1. WiMax RF chip Design April 9, 2007 Ali Fotowat, PhD. Managing Director
KavoshCom Asia R&D
April 9, 2007 1

2. Outline
What is WiMax
Is 3G dead? No way.
Basic WiMax RF Specs
RF System level analysis
Multiband front-end, filter, Tx design
Quadrature errors
Quadrature calibration
Remaining Subcarrier Errors
Conclusions

3. Issues

4. Issues

5. Issues

6. (source: Worldwide Interoperability for Microwave Access Forum)
Wireless Technologies
(source: Worldwide Interoperability for Microwave Access Forum)

7. Mobility vs. User/Link Bit rate Mbps
Wireless Standards
Mobility vs. User/Link Bit rate Mbps

8. What is WiMax?
WIMAX stands for Worldwide Interoperability for Microwave Access
WiMAX refers to broadband wireless networks that are based on the IEEE standard, which ensures compatibility and interoperability between broadband wireless access equipment
WiMAX, 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.

9. IEEE Specifications
802.16a
Uses the licensed frequencies from 2 to 11 GHz
Supports Mesh network
802.16b
Increase spectrum to 5 and 6 GHz
Provides QoS( for real time voice and video service)
802.16c
Represents a 10 to 66GHz
802.16d
Improvement and fixes for a
802.16e
Addresses Mobile needs
Enable high-speed signal handoffs necessary for communications
with users moving at vehicular speeds

10. Issues

11. Issues

12. 802.16 Backhaul for Business 802.11 Non Line of Sight
Point to Multi-point
Point to Point BACKHAUL
802.11
Telco Core Network or Private (Fiber) Network
FRACTIONAL E1/T1 for SMALL BUSINESS
E1/T1+ LEVEL
SERVICE ENTERPRISE
INTERNET
BACKBONE

13. Point to Point BACKHAUL Telco Core Network or Private (Fiber) Network
Consumer Last Mile
Non Line of Sight
Point to Multi-point
OUTDOOR
CPE
Point to Point BACKHAUL
802.11
&
INDOOR
CPE
Telco Core Network or Private (Fiber) Network
INTERNET
BACKBONE

14. Telco Core Network or Private (Fiber) Network
802.16e Nomadic / Portable
Non Line of Sight
Point to Multi-point
Line of Sight BACKHAUL
802.16e
802.16e
PC Card
Telco Core Network or Private (Fiber) Network
Laptop Connected
Through e
SEEKS BEST
CONNECTION
INTERNET
BACKBONE
2 to 3 Kilometers Away

16. Issues

19. Adaptive Modulation

22. Issues

27. Key Advantages of Mobile WiMax to 3G
Tolerance to Multipath and Self-Interference
Scalable Channel Bandwidth
Orthogonal Uplink Multiple Access
Support for Spectrally-Efficient TDD
Frequency Selective Scheduling
Fractional Frequency Reuse
Fine Quality of Service (QoS)
Advanced Antenna Technology

28. Outline
What is WiMax
Is 3G dead? No way!
Basic WiMax RF Specs
RF System level analysis
Multiband front-end, filter, Tx design
Quadrature errors
Quadrature calibration
Remaining Subcarrier Errors
Conclusions

29. Issues

30. Issues

31. Issues

32. Issues

34. The shared channel concept

35. Key ideas in HSDPA
The HSDPA concept is based on the following features:
Shared channel transmission
Higher-order modulation
Short transmission time interval (TTI)
Fast link adaptation
Fast scheduling
Fast hybrid automatic-repeat-request (ARQ).

36. Reduced latency Short TTI (Transmission time interval)
Channel codes from the shared code resource are
dynamically allocated every 2 ms or 500 times per
second.
A short TTI reduces roundtrip time and
improves the tracking of channel variations—a feature
that is exploited by link adaptation and channel-
dependent scheduling.

37. Code sharing for shared channel
Shared channel transmission
HS-DSCH is based on shared-channel transmission,
which means that some channel codes and the
transmission power in a cell are seen as a common
resource that is dynamically shared between users in
the time and code domains.
The shared code resource onto which the HS-DSCH is
mapped consists of up to 15 codes. The spreading
factor (SF) is fixed at 16.

38. Issues

39. Increase average speed by serving channels that can be best served!!

40. Issues

41. Issues
QPSK, 16QAM

42. Issues

43. Hybrid Automatic Repeat Request

44. Issues

45. HSDPA Mutli-User Diversity
High data rate
Low data rate
Efficient scheduler

46. Issues

47. Outline
What is WiMax
Is 3G dead? No way!
Basic WiMax RF Specs
RF System level analysis
Multiband front-end, filter, Tx design
Quadrature errors
Quadrature calibration
Remaining Subcarrier Errors
Conclusions

48. Transmitter test set-up

49. Transmitter requirements
(IEEE , 8.5.2)
Transmit power level control
Transmit spectral flatness
Transmit constellation error
Transmit spectral mask
ACPR, maximum output power, spurious and
harmonics are also important

50. Transmitter Spectral Flatness
Spectral lines
Spectral flatness
Spectral lines from -50 to -1 and +1 to +50
+/--2 dB from the measured energy averaged over all active tones
Spectral lines from -100 to -1 and +1 to +100
+2dB/--4dB from the measured energy averaged over all active tones
Adjacent subcarriers
+/--0.1dB

51. Transmitter Constellation Error
Burst type
Relative constellation
error (dB)
BPSK-1/2
-13.0
QPSK-1/2
-16.0
QPSK-3/4
-18.5
16QAM-1/2
-21.5
16QAM-3/4
-25.0
64QAM-2/3
-28.5
64QAM-3/4
-31.0

52. Receiver test set-up

53. Receiver requirements are defined in
IEEE section
Receiver sensitivity
Receiver adjacent and alternate channel
rejection
Receiver maximum input signal
Receiver maximum tolerable signal
Receiver image rejection

54. Receiver Sensitivity (dBm) for 10e-6 BER
Modulation type and coding rate
Bandwidth
64QAM
16QAM
QPSK
BPSK
3/4
2/3
1/2
-75.7
-72.7
-69.8
-68.1
-65.0
24.4
-77.4
-74.4
-71.3
-66.7
22.7
-81.9
-78.9
-78.8
-74.3
-71.2
18.2
-83.7
-80.7
-77.6
-76.1
-73.0
16.4
-88.9
-85.9
-82.8
-81.3
-78.2
11.2
-90.7
-87.7
-84.6
-83.1
-80.3
9.4
-93.7
-87.6
-86.1
-83.0
6.4
1.75MHz
3.5MHz
7.0MHz
10.0MHz
20.0MHz
Rx SNR (dB)

56. Outline
What is WiMax
Is 3G dead? No way!
Basic WiMax RF Specs
RF System level analysis
Multiband front-end, filter, Tx design
Quadrature errors
Quadrature calibration
Remaining Subcarrier Errors
Conclusions

57. Examples of Implementation Trials
Almost all of the implementations use Direct Conversion method
Direct Conversion pros and cons:
Advantages:
High level of integration
Low power consumption
Disadvantages:
DC offset problem
Higher flicker noise
Poor quadrature matching

58. For WLAN / WiMAX [B. Farahani-05]
Direct Conv Architecture
For WLAN / WiMAX [B. Farahani-05]

59. Frequency Synthesizer Specifications [B. Farahani-05]
Direct Conv Architecture
Frequency Synthesizer Specifications [B. Farahani-05]
1.The phase noise requirement for PLL :
offset frequency
2.The settling time should be better than:
5µs

60. Block Requirements for Direct Conversion WiMAX Receiver [B
Block Requirements for Direct Conversion WiMAX Receiver [B. Farahani-05]

61. Block Requirements for Direct Conversion WiMAX Receiver [B
Block Requirements for Direct Conversion WiMAX Receiver [B. Farahani-05]

62. Different signal levels in Zero-IF receiver for WiMAX system [B
Different signal levels in Zero-IF receiver for WiMAX system [B. Farahani-05]

63. Outline
What is WiMax
Is 3G dead? No way!
Basic WiMax RF Specs
RF System level analysis
Multiband front-end, filter, Tx design
Quadrature errors
Quadrature calibration
Remaining Subcarrier Errors
Conclusions

64. Good idea to support both WiMax and WLAN
WiMAX needs are more stringent than WLAN:
Tx EVM requirement is smaller for WiMAX
Because 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 centers
I/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

65. Multi-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 Receiver
Multi band including: GHz, GHz, GHz
Two stage & programmable LNA instead of wideband LNA
The gain of dB approximately constant for all bands
Utilizes Gilbert cell as a mixer
No report about synthesizer and filters
In 1.8v IBM 0.18µm CMOS process.

66. Two stage wide-band programmable LNA

67. Performance summary

68. Dual-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 CMOS
Fractional-N synthesizer achieves 0.6˚(0.7˚) integrated phase error at 5GHz (2.4GHz)
Digital calibration eliminates I/Q mismatch and carrier leakage
Programmable BW filters are used
Achieves EVM of -35dB in both transmit and receive paths
Achieves sideband suppression better than 45 dB & LO leakage lower than -30dBc

69. System Architecture uses TDD

70. GM-C filter and its OTA enable continuous tuning either 2-15 MHz or 10-25MHz

71. Programmable RF amplifier

72. Rx EVM vs. input power for different bands

73. Measured performance summary

74. 5 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 architecture
The dual-mode receiver is fabricated in a 0.25µm IBM BiCMOS6HP SiGe process
The frequency synthesizer uses VCO running at half the RF frequency to help reduce dc offset due to LO feedthrough
DC offset calibration is performed on chip
The receiver employs GM-C base-band filter with tunable cut-off frequency of either 5 or 10 MHz
The chip adopts a 3 wire serial bus interface to control all the functions
The chip consumes 360mW from 3V power supply
Two mode double gain LNA is used

75. VCO with Auto Calibration Circuit
Frequency fine tuning is achieved by hyper-abrupt varactor
An auto calibration circuit is implemented to select the appropriate band based on process variations

76. Performance Summary

77. A 2.4GHz Direct Conversion Transmitter for
Wimax Applications, C. Masse, Analog Devices
Old article but explains the integration problems

78. The I & Q analog baseband
Signal Generation
Dual 14-bit DAC
IQ modulator.
direct up-conversion at the RF frequency
remove the alias at multiples of the sampling frequency
Low-pass Filters
external fractional-N synthesizer
LO (a continuous signal
with minimal Phase error)
VGA with about 50dB
of gain control range.
amplify or attenuate the composite RF signal out of the modulator
Precise output power
control is achieved.
RMS power detector

79. A direct up-conversion architecture is attractive because:
Wimax OFDM has no active sub-carrier at the origin, direct up-conversion produces less mixing product spurs
Requires fewer filters which is important when dealing with such wideband signals
The lower number of parts helps minimize the current consumption.

80. Performance Summary
WiMax OFDM modulation used here is a10MHz, 64QAM, 256-OFDM signal

82. A 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

83. VGA First Stage of VGA: Second Stage of VGA: Totally:
The gain control is realized with a current steering structure
Second Stage of VGA:
The primary coil of the on-chip transformer serves as the load inductor
for a differential-to-single-ended conversion
Totally:
16 gain steps are achieved

84. Outline
What is WiMax
Is 3G dead? No way!
Basic WiMax RF Specs
RF System level analysis
Multiband front-end, filter, Tx design
Quadrature errors
Quadrature calibration
Remaining Subcarrier Errors
Conclusions

85. Slides 85-107 in this section based on Dr. Earl McCune’s presentation
Complexities in Quadrature signal generation for OFDM Transmitters and Receivers
What is Quadrature Modulation?
First-Order Error Sources
Individual Effects of error sources
Proposed New Calibration Procedure
Slides in this section based on Dr. Earl McCune’s presentation

86. Quadrature Modulator
Converting the quadrature modulation equation as a block diagram gives the familiar form

87. Realistic QM Model
There are nine different first-order error terms

88. Realistic QM Math Gain mismatch & Quadrature error Carrier leakages
Data leakages
DC offsets

89. ‘Ideal’ Error Values

90. Individual Error Analyses
Modulation offsets
Carrier Offsets
Modulation gain errors
Carrier magnitude errors
LO quadrature error
Modulator output compression

91. Standard - Use SSB Case
A simple, constant-envelope signal (LSB is shown)

92. Modulation (I,Q) Offsets
Cause carrier leakage
AM also results

93. Carrier Offsets No effect on RF spectrum data leakage present
amplitude variations on output signal

94. Modulation Gain Errors
Causes image sideband
AM at double the ‘data’ frequency

95. Carrier Magnitude Errors
Carrier magnitude mismatch - looks identical to IQ gain mismatch

96. LO Quadrature Error Causes image sideband
AM is phase shifted from gain-mismatch cases

97. Output Compression P1dB effect on QM output
gain mismatch case
data leakage case
P1dB effect on QM output
adds intermodulation sidebands

98. Proposed CAL Procedure
Minimize interactivity among adjustments
Based on simple SSB signal
Five steps (in order!):
null data leakage
null carrier leakage
zero quadrature error
match path gains
set signal level (for signals with AM)

99. Error-Combination Case
data leakage
RF signal

100. Begin with ideal modulator SSB drive signals
1. Ideal Modulator Drive
Begin with ideal modulator SSB drive signals

101. 2. Null Data Leakage Zero the carrier offset terms OC and OS
RF signal
Zero the carrier offset terms OC and OS

102. 3. Null Carrier Leakage Adjust data offsets
tradeoff until carrier is nulled
RF signal

103. 4. Minimize Image Sideband
Adjust quadrature error
image sideband minimizes at fe = 0
may need 0.1 dB resolution
RF signal

104. 5. Match Path Gains Adjust one data-gain term (choose AI, for example)
null the image sideband
compression distortion also disappears
RF signal

105. 6. Set Overall Gain Apply I(t) and Q(t) for an envelope-varying signal
Turn down gains AI and AQ together to drop sidebands
effects a backoff from the P1dB of the summer

106. Limited-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 degree
2 degrees

107. Conclusions
Quadrature modulation is a rectangular (Cartesian) method of implementing signals
Real Quadrature-Modulators are not as simple as the textbooks (or databooks!) claim
Quadrature modulators can be tamed if all ports are available to the design engineer
Output Noise Figure is a major problem
Linear modulations tend to require large output (and input) backoffs
All error terms are, in general, functions of temperature!

108. Outline
What is WiMax
Is 3G dead? No way!
Basic WiMax RF Specs
RF System level analysis
Multiband front-end, filter, Tx design
Quadrature errors
Quadrature calibration
Remaining Subcarrier Errors
Conclusions

109. How much spur rejection is needed?
For 45 dB rejection, the gain matching should be better than 0.1 dB
and the phase matching better than 0.5 degree.
Some manufacturers require better than 55 dB for individual
blocks!

110. ASt cos(wStt) + AWe cos (wWet)
Strong and weak duo
ASt cos(wStt) + AWe cos (wWet)
A two tone signal in time domain.

111. A quadrature transmitter overall block diagram

112. Transmitter features Allow offset adjustment blocks at the inputs
A programmable low pass filter corner frequency for all WiMax needs
Baseband 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 mixer
A very linear combiner
A programmable attenuator (20 dB range in 1 dB steps)
An integrated power level detector

113. Strong and weak duo is not a duo!f
Strong 3rd Harmonic
Weak 3rd
Harmonic
Undesired sideband
Desired sideband
LO feed-through
 A typical shape of spectrum out of a quadrature transmitter

114. LO feed through minimization process
4 MHz LO
Feed-
through
Use 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-through
Adjust offset controls till lo feed-through is minimized (2 dimensional search)

115. The Side-band minimization processf
4 MHz
Minimized LO feed-through
Undesired sideband
Desired sideband
Change the input frequency to 2 MHz
In 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 limited
by the 3rd harmonic levels which is set by the transmit mixer

116. Outline
What is WiMax
Is 3G dead? No way!
Basic WiMax RF Specs
RF System level analysis
Multiband front-end, filter, Tx design
Quadrature errors
Quadrature calibration
Remaining Subcarrier Errors
Conclusions

117. Will this calibration work across the band?
In single carrier systems single frequency
interference and even non-flat noise spectrum can be
tolerated.
In OFDM systems S/N has to be fixed across the band
Optimum G and  do not stay the same vs.
frequency!
What are the sources of the error vs. frequency?

118. Edge of the band matching in filters
C+C
R+R Q
Iout
Qout
In a 1st order filter 0.5% mismatch in components results in
1.14 degree phase error at the edge of the band.
In a 5th order filter assuming random component values the
expected phase error will be 3.6 degrees at the edge of the
band.

119. OFDM base-band spectrum
Filter gain response
Filter phase response
System gain mismatch
System phase mismatch
OFDM base-band spectrum

120. Broadband 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!

121. DSP
Att.
Poly I Q
Sin
VGA j
Ortho. Tester
BB filter
The solution is using base-band filter loop back techniques and saving frequency dependent calibration values in the DSP

122. Outline
What is WiMax
Is 3G dead? No way!
Basic WiMax RF Specs
RF System level analysis
Multiband front-end, filter, Tx design
Quadrature errors
Quadrature calibration
Remaining Subcarrier Errors
Conclusions

123. What 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