All All - - Digital Quadrature Modem Digital Quadrature Modem for High Speed Wireless for High Speed Wireless Communications Communications Abhishek Mitra 19991003 Abhishek Mitra 19991003 IIIT Allahabad IIIT Allahabad 27 27 th th Jan 2003 to 22 Jan 2003 to 22 nd nd May 2003 May 2003 Under guidance of Mr. Vishwajit Mitra HFCL R&D
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High Speed Digital Quadrature Modem for Digital Radiosalumni.cs.ucr.edu/~amitra/modem.pdf · All-Digital Quadrature Modem for High Speed Wireless Communications Abhishek Mitra 19991003
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AllAll--Digital Quadrature Modem Digital Quadrature Modem for High Speed Wireless for High Speed Wireless
2727thth Jan 2003 to 22Jan 2003 to 22ndnd May 2003May 2003
Under guidance of
Mr. Vishwajit Mitra
HFCL R&D
ReferencesReferences
§§ A VLSI Architecture for a HighA VLSI Architecture for a High--Speed AllSpeed All--Digital Digital Quadrature Modulator and Demodulator for Digital Quadrature Modulator and Demodulator for Digital Radio Applications. ”Henry Radio Applications. ”Henry SamueliSamueli and Bennett and Bennett C. Wong”, IEEE Journal on Selected Areas in C. Wong”, IEEE Journal on Selected Areas in Communications, Vol8, No 8, October ‘90Communications, Vol8, No 8, October ‘90§§ A BPSK / QPSK Timing A BPSK / QPSK Timing –– Error Detector for Error Detector for
Sampled Receivers. “Floyd M Gardner”, IEEE Sampled Receivers. “Floyd M Gardner”, IEEE Transactions on Communications, Vol. COMTransactions on Communications, Vol. COM--34, 34, No 5, May ‘86 No 5, May ‘86 §§ A survey of CORDIC algorithms for FPGA based A survey of CORDIC algorithms for FPGA based
computers. “Ray computers. “Ray AndrakaAndraka”, FPGA 98, Monterey, ”, FPGA 98, Monterey, CA, USACA, USA
ModemModem
–– QPSK (Quadrature Phase Shift Keying)QPSK (Quadrature Phase Shift Keying)–– 10MHz Data rate10MHz Data rate–– IF at 10MHzIF at 10MHz–– UptoUpto 40MHz Data rate40MHz Data rate–– Reconfigurable Devices (FPGA)Reconfigurable Devices (FPGA)–– Digital FilteringDigital Filtering–– Digital Modulation and DemodulationDigital Modulation and Demodulation–– Applications: Point to Point W/Less Links and Satellite Applications: Point to Point W/Less Links and Satellite
Communications.Communications.
QPSK ModulationQPSK Modulation
Input (I ) Mixer (Q) Mixer Sum Sum (solved) 00
ctωsin−
ctωcos−
ctct ωω cossin
)135sin( °−ctω
01
ctωsin−
ctωcos ctct ωω cossin +
)135sin( °+ctω
10
ctωsin
ctωcos−
ctct ωω cossin − )45sin( °−ctω
11 ctωsin
ctωcos ctct ωω cossin + )45sin( °+ctω
Waveform Sample Tn Sample
Tn+1 Sample Tn+2
Sample Tn+3
Sine (Q) Cosine (I)
0 1
1 0
0 -1
-1 0
ModulatorModulator
§§ 32 bit RRC filtering32 bit RRC filtering§§ IF at 10MHzIF at 10MHz§§ Analog LPF with B/W 13MHzAnalog LPF with B/W 13MHz§§ 10bit precision digital output10bit precision digital output§§ Parallel multiplication (LUT based)Parallel multiplication (LUT based)§§ When I carrier is sampled ‘1’ or ‘When I carrier is sampled ‘1’ or ‘--1’, the Q carrier 1’, the Q carrier
is sampled ‘0’ which means at any sampling is sampled ‘0’ which means at any sampling instant we need to process only one of the carrier. instant we need to process only one of the carrier. §§ PRBS Generator used to generate Random PRBS Generator used to generate Random
SequenceSequence
ModulatorModulator k=0,1,2,3,4,5,6,7,8
Block Diagram (Modulator Section)
h4k
h4k + 1
-h4k +2
-h4k +3
Idata
Qdata
Clk 4X
4 – To – 1 Multiplexer
D / A Converter
IF Output
Gain Block
PtoSConverter
Digital FilterDigital Filter§§ Root Raised Cosine (Low Pass)Root Raised Cosine (Low Pass)§§ Avoids ISIAvoids ISI§§ Digital Filter Coefficients:Digital Filter Coefficients:
7 4 7 4 --7 7 --16 16 --12 9 33 38 8 12 9 33 38 8 --48 48 --93 93 --77 27 203 390 511 77 27 203 390 511 511 390 203 27 511 390 203 27 --77 77 --94 94 --48 8 38 33 9 48 8 38 33 9 --12 12 --16 16 --7 4 77 4 7§§ Transmit filter has to meet the following specifications:Transmit filter has to meet the following specifications:§§ Operating FrequencyOperating Frequency : : 10 MHz.10 MHz.§§ No. of TapsNo. of Taps :: 3232§§ Roll of factor Roll of factor ?? : : 0.30.3§§ Pass band BandwidthPass band Bandwidth :: 13 MHz13 MHz
§§ Error = Sum of both I and Q channels Error = Sum of both I and Q channels §§ 10 bit digital error output converted to 10 bit digital error output converted to
analoganalog§§ Error signal is used to control the VCXOError signal is used to control the VCXO
Carrier RecoveryCarrier Recovery
§§ Carrier ambiguities result in a rotating constellationCarrier ambiguities result in a rotating constellation§§ DeDe--rotation is neededrotation is needed§§ Using CORDIC algorithms (iterative algorithm for Using CORDIC algorithms (iterative algorithm for
coordinate rotation)coordinate rotation)§ If the rotation angle is limited to tan( 2-i) then
multiplication by tangent term becomes a shift operation.
§ The final equation results in
]tan[cos']tan[cos'
φφφφ
XYYYXX
+=−=
2..[1 diYiXiKiXi −=+ -i]
2..[1 diXiYiKiYi +=+ -i]
Carrier RecoveryCarrier Recovery
Before After
CMA EqualizerCMA Equalizer
§§ Iterative Algorithm, Step size = 2Iterative Algorithm, Step size = 2§§ The algorithm fits the constellation points to The algorithm fits the constellation points to
a circle of constant magnitude.a circle of constant magnitude.§§ Dependent on the accuracy of the AGCDependent on the accuracy of the AGC§§ Decision directed algorithmDecision directed algorithm§§ C=C=C+cmaerrorC+cmaerror§§ CmaCma error = (Radius CMA)^2 error = (Radius CMA)^2 –– (Radius C)^2(Radius C)^2
FPGAFPGA
§§ Reprogrammable Digital HardwareReprogrammable Digital Hardware§§ Extremely Customizable and parallel executionExtremely Customizable and parallel execution§§ Very fast speed (150 MHz)Very fast speed (150 MHz)§§ Very low design turnaround timeVery low design turnaround time§§ Simulation / Synthesis ToolsSimulation / Synthesis Tools§§ Testing, Debugging, and ImplementationTesting, Debugging, and Implementation§§ XC 2S 200 (200 K system gates)XC 2S 200 (200 K system gates)§§ XC 2V 1500 (1.5 M system gates)XC 2V 1500 (1.5 M system gates)
ImplementationImplementation
§§ Quadrature Modulator (Tested)Quadrature Modulator (Tested)§§ Quadrature Demodulator (Tested)Quadrature Demodulator (Tested)§§ PRBS generator (24 bit Maximal Length)PRBS generator (24 bit Maximal Length)§§ Digital FiltersDigital Filters