6(source: Worldwide Interoperability for Microwave Access Forum) Wireless Technologies(source: Worldwide Interoperability for Microwave Access Forum)
7Mobility vs. User/Link Bit rate Mbps Wireless StandardsMobility vs. User/Link Bit rate Mbps
8What is WiMax?WIMAX stands for Worldwide Interoperability for Microwave AccessWiMAX refers to broadband wireless networks that are based on the IEEE standard, which ensures compatibility and interoperability between broadband wireless access equipmentWiMAX, which will have a range of up to 31 miles, is primarily aimed at making broadband network access widely available without the expense of stringing wires (as in cable-access broadband) or the distance limitations of Digital Subscriber Line.
9IEEE Specifications802.16aUses the licensed frequencies from 2 to 11 GHzSupports Mesh network802.16bIncrease spectrum to 5 and 6 GHzProvides QoS( for real time voice and video service)802.16cRepresents a 10 to 66GHz802.16dImprovement and fixes for a802.16eAddresses Mobile needsEnable high-speed signal handoffs necessary for communicationswith users moving at vehicular speeds
12802.16 Backhaul for Business 802.11 Non Line of Sight Point to Multi-pointPoint to Point BACKHAUL802.11Telco Core Network or Private (Fiber) NetworkFRACTIONAL E1/T1 for SMALL BUSINESSE1/T1+ LEVELSERVICE ENTERPRISEINTERNETBACKBONE
13Point to Point BACKHAUL Telco Core Network or Private (Fiber) Network Consumer Last MileNon Line of SightPoint to Multi-pointOUTDOORCPEPoint to Point BACKHAUL802.11&INDOORCPETelco Core Network or Private (Fiber) NetworkINTERNETBACKBONE
14Telco Core Network or Private (Fiber) Network 802.16e Nomadic / PortableNon Line of SightPoint to Multi-pointLine of Sight BACKHAUL802.16e802.16ePC CardTelco Core Network or Private (Fiber) NetworkLaptop ConnectedThrough eSEEKS BESTCONNECTIONINTERNETBACKBONE2 to 3 Kilometers Away
27Key Advantages of Mobile WiMax to 3G Tolerance to Multipath and Self-InterferenceScalable Channel BandwidthOrthogonal Uplink Multiple AccessSupport for Spectrally-Efficient TDDFrequency Selective SchedulingFractional Frequency ReuseFine Quality of Service (QoS)Advanced Antenna Technology
28OutlineWhat is WiMaxIs 3G dead? No way!Basic WiMax RF SpecsRF System level analysisMultiband front-end, filter, Tx designQuadrature errorsQuadrature calibrationRemaining Subcarrier ErrorsConclusions
35Key ideas in HSDPAThe HSDPA concept is based on the following features:Shared channel transmissionHigher-order modulationShort transmission time interval (TTI)Fast link adaptationFast schedulingFast hybrid automatic-repeat-request (ARQ).
36Reduced latency Short TTI (Transmission time interval) Channel codes from the shared code resource aredynamically allocated every 2 ms or 500 times persecond.A short TTI reduces roundtrip time andimproves the tracking of channel variations—a featurethat is exploited by link adaptation and channel-dependent scheduling.
37Code sharing for shared channel Shared channel transmissionHS-DSCH is based on shared-channel transmission,which means that some channel codes and thetransmission power in a cell are seen as a commonresource that is dynamically shared between users inthe time and code domains.The shared code resource onto which the HS-DSCH ismapped consists of up to 15 codes. The spreadingfactor (SF) is fixed at 16.
49Transmitter requirements (IEEE , 8.5.2)Transmit power level controlTransmit spectral flatnessTransmit constellation errorTransmit spectral maskACPR, maximum output power, spurious andharmonics are also important
50Transmitter Spectral Flatness Spectral linesSpectral flatnessSpectral lines from -50 to -1 and +1 to +50+/--2 dB from the measured energy averaged over all active tonesSpectral lines from -100 to -1 and +1 to +100+2dB/--4dB from the measured energy averaged over all active tonesAdjacent subcarriers+/--0.1dB
53Receiver requirements are defined in IEEE sectionReceiver sensitivityReceiver adjacent and alternate channelrejectionReceiver maximum input signalReceiver maximum tolerable signalReceiver image rejection
54Receiver Sensitivity (dBm) for 10e-6 BER Modulation type and coding rateBandwidth64QAM16QAMQPSKBPSK3/42/31/2-75.7-72.7-69.8-68.1-65.024.4-77.4-74.4-71.3-66.722.7-81.9-78.9-78.8-74.3-71.218.2-83.7-80.7-77.6-76.1-73.016.4-88.9-85.9-82.8-81.3-78.211.2-90.7-87.7-84.6-83.1-80.39.4-93.7-87.6-86.1-83.06.41.75MHz3.5MHz7.0MHz10.0MHz20.0MHzRx SNR (dB)
56OutlineWhat is WiMaxIs 3G dead? No way!Basic WiMax RF SpecsRF System level analysisMultiband front-end, filter, Tx designQuadrature errorsQuadrature calibrationRemaining Subcarrier ErrorsConclusions
57Examples of Implementation Trials Almost all of the implementations use Direct Conversion methodDirect Conversion pros and cons:Advantages:High level of integrationLow power consumptionDisadvantages:DC offset problemHigher flicker noisePoor quadrature matching
59Frequency Synthesizer Specifications [B. Farahani-05] Direct Conv ArchitectureFrequency Synthesizer Specifications [B. Farahani-05]1.The phase noise requirement for PLL :offset frequency2.The settling time should be better than:5µs
60Block Requirements for Direct Conversion WiMAX Receiver [B Block Requirements for Direct Conversion WiMAX Receiver [B. Farahani-05]
61Block Requirements for Direct Conversion WiMAX Receiver [B Block Requirements for Direct Conversion WiMAX Receiver [B. Farahani-05]
62Different signal levels in Zero-IF receiver for WiMAX system [B Different signal levels in Zero-IF receiver for WiMAX system [B. Farahani-05]
63OutlineWhat is WiMaxIs 3G dead? No way!Basic WiMax RF SpecsRF System level analysisMultiband front-end, filter, Tx designQuadrature errorsQuadrature calibrationRemaining Subcarrier ErrorsConclusions
64Good idea to support both WiMax and WLAN WiMAX needs are more stringent than WLAN:Tx EVM requirement is smaller for WiMAXBecause of Higher number of subcarriers in WiMAX OFDM, Phase noise of the VCO should be smaller for WiMAX which is on the order of 1˚ at arbitrary channel centersI/Q sideband rejection requirement is more stringent than WLAN which is of the order of 35dB.To cover different BW in WiMAX, programmable analog channel selection filters are required
65Multi-band RF front-end for 4G WiMAX and WLAN applications [C Multi-band RF front-end for 4G WiMAX and WLAN applications [C.Garuda-06]Direct Conversion ReceiverMulti band including: GHz, GHz, GHzTwo stage & programmable LNA instead of wideband LNAThe gain of dB approximately constant for all bandsUtilizes Gilbert cell as a mixerNo report about synthesizer and filtersIn 1.8v IBM 0.18µm CMOS process.
68Dual-band GHz, GHz, 0.18µm CMOS Transceiver for a/b/g and d/e(WiBro) [I.Vassiliou, et.al, Broadcom-06]Direct Conversion double band transceiver is fabricated in a 0.18µm 1P6M CMOSFractional-N synthesizer achieves 0.6˚(0.7˚) integrated phase error at 5GHz (2.4GHz)Digital calibration eliminates I/Q mismatch and carrier leakageProgrammable BW filters are usedAchieves EVM of -35dB in both transmit and receive pathsAchieves sideband suppression better than 45 dB & LO leakage lower than -30dBc
745 GHz Dual-Mode WiMAX/WLAN Direct Conversion Receiver [Y. Zhou, Instit 5 GHz Dual-Mode WiMAX/WLAN Direct Conversion Receiver [Y. Zhou, Instit. for Infocom Res-06]Direct Conversion architectureThe dual-mode receiver is fabricated in a 0.25µm IBM BiCMOS6HP SiGe processThe frequency synthesizer uses VCO running at half the RF frequency to help reduce dc offset due to LO feedthroughDC offset calibration is performed on chipThe receiver employs GM-C base-band filter with tunable cut-off frequency of either 5 or 10 MHzThe chip adopts a 3 wire serial bus interface to control all the functionsThe chip consumes 360mW from 3V power supplyTwo mode double gain LNA is used
75VCO with Auto Calibration Circuit Frequency fine tuning is achieved by hyper-abrupt varactorAn auto calibration circuit is implemented to select the appropriate band based on process variations
77A 2.4GHz Direct Conversion Transmitter for Wimax Applications, C. Masse, Analog DevicesOld article but explains the integration problems
78The I & Q analog baseband Signal GenerationDual 14-bit DACIQ modulator.direct up-conversion at the RF frequencyremove the alias at multiples of the sampling frequencyLow-pass Filtersexternal fractional-N synthesizerLO (a continuous signalwith minimal Phase error)VGA with about 50dBof gain control range.amplify or attenuate the composite RF signal out of the modulatorPrecise output powercontrol is achieved.RMS power detector
79A direct up-conversion architecture is attractive because: Wimax OFDM has no active sub-carrier at the origin, direct up-conversion produces less mixing product spursRequires fewer filters which is important when dealing with such wideband signalsThe lower number of parts helps minimize the current consumption.
80Performance SummaryWiMax OFDM modulation used here is a10MHz, 64QAM, 256-OFDM signal
82A CMOS transmitter front-end with digital power control for WiMAX 802 A CMOS transmitter front-end with digital power control for WiMAX e applications Y.H. Liu, H.C. Chen
83VGA First Stage of VGA: Second Stage of VGA: Totally: The gain control is realized with a current steering structureSecond Stage of VGA:The primary coil of the on-chip transformer serves as the load inductorfor a differential-to-single-ended conversionTotally:16 gain steps are achieved
84OutlineWhat is WiMaxIs 3G dead? No way!Basic WiMax RF SpecsRF System level analysisMultiband front-end, filter, Tx designQuadrature errorsQuadrature calibrationRemaining Subcarrier ErrorsConclusions
85Slides 85-107 in this section based on Dr. Earl McCune’s presentation Complexities in Quadrature signal generation for OFDM Transmitters and ReceiversWhat is Quadrature Modulation?First-Order Error SourcesIndividual Effects of error sourcesProposed New Calibration ProcedureSlides in this section based on Dr. Earl McCune’s presentation
86Quadrature ModulatorConverting the quadrature modulation equation as a block diagram gives the familiar form
87Realistic QM ModelThere are nine different first-order error terms
88Realistic QM Math Gain mismatch & Quadrature error Carrier leakages Data leakagesDC offsets
91Standard - Use SSB CaseA simple, constant-envelope signal (LSB is shown)
92Modulation (I,Q) Offsets Cause carrier leakageAM also results
93Carrier Offsets No effect on RF spectrum data leakage present amplitude variations on output signal
94Modulation Gain Errors Causes image sidebandAM at double the ‘data’ frequency
95Carrier Magnitude Errors Carrier magnitude mismatch - looks identical to IQ gain mismatch
96LO Quadrature Error Causes image sideband AM is phase shifted from gain-mismatch cases
97Output Compression P1dB effect on QM output gain mismatch casedata leakage caseP1dB effect on QM outputadds intermodulation sidebands
98Proposed CAL Procedure Minimize interactivity among adjustmentsBased on simple SSB signalFive steps (in order!):null data leakagenull carrier leakagezero quadrature errormatch path gainsset signal level (for signals with AM)
100Begin with ideal modulator SSB drive signals 1. Ideal Modulator DriveBegin with ideal modulator SSB drive signals
1012. Null Data Leakage Zero the carrier offset terms OC and OS RF signalZero the carrier offset terms OC and OS
1023. Null Carrier Leakage Adjust data offsets tradeoff until carrier is nulledRF signal
1034. Minimize Image Sideband Adjust quadrature errorimage sideband minimizes at fe = 0may need 0.1 dB resolutionRF signal
1045. Match Path Gains Adjust one data-gain term (choose AI, for example) null the image sidebandcompression distortion also disappearsRF signal
1056. Set Overall Gain Apply I(t) and Q(t) for an envelope-varying signal Turn down gains AI and AQ together to drop sidebandseffects a backoff from the P1dB of the summer
106Limited-Access Bounds If full access to all error terms is not available, then the achievable performance will be limited.Ex. : quad error limitation (no access to quadrature error)1 degree2 degrees
107ConclusionsQuadrature modulation is a rectangular (Cartesian) method of implementing signalsReal Quadrature-Modulators are not as simple as the textbooks (or databooks!) claimQuadrature modulators can be tamed if all ports are available to the design engineerOutput Noise Figure is a major problemLinear modulations tend to require large output (and input) backoffsAll error terms are, in general, functions of temperature!
108OutlineWhat is WiMaxIs 3G dead? No way!Basic WiMax RF SpecsRF System level analysisMultiband front-end, filter, Tx designQuadrature errorsQuadrature calibrationRemaining Subcarrier ErrorsConclusions
109How much spur rejection is needed? For 45 dB rejection, the gain matching should be better than 0.1 dBand the phase matching better than 0.5 degree.Some manufacturers require better than 55 dB for individualblocks!
110ASt cos(wStt) + AWe cos (wWet) Strong and weak duoASt cos(wStt) + AWe cos (wWet)A two tone signal in time domain.
112Transmitter features Allow offset adjustment blocks at the inputs A programmable low pass filter corner frequency for all WiMax needsBaseband gain with G adjustment (1 dB range in 0.1 dB resolution)Control on the local oscillator phase (3 degree range with 0.1 degree control)A very linear transmit mixerA very linear combinerA programmable attenuator (20 dB range in 1 dB steps)An integrated power level detector
113Strong and weak duo is not a duo! fStrong 3rd HarmonicWeak 3rdHarmonicUndesired sidebandDesired sidebandLO feed-through A typical shape of spectrum out of a quadrature transmitter
114LO feed through minimization process 4 MHzLOFeed-throughUse a 4 MHz baseband tone. Reduce the input level so that the upper side band is more than 60 dB down or into the noise level.The detector gives a 4 MHz LO feed-through, and a 8 MHz upper side band.Use a low frequency bandpass filter to detect the LO feed-throughAdjust offset controls till lo feed-through is minimized (2 dimensional search)
115The Side-band minimization process f4 MHzMinimized LO feed-throughUndesired sidebandDesired sidebandChange the input frequency to 2 MHzIn crease the input level. The LSB and USB will both increase.The detector detects a 4MHz USB, a 8 MHz USB 3rd harmonic.The 4 MHz LSB 3rd harmonic falls on the undesired signal!Adjust G and to minimize the USB. The rejection is limitedby the 3rd harmonic levels which is set by the transmit mixer
116OutlineWhat is WiMaxIs 3G dead? No way!Basic WiMax RF SpecsRF System level analysisMultiband front-end, filter, Tx designQuadrature errorsQuadrature calibrationRemaining Subcarrier ErrorsConclusions
117Will this calibration work across the band? In single carrier systems single frequencyinterference and even non-flat noise spectrum can betolerated.In OFDM systems S/N has to be fixed across the bandOptimum G and do not stay the same vs.frequency!What are the sources of the error vs. frequency?
118Edge of the band matching in filters C+CR+RQIoutQoutIn a 1st order filter 0.5% mismatch in components results in1.14 degree phase error at the edge of the band.In a 5th order filter assuming random component values theexpected phase error will be 3.6 degrees at the edge of theband.
119OFDM base-band spectrum Filter gain responseFilter phase responseSystem gain mismatchSystem phase mismatchOFDM base-band spectrum
120Broadband gain mismatch compensated by G but residual gain error still at the band edge! Broadband phase mismatch compensated by control with LLL, but residual phase error still at the band edge!
121DSPAtt.PolyIQSinVGAjOrtho. TesterBB filterThe solution is using base-band filter loop back techniques and saving frequency dependent calibration values in the DSP
122OutlineWhat is WiMaxIs 3G dead? No way!Basic WiMax RF SpecsRF System level analysisMultiband front-end, filter, Tx designQuadrature errorsQuadrature calibrationRemaining Subcarrier ErrorsConclusions
123What did we learn?WiMax is the best the telecom technology offers, but 3G is in hot pursuit!The testing methodology to separate the RF and DSP designers is very critical.Direct Conv. with new VCO/synthesizer techniques is the way to go.The Quadrature calibrations issues are the key to success.123