AirExpress: Enabling Seamless In-band Wireless
Multi-hop Transmission
Joint work with Bo Chen, Ouyang Zhang, and Kannan Srinivasan
Yue Qiao
Computer Science and Engineering
Email: qiaoyu@cse.ohio-state.edu
Sep. 10 2015
1
Wired Ethernet
Source
Router B
Router A
Client
Router C
Cut-through Routing
2
Problem
Increased Throughput
Decreased Latency
Source
Router B
Router A
Destination
Router C
3
Wireless In-band Cut-through
Routing?
AirExpress
4
Challenges

Various types of interference
Source
Full-Duplex
Cross-Hop Interference
Self Interference
Router B
Router A
Forwarder
Interference
AirExpress: Estimate Channel
Only Once
at One
Measure
Inefficient
Each Interfereing
and Impractical
Channel?
Node
Router C
Destination
5
Forwarder Interference
Received Signal
x(t )
Self Interference
hs x(t )
Forwarder Interference
A
B
C
D
hf x(t )
FI
SI
time
6
Forwarder Interference
Received Signal
x(t )
Self Interference
hs x (t )
Forwarder Interference
A
B
C
hs
D
hf x(t )
FI
SI
time
7
Forwarder Interference
Received Signal
x(t )
Self Interference
hs x (t )
Forwarder Interference
A
B
C
D
hs  h f
hf x(t )
FI
SI Superposed Channel?
Measure the
time
8
Forwarding Channel
Forwarding Channel
hCB
Node C  C
ADC
Node B  B
ADC
DAC
hBC
DAC
hBC e j (B C )t x(t )
x(t )
hCB hBC e j (B C )t e j (C B ) t x(t )
 hCB hBC x(t )
No
Measure
frequency
the Superposed
offset for theChannel
forwarding
is Possible!
channel!
9
Our Solution

Self Interference

Forwarder Interference
Full-Duplex Technique
Superposed Channel

Cross-Hop Interference
10
Cross-Hop Interference
Challenge: Various cross-hop channels
A
B
C
D
E
(1)
A
B
C
D
E
Causal
(2)
A
B
C
D
E
(3)
A
B
C
D
Non-causal
E
11
Unified Cancellation Module
Design Philosophy
Causal: cancel out (SI, FI and CHI)
Non-causal: multipath components
Method: Hierarchical Cancellation Structure
A
B
Training
Receiving
Forwarding
C
D
E
Transmitting
Inactive
12
Unified Cancellation Module
Design Philosophy
Causal: cancel out (SI, FI and CHI)
Non-causal: multipath components
Method: Hierarchical Cancellation Structure
A
B
Training
Receiving
Forwarding
C
D
E
Transmitting
Inactive
13
Unified Cancellation Module
Design Philosophy
Causal: cancel out (SI, FI and CHI)
Non-causal: multipath components
Method: Hierarchical Cancellation Structure
A
B
Training
Receiving
Forwarding
C
D
E
Transmitting
Inactive
14
Unified Cancellation Module
Design Philosophy
Causal: cancel out (SI, FI and CHI)
Non-causal: multipath components
Method: Hierarchical Cancellation Structure
A
B
Training
Receiving
Forwarding
C
D
E
Transmitting
Inactive
15
Have
Simultaneous
Successfully
Transmission
Dealt with
Interference
Possible
16
Drawback
Noise accumulation
10010010010001
Error propagation
11110100001111
Packet Discarded due to high Error rate
17
Virtual-Hop Design
Long paths are divided into smaller Virtual-Hops
Hops 1
Virtual-Hop 1
Virtual-Hop 2
Other considerations:
bottleneck link, latency....
18
MAC and Routing

Matching MAC and Routing Algorithms are
designed for the AirExpress.

Radios in the network can support AirExpress
system through distributed coordination. It also
includes the Hierarchical Cancellation Structure
realization and multi-hop link build and release

Details in paper
19
Implementation

NI PXIe 1082

2.4GHz center frequency and 20MHz bandwidth

Unified cancellation block
20
Implementation

Indoor: 20 locations, single flow
21
Throughput Gain over TDMA
1.71
2.39
2.75
Hierachical cancellation structure works nicely!
Throughput gain increases as the number of
hops increases
22
Large Network Evaluation

NS3, 100 nodes in a 1000m X 1000m area, multi
flows

TDMA vs AirExpress

Virtual hop size: 2~5

The cancellation residue interference information is
based on the real testbed measurements
23
Network Performance
4-hop AirExpress
outperforms
TDMA
Throughput
performance
of AirExpress
andby
TDMA
~2X on average
24
Network Performance
TDMA has a 3X delay over 4-hop
Delay performance of AirExpress andTDMA
AirExpress
25
Network Performance
>4 didgain
not further
increase
thedifferent
Throughput
of AirExpress
with
performance virtual-hop
due to noise
accumulation
size
26
Conclusion and Looking forward
 AirExpress: First fully functional in-band wireless
cut-through system
 The fundamental description of transmission
pattern in wireless ad hoc network is challenged.
 A building block of future multi-hop network
architecture
30
Q&A
Thanks
31