VSMC MIMO: A Spectral Efficient Scheme for
Cooperative Relay in Cognitive Radio Networks
Chao Kong1, Zengwen Yuan1, Xushen Han1, Feng Yang1,
Xinbing Wang1,2, Tao Wang3, Songwu Lu4
1 School
of Electronic, Info. & Electrical Engineering, Shanghai Jiao Tong University
2National Mobile Communications Research Laboratory, Southeast University
3EECS School, Peking University, China
4Dept. of Computer Science, University of California, Los Angeles
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Outline
 Introduction
 Fundamental Components of VSMC MIMO scheme
 VSMC MIMO Scheme in Cooperative MIMO Relay
 Resource Allocation
 MAC Layer Design
 Testbed Description and Experimental Results
 Conclusion
VSMC MIMO: A Spectral Efficient Scheme for Cooperative Relay in Cognitive Radio Networks
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Introduction
 SISO (single input single output)
 MIMO (Multiple-Input Multiple-Output)
 spatial multiplexing gain
 diversity gain
VSMC MIMO: A Spectral Efficient Scheme for Cooperative Relay in Cognitive Radio Networks
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Introduction
 Traditional MIMO schemes are designed mainly for the scenario
of contiguous spectrum ranges.
 In cognitive radio networks, the available spectrum is discontiguous,
making traditional MIMO schemes inefficient for spectrum usage.
 We proposed a new scheme: VSMC MIMO (Variable Numbers of
Streams on Multiple Channels of MIMO system)
VSMC MIMO: A Spectral Efficient Scheme for Cooperative Relay in Cognitive Radio Networks
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Introduction
 Simple example of VSMC MIMO
The arrow lines indicate the numbers of data streams between transmitters and receivers,
and do not mean the actual signals at the antennas. All the figures in this paper adopt this
approach for simpler illustration.
VSMC MIMO: A Spectral Efficient Scheme for Cooperative Relay in Cognitive Radio Networks
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Introduction
Contributions:
 Designed a feasible and efficient scheme for MIMO nodes to transmit
variable numbers of streams on multiple channels.
 Solved resource allocation problem to implement cooperative MIMO
relay in cognitive radio networks.
 Built a system of cooperative MIMO relay that involved VSMC
MIMO scheme based on USRP platform.
VSMC MIMO: A Spectral Efficient Scheme for Cooperative Relay in Cognitive Radio Networks
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Outline
 Introduction
 Fundamental Components of VSMC MIMO scheme
 VSMC MIMO Scheme in Cooperative MIMO Relay
 Resource Allocation
 MAC Layer Design
 Testbed Description and Experimental Results
 Conclusion
VSMC MIMO: A Spectral Efficient Scheme for Cooperative Relay in Cognitive Radio Networks
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Fundamental Components
To implement VSMC MIMO scheme, we have two
considerations.
 One radio should transmit or receive signals on multiple channels
----- D-OFDM scheme (Discontiguous Orthogonal Frequency
Division Multiplexing)
 Interferences of several concurrent transmissions on the same
channels should be eliminated.
----- view transmitters and receivers as a “virtual MIMO” system,
thus adopting existing pre-coding methods
VSMC MIMO: A Spectral Efficient Scheme for Cooperative Relay in Cognitive Radio Networks
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Fundamental Components
 Discontiguous OFDM
Sub-channel 1
One channel
Sub-channel 2
Sub-channel 3
Sub-channel 4
Traditional OFDM
Discontiguous OFDM
VSMC MIMO: A Spectral Efficient Scheme for Cooperative Relay in Cognitive Radio Networks
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Fundamental Components
 Combined D-OFDM scheme with MIMO techniques to
implement VSMC MIMO scheme
 Four basic components in VSMC MIMO scheme




Baseband Signal Processing
Preambles on Part of Subcarriers
Coding Methods in VSMC MIMO scheme
Channel States Information
VSMC MIMO: A Spectral Efficient Scheme for Cooperative Relay in Cognitive Radio Networks
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Fundamental Components
 Baseband Signal Processing
 A low-pass filter is used to get the desired signal on certain channels in baseband
while eliminate signal on other channels.
 Frequency range of one desired channel is [ f1 , f 2 ]
f  f
 Shift frequency by 2
 Pass a low-pass filter
 Shift back by  f 2 f
1
1
2
2
 Preambles on Part of Subcarriers
 the preambles for time synchronization and training sequences for channel estimation
are added individually on the corresponding subcarriers.
 The preambles are used to detect the beginning of each packet and calculate the
carrier frequency offset.
VSMC MIMO: A Spectral Efficient Scheme for Cooperative Relay in Cognitive Radio Networks
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Fundamental Components
Coding Methods in VSMC MIMO scheme


adopt V-BLAST pre-coding method
adopt Zero-Forcing(ZF) decoding algorithm
Channel States Information (CSI)

adopt Linear Least Square Estimate (LLSE)
VSMC MIMO: A Spectral Efficient Scheme for Cooperative Relay in Cognitive Radio Networks
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Outline
 Introduction
 Fundamental Components of VSMC MIMO scheme
 VSMC MIMO Scheme in Cooperative MIMO Relay
 Resource Allocation
 MAC Layer Design
 Testbed Description and Experimental Results
 Conclusion
VSMC MIMO: A Spectral Efficient Scheme for Cooperative Relay in Cognitive Radio Networks
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Cooperative MIMO Relay
 An example of implementing cooperative MIMO relay in cognitive
radio networks as shown in Fig. 2.
 Data rate of each stream on one channel is 10 Kbps.
 MIMO nodes try to obtain their spatial multiplexing gain to achieve
larger throughput.
VSMC MIMO: A Spectral Efficient Scheme for Cooperative Relay in Cognitive Radio Networks
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Cooperative MIMO Relay
 Select R1 as a relay, adopt the VSMC MIMO scheme in implementing
cooperative MIMO relay.
 Relay transmission process is divided into two time slots.
VSMC MIMO: A Spectral Efficient Scheme for Cooperative Relay in Cognitive Radio Networks
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Outline
 Introduction
 Fundamental Components of VSMC MIMO scheme
 VSMC MIMO Scheme in Cooperative MIMO Relay
 Resource Allocation
 MAC Layer Design
 Testbed Description and Experimental Results
 Conclusion
VSMC MIMO: A Spectral Efficient Scheme for Cooperative Relay in Cognitive Radio Networks
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Resource Allocation




We move on to the network perspective
There are N Secondary User(SU)s, served by one AP in a certain area.
We only consider downlink transmission
We have to allocate spectrum and select relay nodes
Resource allocation problem is solved by three steps.
A. Network Model
B. Problem Formulation
C. Heuristic Solution
VSMC MIMO: A Spectral Efficient Scheme for Cooperative Relay in Cognitive Radio Networks
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Resource Allocation
 A. Network Model
A graph
denotes the network
represents the
：AP node
N  1 nodes
: SU nodes
represents direct links between nodes
: exits a link between node
and node
: otherwise
: direct link is symmetrical
denotes data demands that SU nodes need to
receive from AP.
VSMC MIMO: A Spectral Efficient Scheme for Cooperative Relay in Cognitive Radio Networks
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Resource Allocation
 A. Network Model
Antenna matrix
: number of antennas equipped with
Frequency range [ f1 , f 2 ] is divided into K channels with same
bandwidth (channel 1, 2,..., k ) .
Channel matrix
: the channel
: unavailable
is available at node
VSMC MIMO: A Spectral Efficient Scheme for Cooperative Relay in Cognitive Radio Networks
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Resource Allocation
 B. Problem Formulation
Channel assignment matrix :
: channel
is assigned to link
: otherwise
The constraints of channel assignment:
VSMC MIMO: A Spectral Efficient Scheme for Cooperative Relay in Cognitive Radio Networks
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Resource Allocation
 B. Problem Formulation
Data streaming matrix :
where
: number of data streams transmitted over
on channel
The constraints of data stream:
VSMC MIMO: A Spectral Efficient Scheme for Cooperative Relay in Cognitive Radio Networks
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Resource Allocation
 B. Problem Formulation
Relay matrix :
: node
works as a relay for node
: otherwise
The constraints for relays:
1)Channel availability:
2)Half duplex:
VSMC MIMO: A Spectral Efficient Scheme for Cooperative Relay in Cognitive Radio Networks
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Resource Allocation
 B. Problem Formulation
Data transmission rate matrix :
where
: data transmission rate of link
on channel
: constant transmission rate for each data stream
Data transmission rate for link
:
VSMC MIMO: A Spectral Efficient Scheme for Cooperative Relay in Cognitive Radio Networks
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Resource Allocation
 B. Problem Formulation
Throughput for SU nodes:
If node
receives data from
the average throughput of link
If node
on channel
on channel
directly,
is
acts as a relay , then
VSMC MIMO: A Spectral Efficient Scheme for Cooperative Relay in Cognitive Radio Networks
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Resource Allocation
 B. Problem Formulation
We divide nodes into three categories: direct-receiving nodes,
relay nodes and relay-assisted nodes, and calculate their throughputs.
1) direct-receiving node:
2) Relay node:
VSMC MIMO: A Spectral Efficient Scheme for Cooperative Relay in Cognitive Radio Networks
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Resource Allocation
 B. Problem Formulation
3) relay-assisted node:
We combine three categories together:
Our final problem:
VSMC MIMO: A Spectral Efficient Scheme for Cooperative Relay in Cognitive Radio Networks
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Resource Allocation
 C. Heuristic Solution for Resource Allocation
The optimization problem (15) is complex and difficult to obtain
the close-form solution, so we propose a heuristic solution.
1) Find potential relay nodes and relay-assisted nodes
in direct transmission
2) Select relays and allocate spectrum resource
VSMC MIMO: A Spectral Efficient Scheme for Cooperative Relay in Cognitive Radio Networks
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Resource Allocation
 C. Heuristic Solution for Resource Allocation
Direct transmission:
The optimization problem (15) can be simplified to
We can transform problem (16) to a max flow problem, thus we can
solve it by various kinds of methods, such as Hungarian Algorithm.
VSMC MIMO: A Spectral Efficient Scheme for Cooperative Relay in Cognitive Radio Networks
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Resource Allocation
 C. Heuristic Solution for Resource Allocation
Max flow problem:
AP
Sink node
k channel nodes
N SU nodes
VSMC MIMO: A Spectral Efficient Scheme for Cooperative Relay in Cognitive Radio Networks
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Resource Allocation
 C. Heuristic Solution for Resource Allocation
After solving the max-flow problem, we can get the direct
transmission throughput of each node
 If
 If
, add
, add
to potential relay set
to potential relay-assisted set
In
, we first calculate relay potential
Where
, if
,
is removed from
In , we calculate data gap
Where
VSMC MIMO: A Spectral Efficient Scheme for Cooperative Relay in Cognitive Radio Networks
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Resource Allocation
 C. Heuristic Solution for Resource Allocation
Another Max flow problem:
g1
p1
g2
p2
pm
Sink node
Source node
gn
Nodes in set
Channel nodes
Satisfying:
Nodes in set
VSMC MIMO: A Spectral Efficient Scheme for Cooperative Relay in Cognitive Radio Networks
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Outline
 Introduction
 Fundamental Components of VSMC MIMO scheme
 VSMC MIMO Scheme in Cooperative MIMO Relay
 Resource Allocation
 MAC Layer Design
 Testbed Description and Experimental Results
 Conclusion
VSMC MIMO: A Spectral Efficient Scheme for Cooperative Relay in Cognitive Radio Networks
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MAC Layer Design
Using VSMC MIMO scheme to implement cooperative MIMO relay in
cognitive radio networks needs good organization and coordination of all
the nodes in the network. To make cooperative MIMO relay scheme
workable and reliable, we propose a MAC layer design.


The whole network is in global
synchronization
The transmission process is based
on frames
VSMC MIMO: A Spectral Efficient Scheme for Cooperative Relay in Cognitive Radio Networks
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Outline
 Introduction
 Fundamental Components of VSMC MIMO scheme
 VSMC MIMO Scheme in Cooperative MIMO Relay
 Resource Allocation
 MAC Layer Design
 Testbed Description and Experimental Results
 Conclusion
VSMC MIMO: A Spectral Efficient Scheme for Cooperative Relay in Cognitive Radio Networks
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Testbed Description
We implement cooperative MIMO relays system based on the existing
codes of MIMO technology introduced in [17] on USRP—LABVIEW
platform.
NI USRP 2920 series
VSMC MIMO: A Spectral Efficient Scheme for Cooperative Relay in Cognitive Radio Networks
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Testbed Description
Transmitting process
VSMC MIMO: A Spectral Efficient Scheme for Cooperative Relay in Cognitive Radio Networks
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Testbed Description
Receiving process
VSMC MIMO: A Spectral Efficient Scheme for Cooperative Relay in Cognitive Radio Networks
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Experiments
Experiment parameters
I/Q rate: 400 K/second
Carrier frequency: 915 MHz
Pulse shaping samples per symbol: 8
Channel: composed of 20 subcarriers (2 pilot subcarriers)
Guard subcarriers: 12
Each data stream supports about 10 Kbps
Packet: 2880 bits
Each run transfers 20 packets in downlink
Experiment times: 100
VSMC MIMO: A Spectral Efficient Scheme for Cooperative Relay in Cognitive Radio Networks
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Experiments
Experiments of two end users
Traffic demand for D : 25 Kbps
Traffic demand for R : 10 Kbps
Setup 1:
Setup 2:
VSMC MIMO: A Spectral Efficient Scheme for Cooperative Relay in Cognitive Radio Networks
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Experiments
Experimental results of two end users
Set up 2:
Set up 1:
Theoretical throughput gain: 50% Theoretical throughput gain: 75%
Average throughput gain: 34.74% Average throughput gain: 55.02%
VSMC MIMO: A Spectral Efficient Scheme for Cooperative Relay in Cognitive Radio Networks
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Experiments
Experiments of three end users
Traffic demand for D1: 15 Kbps
Traffic demand for D2: 15 Kbps
Traffic demand for R: 0 Kbps
Experimental results:
Theoretical throughput gain: 50%
Average throughput gain: 34.11%
VSMC MIMO: A Spectral Efficient Scheme for Cooperative Relay in Cognitive Radio Networks
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Experiments
Evaluation of Experimental results
The gap between the theoretical analysis and experimental results are
caused mainly by these reasons:
Relay nodes need time to switch from receiving mode to transmitting
mode
Relay nodes have to spend time on decoding and recoding message
Signal processing rate of USRP—LabVIEW platform is slow and
variable
VSMC MIMO: A Spectral Efficient Scheme for Cooperative Relay in Cognitive Radio Networks
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Outline
 Introduction
 Fundamental Components of VSMC MIMO scheme
 VSMC MIMO Scheme in Cooperative MIMO Relay
 Resource Allocation
 MAC Layer Design
 Testbed Description and Experimental Results
 Conclusion
VSMC MIMO: A Spectral Efficient Scheme for Cooperative Relay in Cognitive Radio Networks
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Conclusions
 We proposed VSMC MIMO scheme.
 We solved the resource allocation problem in implementing
cooperative MIMO relay in cognitive radio networks and designed a
MAC protocol.
 We built a cooperative MIMO relay system based on USRP.
 In the future, we may enlarge the scale of the testbed to evaluate the
performance of VSMC MIMO in large-scale MIMO relays system.
VSMC MIMO: A Spectral Efficient Scheme for Cooperative Relay in Cognitive Radio Networks
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Thank you!
VSMC MIMO: A Spectral Efficient Scheme for Cooperative Relay in Cognitive Radio Networks
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