog > 0 dBW/Hz Eb Eb No No f 8 Eb/No f Copyright 2010 AIRCOM International P-CPICH as Measurement Reference CPICH RSCP Received Signal Code Power (in dBm). Power measurement of the CPICH Received energy per chip divided by the power density in the band (in dB) UTRA carrier received wide band power, including thermal noise and noise generated in the RSSI receiver CPICH Ec/No CPICH Ec/No = CPICH RSCP UTRA carrier RSSI RSCP •The UE measures the RSCP on the Primary-CPICH. The reference point for the measurement is the antenna connector of the UE. •The received code power may be high, but it does not necessarily indicate the quality of the received signal RSSI •On the overall noise level. •The UE measures the received wide band power, which includes thermal noise and receiver generated noise. •The reference point for the measurements is the antenna connector of the UE. 9 Copyright 2010 AIRCOM International Spreading • If the Bit Rate is Rb, the Chip Rate is Rc, the energy per bit Eb and the energy per chip Ec then Eb Ec Rc Rb • We say the Processing Gain Gp is equal to: PG dB 3840 10 log10 ( ) Rkbps • Commonly the processing gain is referred to as the Spreading Factor 10 Copyright 2010 AIRCOM International Assessing interference Eb/No =SNR +PG W/Hz Ec Io With Noise f If the Eb/No for voice is 9dB what is the SNR? 9 = SNR + 25 f Eb No 9 – 25 = -16 11 Copyright 2010 AIRCOM International Eb/N0 • Ratio of energy per bit over noise (plus interference) spectral density • Value required to reach target quality (BLER) for related service Eb N0 12 PG dB dB Ec N0 dB Copyright 2010 AIRCOM International Processing Gain and Required Eb/N0 Required Signal Power Eb/No= + 4 dB Processing Gain Eb/No= + 2 dB Eb/No= + 1 dB Noise level (ex. -105 dBm) -9 dB - 16 dB NRT 384 kbps +10 dB RT 64 kbps +18 dB Voice 12.2 kbps +25 dB - 21 dB 13 Copyright 2010 AIRCOM International CDMA Spreading Essentially Spreading involves changing the symbol rate on the air interface Spreading Despreading P P Channel f f P Tx Bit Stream P f f Air Interface Chip Stream Rx Bit Stream P f Identica Code Chip Stream 14 l codes Code Chip Stream Copyright 2010 AIRCOM International Spreading and Despreading 1 Tx Bit Stream Spreading X -1 Code Chip Stream Air Interface Chip Stream Despreading X Code Chip Stream Rx Bit Stream 15 Copyright 2010 AIRCOM International Channelisation Codes • Channelisation codes are orthogonal and hence provide channel separation. • Number of codes available is dependent on length of code. • Channelisation codes require an equal number of 1‟s and 1‟s to be orthogonal. • This is because we use integration to demodulate the signal • Channelisation codes are used to spread the signal. 16 Copyright 2010 AIRCOM International OVSF codes • Orthogonal Variable Spreading Factor Codes can be defined by a code tree: Cch,4,0 =(1,1,1,1) Cch,2,0 = (1,1) Cch,4,1 = (1,1,-1,-1) Cch,1,0 = (1) Cch,4,2 = (1,-1,1,-1) Cch,2,1 = (1,-1) Cch,4,3 = (1,-1,-1,1) SF = 1 SF = 2 SF = 4 • SF = Spreading Factor of code (maximum 512 for UMTS) 17 Copyright 2010 AIRCOM International Channelisation Code Generation • Channelisation codes can be generated from a • x Hadamard matrix A Hadamard matrix is:x x x • Where x is a Hadamard matrix of the previous level • For example 4 chip codes are: Note: These two codes correlate • 1,1,1,1 • 1,-1,1,-1 if they are time shifted • 1,1,-1,-1 • 1,-1,-1,1 18 Copyright 2010 AIRCOM International UMTS Frame Structure • • • • Frame Period Tf = 10ms Frames are used for channel format control 15 slots, #0…#14 Slots are used for power control, & synchronisation Tslot = 666.7 s = 2560 chips #0 #1 #2 #i #14 Tf = 10ms = 38400 chips In one second =38400 x100 = 3840000=3.84Mbps 19 Copyright 2010 AIRCOM International Capacity Limiting Factors Factors Limiting Capacity: Orthogonality • Example: Eb/No = 4 dB, i = 0.6, 12200bps Orthogonality 0 0.2 0.4 0.6 0.8 1.0 Pole Capacity 963 1100 1280 1534 1914 2548 Pole Capacity (kbps) 2000 1000 0 20 0.5 1 Orthogonali ty Copyright 2010 AIRCOM International