The document presents a method for old wireless standards like 802.11n to achieve throughputs comparable to the latest standards like 802.11ac using a technique called nested spatial mapping with a single antenna. It works by attenuating the signal of a second stream and combining it with the first stream before transmission through a single antenna. This allows the signal to be decoded like 256QAM in 802.11ac while staying within the specifications of 802.11n. Simulation results show it can achieve over 20% higher throughput than standard 802.11n. Implementation requires only minor firmware changes and is backwards compatible.

1. Good afternoon. Nice to see you.
I am Ealwan Lee working at GCT Semiconductor mainly for wireless
connectivity solution.
Today I am going to present a method in this formal meeting, for old
standard to catch up the throughput of the latest one.
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2. To this end I'll explain why it is needed and how it can be done and will give
some numerical results which may motivate the old WLAN IP owners, whether
they are IP providers or customers, to maximize the usage of what they have
right now in a more diversified applications.
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3. WLAN, have evolved in the 21st centuries leading the trends of the wireless
communication technology such as OFDM + BICM beating the TCM then
MIMO and channel bonding and finally 256QAM modulation in the aspects of
physical layer. However, in each development the market transition time has
come much later than expected by the so called market analyst only because
of its implementation issues with a view to its performance/cost ratio. Simply
speaking it took more time from 11g to 11n than the previous change from
11b to 11g. I am not sure whether we are in the 11ac or 11n mainly in the
newly growing IoT connectivity market. For example, actually iphone4, the
legendary phone, could not have got the WiFi certificate of 11n if WiFi
alliance strictly followed the rules stated by the IEEE 802.11 working group.
However, 2 antenna was somewhat burden for mobile devices at that time
and WiFi alliance coined another categories or profiles for this big guy to
differentiate its new product from the old one with a single stream WiFi
certified n. 11ac is in the same stage in my opinion. It does not enforce multi
antenna because they got some lessons from the deployment of 11n market
but 80MHz bandwidth is mandatory instead so it cannot be even launched in
2.4 GHz ISM band. However some big guys already started to use one of the
key features, 256QAM, with their own branding names such as turbo QAM
and something like that. But anyway still it is illegal, not allowed, in the syntax
of 802.11ac specification and here comes in what GCT has invented and are
ready to open to any future partners. It uses a single antenna like a single
stream 11n but it can provide a throughput equal to that of 256QAM of 11ac
still staying in the grammars of 11n.
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4. Now I'll tell you about this method in more detail.
The concept is really simple. Starting from any two antenna MIMO IP. we want
to remove the physical antenna as a final goal. To this purpose we attenuate
the amplitude of the second stream significantly from the equal power level.
Then simply combine it and transmit through only a single antenna. The
combination can be done in the time domain (here), but if it is done in the
frequency domain inside the IP before the IFFT block with a little modification,
then a whole block for the 2nd stream can go away.
With this simple technique, the second stream has been nested into the
minimum constellation distance of the first stream like this. Then, people will
be ask whether such a composite single stream signal can be decoded. The
answer is yes.
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5. Actually the constellation of 256 QAM and our proposed nested spatial
mapping of 2x2 16QAM is quite similar to each other. Both of them have 256
distinct constellation points in the I/Q eye diagrams.
However, the labelling of the point is different. BICM(Simply a gray mapping)
number the constellation point so that only a single bit change is
encountered with the neighbor constellation in any direction of up/down/left
and right. The proposed nested spatial mapping has this property only inside
each stream, respectively, but not between the streams. So, when the two
neighbors lie across the stream, then two bit change is encountered. This
cannot be avoided if we stick to the grammars of old MIMO standard spec.
The bit error rate becomes large if the neighbor distance gets small or the
noise level increase and the other ways. So, if we control the attenuation ratio
of the two stream then we can balance the bit error rate inside the stream
and between the streams.
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6. Now, it's time to check the optimal value of nesting factor for the link
performance. First we fix the SNR to some proper level and sweep only the
nesting factor. For 2-stream QSPK which is equivalent to 16QAM still in 11n ,
the optimal nesting point is around 6 dB. For 2-stream 16QAM, it is
equivalent to 256QAM of 11ac, the optimal nesting point is slightly below 12
dB. Actually both of them lie slightly above 6 dB and 12 dB because of the
non-optimal mapping compared with BICM-style mapping.
Here is the region where inter-stream error is dominant and here is the region
where intra-stream error is dominant over the inter-stream error. So, if we
extrapolate from each extreme case with two lines, we can find a cross point
of the two lines. Its cross point is nearby the optimal point. So, with this
condition we can estimate an optimal point for each SNR.
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7. Simply a simulation results will do all the things to persuade the technical
guys in charge, however some guys are asking for more theoretical ground
for this work and we did dome math to explain the optimal nesting margin of
around 1dB from the nominal nesting factor. We found from this formula that
the nesting margin is relatively robust or insensitive to the channel condition
for example Rx SNR.
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8. With this background knowledge, we now swept the nesting margin and SNR
then gathered the link throughput which shows a big plateau if proper SNR is
given.
You have to note that it has been obtained with 1 antenna 11n MIMO WLAN
IP. The peak throughput is higher than the maximum of single stream MCS
(the thick line over there.)
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9. Let's look at the plot in two directions.
For the optimal nesting factor. I have said that is almost constant, but any
way we have cut the optimal curve with a front view of the previous 3D curve.
The peak throughput is much higher than 64QAM by 20% from 72.2 Mbps to
86.6 Mbps.
We can see the robustness of this scheme and even the optimal nesting
factor with the top view. This axis is nesting factor, here 12dB. and we can
see that as the more large SNR is available then the range of the nesting
margin becomes wider.
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10. It’s time to look at a more practical issues including some implementation tips.
The simulation results that I have shown in the previous slide has been
obtained with full ML decoder. This nested spatial mapping cannot be
decoded by very old linear demapper method such as zero forcing or MMSE.
They are based on only linear matrix operation. Few years ago, ML decoder
for 64QAM was considered to be formidable. But, everything has changed.
First only 16QAM ML is enough for this scheme to work. ML decoder for
16QAM is much smaller than 64QAM by the factor of 25%. Even the size of
64QAM ML decoder has been reduced to an acceptable level thanks to a
smart work from the algorithm guys. So nothing to be changed if your
MIMO WLAN IP uses an ML decoder or SIC decoder for dual stream.
Although a MIMO IP is used, only a single pair of I/Q ADC and RF is required
for the received to work. The already made and used legacy 11n WLAN
devices can decode this nested MIMO signals with two receiver chain
configurations. For the second thing, the maximum throughput of the Viterbi
decoder will be the bottle neck for this scheme. Many WLAN IP works at 40
MHz or 80MHz, the multiples of the symbol rate 20Msps. So, the maximum
throughput of the Viterbi decoder, the channel decoder, can be limited up to
80Mbps. which is lower than 86 Mbps of this scheme. In this case, you can
get still some gain without any modification of the Viterbi decoder. Simply do
not use the short guard interval. With long guard interval only, the maximum
throughput is now limited to 78 Mbps, which is lower than 86 Mbps but still
higher than 72 Mbps by 8 % and 80 Mbps Viterbi decoder can handle it
without any overflow and without any design change.
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11. There should be some slight modification of the firmware. First we have to
declare that I can send nested MIMO. It can be declared by setting bit 96 and
bit 97 of HT supported MCS set field then setting the number of antennas
fields (in virtual meaning, physically one) to 2. You should not be confused
with the following field of Tx unequal modulation supported. It is for mixing
different modulation scheme in MIMO which is almost dead.
Then, now we have to tell whether we can receive the nested MIMO. Actually,
if we set the MCS from 8 through 12, then the conventional MIMO WLAN
station will try to send dual streams to us. If we have physical two antennas
we may decode it. But, we have only a single physical antenna and cannot
decoded although the IP itself is MIMO IP that you have designed perfectly.
So, we cannot set this field.
We will keep this field intact but will increase the supported data rate field to
the specific number above 72, the typical number of single stream 11n.
To legacy product it does not make any difference or harm. But, for the
same-kind-of devices like us, they can now tell that we may send the nested
spatial mapping streams to the device over there, ours.
If both parties recognize the feature of each side, then there is nothing to be
done more. Every other things will go as if it is the 11n MIMO IP.
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12. In technical side, we have found out that MIMO can work even with a single
antenna if the two stream are properly scaled to the point of nesting. Then,
no need to envy the 256QAM of 11ac. we can simply achieve the same
number of throughput by using 2x2 16QAM as exemplified in the typical case
of my presentation. Moreover, the algorithm is not so fragile only working
with a very sensitive parameters
in a very narrow region. It is generous and fool-proof to be very pragmatic
for challengers if any. Even I did it.
In marketing side, MIMO WLAN IP developed few years ago but it is not
selling well enough pushed by its successor 11ac and soon coming 11ax.
Actually their rival is single-stream 11n. But, now it can get another chance by
achieving the higher throughput in 2.4 GHz without any violation of the
standard. No need for expensive 80MHz band-width ADC only to pass the
certification for 256QAM.
Before time goes off, Let me repeat the hash keywords of my presentation.
#single antenna MIMO
#nested spatial mapping
#256QAM
#Optimization of nesting margin
#11n vs. 11ac
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13. And, this has been presented in IP-SOC2016 first and you can get a more
detailed information from the following links
if you have any interest in this topic.
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