Challenges In Designing 5 GHz 802.11 ac WIFI Power Amplifiers

CHALLENGES IN DESIGNING
5 GHZ 802.11AC WIFI POWER
AMPLIFIERS

Evolution of IEEE 802.11 Standards
Standard Year
Released
Technology
Details
Frequency Bandwidth Highest
data rate
802.11n 2009 OFDM
(64-QAM),
MIMO
2.4 GHz and
5 GHz
20 and 40
MHz
1×1: 150 Mb/s
4×4: 600 Mb/s
802.11ac 2012 OFDM
(256-QAM),
MIMO,
MU_MIMO
5 GHz only 20, 40, and
80 MHz,
160 MHz
(Optional)
1×1: 866.7
Mb/s
8×8: 6.77 Gb/s
(160 MHz BW)

Drivers for Higher WiFi Data Rates
n Wireless display of high-definition(HD) images and
video.

Linearity Requirement on 802.11ac Transmitter/PA
n 802.11AC VS. 802.11N
u In order to ensure modulation quality, 256-QAM (802.11ac)
requires more stringent EVM requirement than 64-QAM
(802.11n) due to higher constellation density [1]

n 802.11AC VS. 802.11N
[8]

n 802.11AC VS. 802.11N
u In 802.11n, the maximum allowed EVM of the WiFi
transmitter is -28dB or 4%, when using 64 QAM with a
coding rate of 5/6. In order to meet this requirement over
varying temperatures and power supply voltage levels, the
WiFi PA or FEM is typically required to achieve an EVM
below -30.5dB or 3%.
u In 802.11ac, the maximum allowed EVM of the WiFi
transmitter has been reduced to -32dB or 2.5% while the
WiFi PA usually needs to achieve an EVM below -35dB
or 1.8%, when being tested with an 80MHz 11ac MCS9
signal (i.e., 256-QAM modulation with a coding rate of
5/6) [1].

n 802.11AC VS. 802.11N

n 802.11AC VS. 802.11N
u Take RFMD RFFM8505 802.11ac Front End Module (FEM) for
example, it achieves 19dBm power @ 1.8% EVM spec
in TX Mode [1]

n 802.11AC VS. 802.11N
u 11n PA meets 17.5 dBm output power @ 3% EVM, but can
only meet 10dBm output power @ 1.8% EVM
(11ac requirement) [1]
u 11n PA needs re-optimization to meet 11ac EVM at same
output power
u In other words, in terms of EVM, 11ac is the worst case. If
the test result can meet 11ac requirement, it should be able
to meet 11n requirement as well.

Design Challenge 1: Very Low EVM Requirement
n Usually 802.11ac PAs need to achieve better than -35dB
or 1.8% EVM [1]
n Very stringent requirements for PA AM/AM and AM/PM
distortion:
0.3dB Gain Imbalance or 2 Phase Imbalance can cause
1.8% EVM

Design Challenge 1: Very Low EVM Requirement
n Due to very low EVM requirement, the transmitter
adopting Direct Up-conversion architecture need more
careful design. Otherwise, the EVM performance is still
bad even though the PA has good EVM performance [9]

Design Challenge 2: Dynamic Operation and
transient Behavior
n WiFi networks utilize Time Division Duplexing (TDD) –
PA is pulsed on and off during usage (dynamic
operation)
n Dynamic mode has worse linearity performance than
static mode, so dynamic operation needs careful
design of PA transient/thermal behavior [1]

transient Behavior
n Once PA is on, amplitude must be flat during entire
transmission. Otherwise, any rise or droop contributes
to AM/AM distortion and degrades EVM [1]

transient Behavior
n Besides, the PA with dynamic mode needs more careful
design on Vcc ripple and IR drop than static mode,
especially when Pout is maximum. Imperfect Vcc leads
to bad EVM performance as well.

Design Challenge 3: Achieve PAE and Linearity
Simultaneously
n Simple way to improve linearity (EVM) is to increase Icc;
however, not acceptable to customers because of lower
PAE [1]
n Need to achieve PAE & linearity simultaneously:
optimize load, interstage match, bias circuits [1]

Design Challenge 4: Wide Operation Bandwidth
n Wider channel bandwidth of 802.11ac (80/160 MHz):
bias circuit must have sufficient bandwidth to avoid
clipping signal and resulting in distortion [1]
n Very flat gain and very little phase distortion channel to
avoid EVM degradation

n Take RFMD RFFM8505 802.11ac FEM for example, FEM
achieves ~29 dB Gain at various 802.11ac channels,
and gain is very flat up to 19 dBm output power, to
avoid EVM degradation [1]

n RF bandwidth from 5170 to 5835 MHz (~15% fractional
BW), it indicates that PA’s on-die match network
should adopt multi LC section to achieve enough BW
[1]

n Multi LC section match network has wider BW indeed

n But, wider BW leads to smaller Q-factor and more
insertion loss

n According to the following formula, with constant
target TX power, the larger the PA post-loss is, the
larger the PA output power will be
Target TX Power(dBm) = PA output power(dBm) – PA post-loss(dB)

n The larger the PA output power is , the worse linearity
and more current consumption will be [10]

n Also, due to wide RF bandwidth, the off-chip match
networks should be fine-tuned to converge to one
point near 50 ohm in Smith Chart over the whole band
to ensure all the channels have the identical
performance

Design Challenge 5: High Operation Frequency
n In general, due to skin effect, the higher frequency is,
the more insertion loss(IL) will be. Take the
WiFi 2.4 GHz / 5 GHz diplexer for example [11] :

n In 2.4 GHz, the IL is less than 0.5 dB. Nevertheless, in
5 GHz, the IL is more than 0.5 dB [11] :
n As mentioned above, with constant target TX power,
the larger the PA post-loss is, the larger the PA output
power and worse linearity will be

n For a 0201 Size 6.8 nH inductor, its SRF(Self Resonant
Frequency) is about 6 GHz
n Thus, when the operation frequency:
u > 6 GHz => capacitance behavior
u < 6 GHz => inductance behavior
u = 6 GHz => resistance behavior

n For a 0201 Size 1.5 pF capacitor, its SRF(Self Resonant
Frequency) is about 6 GHz
n Thus, when the operation frequency:
u > 6 GHz => inductance behavior
u < 6 GHz => capacitance behavior
u = 6 GHz => resistance behavior

n Hence, when fine-tuning the match networks with 0201
size components, the inductor value should NOT be
larger than 6.8 nH, and the capacitor value should NOT
be larger than 1.5 pF
n Otherwise, the impedance trajectory in Smith Chart will
be unexpected. An inductor behaves like a capacitor,
and a capacitor behaves like an inductor

n The higher operation frequency is, the stronger
parasitic effect will be. In other words, it is more
difficult for 5 GHz opeartion to fine-tune match
networks than 2.4 GHz
n Thus, 5 GHz PA’s load-pull is more sensitive to the
tolerance of LC value, PCB line width, and soldering
quality of front-end components. This may
lead to yield rate issue during mass production

Conclusion
n Challenges in designing 5 GHz 802.11ac WIFI PA iles in :
u Very Low EVM Requirement
u Dynamic Operation and transient Behavior
uAchieve PAE and Linearity Simultaneously
u Wide Operation Bandwidth
u High Operation Frequency

Reference
[1] CHALLENGES IN DESIGNING 5 GHZ 802.11AC WIFI POWER AMPLIFIERS, RFMD
[2] WCN3660 EVM Degradation Issue Technical Note, Qualcomm
[3] SE5516A: Dual-Band 802.11a/b/g/n/ac WLAN Front-End Module, SKYWORKS
[4] 802.11ac Technology Introduction White Paper, RHODE & SCHWARZ
[5] WLAN IEEE 802.11ac Wide bandwidth high speed 802.11ac technology and testing,
RHODE & SCHWARZ
[6] ACPF-7024 ISM Bandpass Filter (2401 – 2482 MHz), AVAGO
[7] WCN36x0(A) RF Matching Guidelines, Qualcomm
[8] MCS Index for 802.11n and 802.11ac Chart
[9] Sources of Error in IQ Based RF Signal Generation
[10] Integration Aids 802.11ac Mobile Wi-Fi Front Ends
[11] Mini filters for multiband devices, TDK

Challenges In Designing 5 GHz 802.11 ac WIFI Power Amplifiers

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