Case Study: Using New Technologies to Design and Test Wireless Receivers
Agenda Architecture of a receiver Basic GPS Receiver Measurements Case Study 1: GPS Simulation How Testing Works Simulation vs. Record and Playback Additional Test Challenges: Multi-Standard Receivers Phase Diversity / MIMO
Receiver Fundamentals Over-the-air signals acquired via antenna Noise floor of antenna is thermal noise -174 dbm/hz at room temp All receivers amplify over-the-air signals with an LNA Noise figure of LNA determines SNR of acquired signal Some receivers apply dynamic gain with an AGC Rx1 LNA LNA has both a Gain and a Noise Figure 90 ADC ADC I Q
Basic Receiver Metric: Sensitivity Definition: lowest power at which receiver achieves minimum performance criteria Can be measured by SNR of IQ signal Can be measured by BER or FER of receiver Each standard specifies minimum sensitivity WLAN: 10% PER at -65 dbm (54 Mbps 802.11g) WiMAX: 0.1% BER at -80 dbm + 10 log (BW) for 64-QAM GSM: 0.1% BER at -102 dbm (GSM900) GPS: Minimum power at which receiver obtains a position fix (C/N 0 of 30 db-hz) Not regulated by an standards body Typical signal power at -130 dbm Typical receiver sensitivity = -142 dbm (position fix) and -160 dbm (tracking)
Sensitivity of Analog FM Receiver Analog FM receiver produces analog audio signal SINAD (Signal to noise and distortion) of single-tone stimulus correlates with NF SINAD Effect of improving receiver noise figure SINAD limited by receiver dynamic range Linear correlation between SINAD and input power Minimum SINAD requirement determines sensitivity sensitivity RF Power
Case Study 1: GPS Receiver Test Challenge: Simulating the real-world environment of a GPS receiver Requirements: Simulation of multi-path and ionosphere/troposphere Simulation of custom receiver movements Simulation of satellite power fading
Overview of the Satellite Constellation P Code C/A Code L5 (1.18 GHz) L2 (1.23 GHz) L1 (1.58 GHz) Receiver typically sees 7 to 10 satellites C/A codes (1 MHz) have RF power of -125 to -135 dbm Commercial GPS: L1 C/A (course acquisition) iti codes Military / Precision GPS: all P codes (L1, L2, and L5) All GPS signals are CDMA-based
How GPS Testing Works Things to Simulate Poor signal strength View of satellites obstructed Position constantly changing GPS toolkit Creates Signal in LabVIEW PXIe-5673 VSG Generates Signal GPS receiver behaves as if it see s real satellites GPS Receiver
Sensitivity Measurements GPS signals are fundamentally weak (-130 dbm) Receiver uses amplifiers to increase signal power Amplifiers fundamentally add noise: called Noise Figure Receiver reports C/N 0 (carrier-to-noise) for each satellite C/N 0 of 30 to 32 db-hz required for position fix Position tracking sensitivity = lowest power level where receiver can achieve position fix (typical is -142 dbm) Signal tracking sensitivity= Lowest power level at which receiver maintains position fix (typical is -160 dbm) Sensitivity = 0 174dB / Hz + C/N + NF receiver
Time to First Fix (TTFF) Measurements GPS receiver s obtain position fix by decoding satellite data TTTF usually measured from Cold Start condition Receiver s memory of location is completely clear Usually takes 30 to 60 seconds for receiver to obtain position fix Requires simulate to generate at least 4 satellites A-GPS (assisted GPS) receivers have faster TTFF times Receiver obtains Ephemeris (satellite information) from cell tower
Approaches to Receiver Test Simulated Data Better power accuracy (no noise added) Simulate custom signal settings Custom content (latitude/longitude or VIDEO for DVB-T) Custom bistream (bistream affects PAPR) Record and Playback Ability to capture real-world impairments (multipath fading, etc.) Is more repeatable that drive/field tests
GPS Simulation NI GPS Toolkit for LabVIEW main features C/A codes in the L1 band (1.57542 GHz) Simulation of up to 12 satellites Up to 24 hours of generation Custom satellite power profiles User-defined custom receiver trajectory Perform receiver measurements such as: Sensitivity determines receiver noise figure Time to first fix (TTFF) Receiver position accuracy (toolkit accurate to within 2 meters)
Example Application: Motion Trajectory 30.5601, -97.7462 30.5575, -97.7448 30.5599, -97.7455 Receiver path defined through waypoints Receiver behaves as if it is moving
Simulating GPS Signals Write Waveform to File Generate Waveform
RF Record and Playback for GPS RF record and playback produces a repeatable field test Signal can be played back from disk with a vector signal generator 2 TB raid drive can playback 25 hours of GPS signal Antenna Amplifier Bandpass Filter Vector Signal Analyzer Disk Array LNA Disk Array Vector Signal Generator Attenuator GPS Receiver
Demo: GPS record and playback Demo Record & playback frequency: 1.57542 GHz Record reference level: l -50 dbm 60 db of gain should amplify power in L1 band to -56 dbm Playback power can be adjusted to achieve C/N target For best results, use direct connect with DC blocker Download code at: www.ni/com/streaming/rf
Multi-Protocol Test Challenge: Testing Multiple Radios on the Same Device Requirements: Generation of GPS navigation signals Generation of FM/RDS radio signal Generation of DVB-T radio signal Measurement of audio/video signals
Example System: Averna URT Radio Standards AM/FM with FM fading simulator RDS and RDBS an TMC over RDS Data Radio Channel (DARC) Sirius / XM IBOC / HD radio DAB DRM Navigation Standards TMC over RDS GPS Constellation Video Standards PAL/NTSC RF Signal Generation DVB-T Player
Multi-protocol Signal Generation Demo Software-Defined Test Signal RF Signal Generator Multiple Receivers
Emerging Technology: Multiple Antennas Phase Diversity Receivers Antenna Phase diversity used in DVB-T, satellite, etc. Rely on array gain to improve receive signal strength MIMO Channel Systems Typically use multiple transmit and multiple receive antennas Rely on both spatial diversity and array gain Examples include: 802.11n, Mobile WiMAX
Phase-Diversity Receivers Each channel is completely synchronized Synchronization requires sharing of LO and sample clocks ADC I Rx0 LNA ADC Q ADC I DSP Rx1 LNA ADC Q 90 2-Channel ZIF Receiver e
Diversity and Signal Power Diversity increases receive signal strength
What is MIMO? Stands for Multiple Input Multiple Output Common Configurations are 2x2 and 4x4 Used ins: 802.11n, WiMAX, LTE Transmitter Free Space Receiver
Architecture of a 4-channel VSA Demo Shared clocks Local oscillator ADC clock NCO on DDC Example shown with PXIe-5663 Rx0 Rx1 Rx2 Rx3 ADC ADC ADC ADC 16 16 16 16 DDC DDC DDC DDC I Q I Q I Q I Q 10 MHz OCXO Shared Local Oscillator Shared ADC Sample Clock
Synchronized VSAs and VSGs Up to 2 VSGs and 4 VSAs concurrently in a 18-slot chassis Phase coherent generation & acquisition with less than 0.1 jitter Data from all channels can be streamed to/from disk Typical 4x2 MIMO Test System
Summary All wireless receivers require similar measurements Sensitivity / BER Multiple methods can be used to generate signal Simulated waveforms Recorded RF waveforms MIMO is an emerging wireless technology Requires phase-coherent generators / analyzers