5.4: A 5GHz CMOS Transceiver for IEEE 802.11a Wireless LAN

28 Pages·2001·0.18 MB·English

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,
1
and Bruce Wooley
Atheros Communications, Sunnyvale, California
1
Stanford University, Stanford, California
Outline
❑
Introduction: 802.11a Wireless LAN
❑
Architecture
❑
Radio Design
• Transmitter
• Receiver
• Frequency Synthesizer
❑
Summary
IEEE 802.11a WLAN
(cid:2)
Frequency: 5 GHz UNII (Unlicensed National
Information Infrastructure)
800mW
200mW
40mW
5.15G 5.25G 5.35G 5.725G 5.825G
(cid:2)
Total UNII Bandwidth: 300 MHz (> IEEE 802.11b)
(cid:2)
Modulation: OFDM
(Orthogonal Frequency Division Multiplexing)
+ BPSK / QPSK / 16QAM / 64QAM
(cid:2)
Data Rate: 6 - 54 Mbps
Spectral-Efficient Modulation
(cid:2)
64-QAM (Quadrature Amplitude Modulation)
— Large signal to noise ratio > 30dB
• Phase noise
• I/Q mismatch
(cid:2)
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 - No off-chip IF filter - LO leakage
Conversion - Single synthesizer - LO pulling
- Quadrature LO
RF
- 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
Tx_in
Synthesizer Control
5GHz
Rx_out
Receiver
Dual Transmit Conversion
Freq(Hz)
dc 1G 4G 5G
LO LO
IF RF
(cid:2) ≠
Radio Frequency (RF) Local Oscillator (LO)
• LO leakage is out of band
• LO pulling by power amplifier is reduced
LO
RF
(cid:2)
Sliding Intermediate Frequency (IF): LO = ---------------
IF
4
• Single synthesizer
• Excellent 1 GHz quadrature for good transmit image
rejection
(cid:2)
Double Image-reject mixers
• Avoid IF filtering of sideband
Transmitter Block Diagram
LO (I) LO (I)
RF IF
TX_I
PA
RF_OUT LO (Q)
IF
5 GHz
TX_Q
LO (Q) LO (I)
RF IF
Dual Receive Conversion
f f
IF RF
Freq(Hz)
dc 1G 3G 4G 5G
LO LO
IF RF
(cid:2)
No external IF filtering
(cid:2)
Channel selection at Baseband with passive
LC filtering
(cid:2)
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
LO LO (I)
RF IF
PGA
off-chip
RX_I
LC LPF
LNA
DAC Offset
RF_IN
Control
DAC
5GHz
off-chip
RX_Q
LC LPF
PGA
LO (Q)
IF