Introduction: 802.11a Wireless LAN. â Architecture .... s Advantages: ⢠Low-cost, high- ... *Van Nee & Prasad, OFDM for Wireless Multimedia Communications,.
5.4: A 5GHz CMOS Transceiver for IEEE 802.11a Wireless LAN David Su, Masoud Zargari, Patrick Yue, Shahriar Rabii, David Weber, Brian Kaczynski, Srenik Mehta, Kalwant Singh, Sunetra Mendis, and Bruce Wooley1
Atheros Communications, Sunnyvale, California 1Stanford University, Stanford, California
Outline ❑ Introduction: 802.11a Wireless LAN ❑ Architecture ❑ Radio Design • Transmitter • Receiver • Frequency Synthesizer ❑ Summary
IEEE 802.11a WLAN ■ Frequency: 5 GHz UNII (Unlicensed National
Information Infrastructure) 40mW
5.15G
800mW
200mW
5.25G
5.35G
5.725G
5.825G
■ Total UNII Bandwidth: 300 MHz (> IEEE 802.11b) ■ Modulation: OFDM
(Orthogonal Frequency Division Multiplexing) + BPSK / QPSK / 16QAM / 64QAM ■ Data Rate: 6 - 54 Mbps
Spectral-Efficient Modulation ■ 64-QAM (Quadrature Amplitude Modulation)
— Large signal to noise ratio > 30dB • Phase noise • I/Q mismatch ■ OFDM (Orthogonal Frequency Division Mux) — Large peak to average power ratio of 52 or
17dB • TX: large power backoff • RX: large dynamic range • Some signal clipping can be tolerated Requires High Linearity
Architecture Architecture Direct Conversion
+
–
- No off-chip IF filter - LO leakage - Single synthesizer - LO pulling - Quadrature LORF - DC offset
Traditional - Low LO leakage - Off-chip IF filter Superheterodyne - Weak LO pulling - Two synthesizers - No quadrature LO - Design flexibility Dual conversion with 1GHz sliding IF
Radio Transceiver Transmitter
5GHz
Synthesizer
Tx_in
Control
Rx_out
Receiver
Dual Transmit Conversion dc
1G LOIF
4G 5G LORF
Freq(Hz)
■ Radio Frequency (RF) ≠ Local Oscillator (LO) • LO leakage is out of band • LO pulling by power amplifier is reduced
LO RF = --------------4
■ Sliding Intermediate Frequency (IF): LO IF • Single synthesizer • Excellent 1 GHz quadrature for good transmit image rejection ■ Double Image-reject mixers • Avoid IF filtering of sideband
Transmitter Block Diagram LORF(I)
LOIF(I) TX_I
PA
RF_OUT 5 GHz
LOIF(Q)
TX_Q LORF(Q)
LOIF(I)
Dual Receive Conversion fRF
fIF dc
1G LOIF
3G
4G LORF
5G
Freq(Hz)
■ No external IF filtering ■ Channel selection at Baseband with passive
LC filtering ■ Very high IF of 1GHz • 3GHz image is 2GHz away from 5GHz signal • Inherent bandpass filtering of 3GHz: –23dBc • RF mixer: 5-4 = 1GHz (IF) and 5+4 = 9GHz • No image-reject mixers
Receiver Block Diagram LORF
LOIF (I) PGA
off-chip LC LPF
LNA
RX_I DAC
RF_IN 5GHz
DAC
off-chip LC LPF
RX_Q PGA
LOIF (Q)
Offset Control
Synthesizer ■ Single synthesizer with sliding IF:
LO RF LO IF = --------------4 ■ Divide-by-four generates quadrature LOIF • Excellent I/Q matching ■ P+/N-well varactor ■ Frequency Plan: RF
5.160 to 5.340 GHz
10 MHz spacing
LORF
4.128 to 4.272 GHz
8 MHz spacing
LOIF
1.032 to 1.068 GHz
2 MHz spacing
Synthesizer Block Diagram
8MHz
PFD
CP
32
off-chip RC LPF
VCO LORF (4GHz)
16/17
Decoder
Channel Select
4
LOIF (1GHz)
5GHz CMOS RF Design ■ Advantages: • Low-cost, high-yield • Multi-layer interconnect makes decent inductors • High-level of integration supports sophisticated digital signal processing* ■ Challenges: • 5 GHz: 0.25µm + narrowband with inductors • No high-Q BPF: architecture + dynamic range • Process/Temp Variation: DSP algorithms • Noise/Power performance limitations * J. Thomson et al, ISSCC 2002, Paper 7.2
Power Amplifier Design ■ Large peak to average ratio (PAR) of
52 or 17dB
■ Signal peaks are infrequent: 0.25dB SNR
degradation when PAR reduced to 6dB for 16-QAM*. ■ Implications: • Poor power efficiency • With 6dB PAR, to obtain 40mW (16dBm) requires Psat of ~22dBm or 160mW • With 17dB PAR, to obtain 40mW (16dBm) requires Psat of ~33dBm or 2W *Van Nee & Prasad, OFDM for Wireless Multimedia Communications, Artech House, 2000
Power Amplifier Topology Vpa = 3.3V L2*
L3 C2
■ Class A operation ■ Cascoded • 3.3V supply voltage Output • Stability ■ Capacitive Level-shift • Metal-2,3,4,5 stacks
Input
M2
M3 L4*
■ Inductive loads ■ Differential • Off-chip balun
Bias * C.P. Yue and S.S. Wong, IEEE JSSC, May 1998
Power Amplifier Schematic Vpa=3.3V L2p
L3p Vout+
M3p
C2p
L1p
L1n
C1p
C1n
L3n
L2n C2n
M2p
M2n
L4p
L4n
Bias
Bias Vin-
Bias
Vout-
M3n
Bias
Vin+
PSAT = 22 dBm
Measured BPSK OFDM Spectrum 16.25MHz
POFDM = 17.8 dBm
Measured Transmit Constellation
64QAM (300kHz) modulated signal
OFDM Output Power (dBm)
Measured Transmit Output Power 18
16
14
Carrier Leak
Spectral Images –51dBc
12
10
–29dBc
6
9
12
18
24
36
Data Rate (Mbps)
48
54
LNA Schematic Vdd Vout M3 Vin+
M4 M1 Lsp
M2
Vin-
Lsn
Receiver NF: LNA to Baseband = 8dB
Programmable Baseband Amplifier Vdd
Vdd
Bias_p VoutR2 Vin+
R1
Vout+ R2
VinBias_n Vos+
Bias_n
offset control
Vos-
Measured Receiver Performance IF Mixer Output (dBm)
10 0 -10
Max. Gain
-20 -30 -40
Min. Gain
-50 -60 -90 -80 -70 -60 -50 -40 -30 -20 -10 RF Input (dBm)
0
Voltage Controlled Oscillator (VCO)
Vc Control
Control M1
M2
Phase Noise (dBc/Hz)
Composite Phase Noise at 5GHz –80 –90
–100 –110
–120 –130 1k
10k
100k 1M Frequency (Hz)
10M
Die Photograph Tx
Synth
Logic
Rx
Bias
Measured Performance TX Output Power Level
22 dBm
RX Chain Noise Figure
8 dB
Phase Noise (∆f=1MHz)
–112 dBc/Hz
Supply Voltages
2.5 V & 3.3 V I/O
TX Chain Power Dissipation
790 mW
RX Chain Power Dissipation
250 mW
Synthesizer Power Dissipation 180 mW Technology
0.25 µm 1P5M CMOS
Package
64-pin LPCC
Die Size
22 mm2
Conclusions ■ IEEE 802.11a radio transceiver in 0.25 µm
standard digital CMOS for 5-GHz WLAN ■ No external IF filter: • TX: double image-reject mixers • RX: very high IF of 1GHz ■ Dual conversion with sliding IF: single
synthesizer ■ Integration of: • transmitter with 22dBm output power • receiver with 8dB noise figure • synthesizer with –112dBc/Hz (∆f=1MHz)
Acknowledgement ■ Support of the Wireless Team at Atheros for
design, layout, and testing. In particular: H. Dieh, J. Kung, R. Popescu, A. Ong, J. Zheng, D. Nakahira, R. Subramanian, J. Kuskin, A. Dao, D. Johnson, C. Lee, L. Thon, P. Husted, W. McFarland, S. Wong, R. Bahr, T. Meng ■ Assistance of TSMC. In particular: S. C. Wong
and B. K. Liew.