Evolotion from GSM to WCDMA

1 © NOKIA TKK_001023.PPT/ 23.10.2000 Introduction to WCDMA (Chapter 3) Harri Holma, Senior Research Engineer IP Mobility Networks Nokia Networks 2 © NOKIA TKK_001023.PPT/ 23.10.2000 Outline • What is spread spectrum and where does it come from • Spreading • Correlator and RAKE receiver • Matched Filter • Uplink and downlink diversity • WCDMA power control • Closed loop • Outer loop • WCDMA handovers • Soft handover • Softer handover • Concluding remarks 3 © NOKIA TKK_001023.PPT/ 23.10.2000 What is Spread Spectrum? • Transmission bandwidth is much larger than the information bandwidth • Bandwidth is not dependant of the information signal • Processing gain = Transmitted bandwidth / Information bandwidth • Classification • Direct Sequence (spreading with pseudo noise (PN) sequence) • Frequency hopping (rapidly changing frequency) • Time Hopping (large frequency, short transmission bursts) 4 © NOKIA TKK_001023.PPT/ 23.10.2000 Where Does Spread Spectrum Come from • First publications, late 40s • First applications: Military from the 50s • Rake receiver patent 1956 • Cellular applications proposed late 70s • Investigations for cellular use 80s • IS-95 standard 1993 • 1997/1998 3G technology choice in ETSI/ARIB/TTA… • 2001/2002 Commercial launch of WCDMA technology … and where is it heading to Very low C/I 5 © NOKIA TKK_001023.PPT/ 23.10.2000 GSM System is TDMA Based f1 f2 f1 f1 f2 f2 f3 f1 f1 f2 f2 f3 f3 f1 f2 f1 f3 f1 M S 1 M S 2 M S 3 M S 4 BTS Time 200 kHz BTS Typical GSM Frequency Usage Pattern MS = Mobile Station Users divide the common frequency by time slots 6 © NOKIA TKK_001023.PPT/ 23.10.2000 UMTS System is CDMA Based f1 f1 f1 f1 f1 f1 f1 f1 f1 f1 f1 f1 f1 f1 f1 f1 f1 f1 MS1 MS2 MS3 MS4 BS Time 5 MHz CDMA Frequency Usage Pattern MS1 MS2 MS3 MS4 BS FDD = Frequency-division duplex • Uplink and Downlink operate in separated frequency bands TDD = Time-division duplex • Uplink (UL) and downlink (DL) use the same frequency band, which is time-shared by the UL and DL All users share the same frequency/time domain 7 © NOKIA TKK_001023.PPT/ 23.10.2000 Frequency Power density (Watts/Hz) Unspread narrowband signal Spread wideband signal W R Processing gain = W/R Processing gain = W/R • A narrowband signal is spread to a wideband signal Processing Gain & Spreading WCDMA WCDMA 5 MHz, 1 carrier 5 MHz, 1 carrier TDMA (GSM) TDMA (GSM) 5 MHz, 25 carriers 5 MHz, 25 carriers • High bit rate means less processing gain and higher transmit power or smaller coverage 8 © NOKIA TKK_001023.PPT/ 23.10.2000 Spreading Data x Code Data Code Code (pseudo noise) Data +1 +1 +1 +1 +1 Symbol -1 -1 -1 -1 -1 ChipChip Despreading Despreading Spectrum Symbol 9 © NOKIA TKK_001023.PPT/ 23.10.2000 Detecting Own Signal. Correlator Code Data after multiplication +1 +1 +1 -1 -1 -1 Own signal +8 -8 Data after Integration Code Data after multiplication +1 +1 +1 -1 -1 -1 Other signal +8 -8 Data after Integration 10 © NOKIA TKK_001023.PPT/ 23.10.2000 WCDMA Codes • Channelisation codes used for channel separation from a single source • same codes in all cells ==> need for scrambling • Scrambling codes separate • Uplink: different mobiles • Downlink: different cells/sectors • Have good interference averaging (correlation) properties +1 -1 Short code +1 Long code -1 +1 -1 Combined code [...]... can rotate signal to any phase and to any amplitude • QPSK symbols carry information in phase • energy splitted to many fingers -> combining • maximal ratio combining corrects channel phase rotation and weights components with channel amplitude estimate RAKE Diversity Receiver Input signal (from RF) I Phase rotator Correlator Delay Equalizer I I Q Q Q Code generators Channel estimator Finger 1 Finger... adjustment command frame reliability info WCDMA Handovers Soft handover • MS handover between different base stations Softer handover • MS handover within one cell between different sectors Hard handover • MS handover between different frequencies or between WCDMA and GSM (or TDD) Received signal strengt BS1 Base station diversity MS BS 1 Threshold BS 2 BS2 BS 3 BS3 Distance from BS1 21 © NOKIA TKK_001023.PPT/... of fraction of symbol duration Concluding Remarks • WCDMA benefits from and also requires: • Fast power control • Due near far problem • Soft/Softer handover • Due frequency reuse of 1 • WCDMA RAKE receiver • Allows various diversity methods • Is well known technology from research and 2nd generation systems 24 © NOKIA TKK_001023.PPT/ 23.10.2000 WCDMA Physical Layer (Chapter 6) 25 © NOKIA TKK_001023.PPT/... in WCDMA due to frequency re-use 1 • open loop power control for initial power setting of the MS • across the air-interface: closed loop power control 1.5 kHz • Eliminates near-far problem • Typically up or down 1 dB, approx 70 dB range (21 dBm to -50 dBm) TPC commands if SIR > (SIR)set then "down" else "up" P1 TPC commands P2 BS MS2 19 © NOKIA TKK_001023.PPT/ 23.10.2000 MS adjusts power according to. .. Antenna 2 Fading Antenna RAKE combining (MRC) RNC 17 © NOKIA TKK_001023.PPT/ 23.10.2000 Time WCDMA Power Control M S 1 MS 3 MS 2 f1 BS MS 1 MS 4 M S 2 Received Power at the BS M S 1 M S 2 M S 3 M S 4 M S 3 M S 4 The purpose of power control is to ensure that each user receives and transmits just enough energy to properly convey information while interfering with other users no more that necessary 18... frame-by-frame (10 ms) • Higher bit rate requires more transmission power • Continuous transmission regardless of the bit rate • • Reduced audible interference to other equipment (nothing to do with normal interference, does not affect the spectral efficiency) GSM audible interference frequency ~217 Hz (=1/4.615 ms) • Admission control in RNC allocates those bit rates that the connection can use on physical... variations • ETSI /WCDMA solution: I-Q/code multiplexing with complex scrambling (Dual channel BPSK) channelization code cD complex scrambling code I DPDCH (data) I+jQ Q DPCCH (control) channelization code cc 31 © NOKIA TKK_001023.PPT/ 23.10.2000 *j G = gain factor I-Q/code Multiplexing in Uplink • Signal constellation before complex scrambling • Depending on G, constellation can be close to BPSK or QPSK... 3G Requirements on Physical Layer • High bit rates up to 2 Mbps • Bandwidth-On-Demand = Flexible variable bit rate • Multi-service = Multiplexing of different services on a single physical connection • Efficient packet data operation = support for all-IP • High spectral efficiency, especially in downlink • How can we fulfill these requirements with WCDMA physical layer? 26 © NOKIA TKK_001023.PPT/ 23.10.2000... Finger 3 Timing (Finger allocation) Matched filter 13 © NOKIA TKK_001023.PPT/ 23.10.2000 Combiner Matched Filter To make a successful despreading, code and data timing must be known Can be detected e.g by a matched filter Predefined (parallel) data When samples of incoming serial data are equal to bits of predefined data, there is a maximum at filter output Tap 127 Tap 126 Tap 0 Register 1 Σ Incoming serial... 23.10.2000 Sample Sample 127 126 Sample 0 Register 2 Delay Profile Estimation with MF •Multipath propagation causes several peaks in matched filter (MF) output •Allocate RAKE fingers to these peaks •Later: track and monitor the peaks 15 © NOKIA TKK_001023.PPT/ 23.10.2000 UL Receiver Diversity (Space Diversity) Amplitude = Antenna 1 = Antenna 2 Antenna RAKE combining (MRC) Fading Time RNC 16 © NOKIA . = W/R • A narrowband signal is spread to a wideband signal Processing Gain & Spreading WCDMA WCDMA 5 MHz, 1 carrier 5 MHz, 1 carrier TDMA (GSM) TDMA (GSM) 5 MHz, 25 carriers 5 MHz, 25 carriers •. Diversity Receiver Correlator Channel estimator Phase rotator Delay Equalizer Code generators Timing (Finger allocation) Finger 1 Finger 2 Finger 3 I Q I Q Combiner I Q Input signal (from RF) Matched filter 14. ETSI/ARIB/TTA… • 2001/2002 Commercial launch of WCDMA technology … and where is it heading to Very low C/I 5 © NOKIA TKK_001023.PPT/ 23.10.2000 GSM System is TDMA Based f1 f2 f1 f1 f2 f2 f3 f1 f1 f2 f2 f3 f3 f1 f2 f1 f3 f1 M S 1 M S 2 M S 3 M S 4 BTS Time 200