WCDMA Air Interface Training Part 2 CDMA Power Control, RAKE Receiver, and Soft Handover

CDMA Reception Issues Unequal received power levels degrade SSMA performance Near-Far Ratio, terrain, RF obstacles, “Turn-the-Corner” effects, ... Multipath fading cancellation Time of Arrival delay spread

Effective Power Control Required Near-far Problem Path Loss Fading

The Power Control Solution Open loop Fast closed loop

CDMA Power Control Open-Loop Power Control Compute Initial Transmit Power Measure received power from BS Read BS transmit power from Broadcast Channel Transmit Access Preamble Access Acknowledged? Increase Transmit Power by 1 dB No Yes MS Begins Uplink TCH Transmission Outer-Loop (slow) Power Control Inner-Loop (fast) Power Control FER Acceptable? Raise Rx Power Target Lower Rx Power Target No Yes Received power > target? Increase MS Transmit Power by 1 dB Decrease MS Transmit Power by 1 dB

CDMA Power Control BS Receive Power time BS Receive Power Target Inner-loop power Control (Initial Receive Power Target) Inner-loop power Control (Updated Receive Power Target) Open-loop Power Control Access Preambles 800 updates/sec (IS-95, cdma2000) 1500 updates/sec (WCDMA) time BS Receive Power Target

Multipath Fading Fast (Rayleigh) Fading Time between fades is related to RF frequency Geometry of multipath vectors Vehicle speed: Up to 2 fades/sec per kilometer/hour Fast (Rayleigh) Fading Composite Received Signal Strength msec Deep fade caused by destructive summation of two or more multipath reflections time (mSec)

Why Spread Spectrum ? f Channel Quality f Channel Quality Frequency selective fading - Frequency Diversity Interference Averaging Forward Error Correction f Channel Quality f Channel Quality In FDMA/TDMA some users on a certain frequency suffer more from fading than other users on other frequencies. Interference averaging is when you can filter out narrow band interference and other wideband interferers. The coding gives us the possibility of Forward Error Correction (FEC). It could be described as adding extra information bits wchich immediately show what the correct bits should be. In spread spectrum techniques al users will suffer the same from fading problems. Frequency diversity in WCDMA is accomplished with spread spectrum technique.

t t Radio Environment h() Multipath Propagation Time Dispersion 1 2 3 2 3 t h() 1 2 3 Multipath propagation is usually a problem. Signals reach the receiver at different time, time dispersion. In WCDMA the RAKE receiver benefits from the multipath propagation.

Delay Profiles a. Impulse Response b. Coherence Windows Narrow band Time Delay Power (dB) b. Coherence Windows Narrow band Broad band

The RAKE Receiver CDMA Mobile Station RAKE Receiver Architecture Each finger tracks a single multipath reflection Also be used to track other base station’s signal during soft handover One finger used as a “Searcher” to identify other base stations Finger #1 Combiner Sum of individual multipath components Finger #2 Finger #N Power measurement of Neighboring Base Stations Searcher Finger

The RAKE Receiver The CDMA Pilot Channel All CDMA standards include a full-time “Pilot” Channel Broadcast by the Base Station Unique to each cell (or sector) Serves as precision coherent phase reference for Downlink channels Equivalent to a continuous-loop transmission of the Cell’s PN code ‘I’ PN Code I/Q Modulator FIR Filter Data All 0’s Pilot Channel Output FIR Filter Orthogonal Code 0 ‘Q’ PN Code

CDMA RAKE Receiver Architecture Carrier Frequency Tracking Loop Rake Receiver “Finger” cos(2fIFt) “I” PN Code (+1/-1) bit rate = chip rate / SF I/Q Demod D Integrate over ‘SF’ chips  De- Interleave Data Viterbi/ Turbo Decoder Decoded Output Bits BPF LPF  D Orthogonal Code (+1/-1) cos(2fRFt) “Q” PN Code (+1/-1) CRC Verification Timing Adj. Correlator Error Indication Pilot Orthogonal Code (all zeros) Other Rake Receiver Finger

The RAKE Receiver  Composite Received Signal time To Viterbi Decoder 50 100 150 200 250 300 350 400 -2 2 4 6 8 10 12 14 16 18 2 3 1 time To Viterbi Decoder 50 100 150 200 250 300 350 400 -2 2 4 6 8 10 12 14 16 18 2 3 3 + Interference 1 1/2-chip delay Correlator Ai 50 100 150 200 250 300 350 400 -2 2 4 6 8 10 12 14 16 18 2 3 2 + Interference 1 1/2-chip delay  Correlator Ai The 1/2-chip delays shown in this diagram have "..." symbols between them, indicating that any arbitrary number of 1/2-chip delays may need to be used. In other words, the receiver selects the delayed multipath signal in any multiple of 1/2-chip which is needed to equalize the delays of each multipath ray which is to be tracked. Note: The reason you need 1/2-chip steps is that if you had 1-chip steps, you may not be able to center the delay of the received signal to within +/- 1/2 chip of the receiver's internal PN generator. And if the received signal is not centered to within +/- 1/2 chip, it will be reduced to interference rather than correlated as signal. Also, the range of the delay line is typically 50 to 100 usec in most RAKE receivers, in steps of 1/2 chip. (ML=Maximum Likelyhood, which is the same as Maximum Ratio combining) 50 100 150 200 250 300 350 400 -2 2 4 6 8 10 12 14 16 18 2 3 1 + Interference 1 1/2-chip delay Correlator Ai PN, Walsh Codes Equal Combining, ML Combining, or Select Strongest

Handover Inter-System Handover Hard Handover Soft Handover Handover from a CDMA system to an Analog or TDMA system Traffic and Control Channels are Disconnected and must be Reconnected Hard Handover When the MS must change CDMA carrier frequency during the Handover Soft Handover Unique to CDMA During Handover, the MS has concurrent traffic connections with two BS’s Handover should be less noticeable Softer Handover Similar to Soft Handover, but between two sectors of the same cell Handover is simplified since timing sectors have identical timing

Soft Handover

CDMA Without Soft Handover MS responding to BS1 power control bits MS responding to BS2 power control bits BS1 Receive Power Target time BS2 Receive Power Target time Trouble zone: Prior to Hard Handover, the MS causes excessive interference to BS2

Monitor Neighbor BS Pilots CDMA Soft Handover CDMA Soft Handover One finger of the RAKE receiver is constantly scanning neighboring Pilot Channels. When a neighboring Pilot Channel reaches the t_add threshold, the new BS is added to the active set When the original Base Station reaches the t_drop threshold, originating Base Station is dropped from the active set Monitor Neighbor BS Pilots Add Destination BS Drop Originating BS

Soft Handover Add/Drop Thresholds Soft Handover Measurement and Decision Eb / N0 t t t Cell 1 T_DROP T_ADD T_ADD Cell 2 Cell 3 time Cell 1 Connected Add Cell 2 Drop Cell 1 Add Cell 3 Drop Cell 3

CDMA With Soft Handover MS responds to power control commands from both BS1 and BS2 MS responding to BS1 power control commands BS1 BS2 Action 0 0 Reduce power 0 1 Reduce power 1 0 Reduce power 1 1 Increase power MS responding to BS2 power control commands BS1 Receive Power Target 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 time BS2 Receive Power Target 2 2 2 2 1 1 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 time

CDMA Soft Handover Key points to remember about Soft Handover SSMA used to distinguish all transmitters in a Cellular CDMA system Fast power control is required to sustain SSMA performance When fast power control is used, soft handover is essential Allows MS to operate in most conservative power control mode Soft handover provides performance benefits “Seamless” coverage at cell fringes Handover may be less noticeable to the user Increases apparent system capacity when system is lightly loaded Soft handover also degrades system capacity Uses redundant physical layer resources from adjacent or overlapping cells