Transcript Slide 1

CSI-SF: Estimating Wireless Channel State
Using CSI Sampling & Fusion
Riccardo Crepaldi∗, Jeongkeun Lee†, Raul Etkin†, Sung-Ju Lee†, Robin Kravets
∗University of Illinois at Urbana-Champaign
†Hewlett-Packard Laboratories
How do systems describe the channel quality?
 PHY Layer enhancements:
MIMO technology
OFDM
Channel Bonding
AGC
RF
AGC
Rate adaptation
Channel selection
Optimal antenna scheme selection
Best AP selection
What MIMO
Spatial
Mapping
(Q)
Channel
(H)
The
Channel
quality is…
Pros
Finer granularity compared to SNR
Fading and phase shift information for each subcarrier
Tighter bound with channel performance [Halperin’10]
Cons
CSI is significantly larger than SNR
CSI Describes a specific MIMO configuration
CSI for multiple MIMO configurations must be measured for a
complete channel characterization
Y(t)
RX
A real networking problem: Rate Adaptation

RF

Y(t) = H ´Q ´ X(t)+Z(t)


Ex: Estimate of a 2x3 configuration using two 1x3
received packets
Rate [Mbps]
2
Packet 2
(Tx: B, Rx: ABC)
3
CSI2
Q2
Q1†
3
200
150
100
50
0
5
200
150
100
50
0
5
200
150
100
50
0
5
Small or no gaps
Monotonic behavior
2x3 – 20MHz
Worst link
Best link
7
9
Large gaps
11
13
15
17
19
21
23
25
27
Non-monotonic beavior
29
2x3 – 40MHz
7
9
11
13
15
17
19
21
23
25
27
29
3x3 – 20MHz
7
56
9
11
13
15
17 19
SNR [dB]
21
23
25
27
29
150
0
Power scale
factor
Rate[Mbps]
Q1†
(effective SNR based on CSI-SF estimates)50
Estimated
CSI
Example of CSI-SF combining two 1x3 packets in a 2x3
packets
Packet 1: 1x3 – Ant A
20
2
3
4
5
6
7
8
9 10 11 12 13 14 15 16 17 18 19 20
e2SNR [dB]


SNR is not a good metric, eSNR and e2SNR are better
Compared to standard eSNR, e2SNR uses estimates
instead of real samples thus limiting the overhead


Hypoyetical channel variation
Overhead required to rate control to converge using different
metrics
Packet 1: 1x3 – Ant B
tx: B
rx: A
tx: A
rx: C
tx: A
rx: B
e2SNR − Worst
e2SNR − Best
100
0
1
tx: A
rx: A
eSNR [dB]
e2SNR
1

eSNR − Worst
eSNR − Best
eSNR
100
(effective SNR based on real samples)50
56
1
Metrics: SNR, effective SNR, estimated effective SNR
What’s a good metric?
CSI
56
M x N x W complex matrix
W= Number of OFDM subcarriers
M=3, N=3
W=56 (20MHz) or W=114 (40MHz)

Symbol
Mapping
3
Channel State Information (CSI):
RF
X(t)
CSI1
Chan Estimation
Current 802.11n devices:
RX: N receivers
Symbol Mapping
Packet 1
(Tx: A, Rx: ABC)
FFT
One scalar value per packet
Accurate only for narrow-band channel
CSI-SF enables estimation of uninvestigated channel
Z(t)
configurations

[M spatial streams]
Chan Estimation
Signal-to-Noise Ratio (SNR):
TX: M parallel data streams
Channel quality
Encoding
AGC
RF
 Choices based on:
Bitstream
FFT
Demap
configuration
should I choose?

Demap
MIMO Decoding
 Adaptive protocols for:

Chan Estimation
[N Input streams]
RF




FFT
Rate[Mbps]



tx: B
rx: B
tx: B
rx: C
SNR [db]
15
10
5
0
Packet 3: 2x3 – Ant AB
20
Real CSI
SNR [db]
15
10
Estimated CSI
5
0
−5
10
20
30
10
20
30
10
20
30
10
OFDM subcarrier index
20
30
10
20
30
10
20
30