vsmc mimo: a spectral efficient scheme for cooperative...

VSMC MIMO: A Spectral Efficient Scheme for

Cooperative Relay in Cognitive Radio Networks

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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, China4Dept. 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

2VSMC MIMO: A Spectral Efficient Scheme for Cooperative Relay in Cognitive Radio Networks

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 3

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)


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.

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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

Introduction

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Outline

Introduction



Resource Allocation

MAC Layer Design


Conclusion


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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


Fundamental Components

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Discontiguous OFDM



One channel

Sub-channel 1

Sub-channel 2

Sub-channel 3

Sub-channel 4

Traditional OFDM Discontiguous OFDM

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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



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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

Shift frequency by

Pass a low-pass filter

Shift back by

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.



1 2[ , ]f f

1 2

2

f f

1 2

2

f f

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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)



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Outline

Introduction



Resource Allocation

MAC Layer Design


Conclusion


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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.


Cooperative MIMO Relay

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Select R1 as a relay, adopt the VSMC MIMO scheme in implementing

cooperative MIMO relay.

Relay transmission process is divided into two time slots.


Cooperative MIMO Relay

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Outline

Introduction



Resource Allocation

MAC Layer Design


Conclusion


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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

Resource allocation problem is solved by three steps.

A. Network Model

B. Problem Formulation

C. Heuristic Solution

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A. Network Model


Resource Allocation

A graph denotes the network

represents the nodes

：AP node : SU nodes

represents direct links between nodes

: exits a link between node and node

: otherwise

: direct link is symmetrical

1N

denotes data demands that SU nodes need to

receive from AP.

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A. Network Model


Resource Allocation

Antenna matrix

: number of antennas equipped with

Frequency range is divided into channels with same

bandwidth (channel ) .

K

Channel matrix

: the channel is available at node

: unavailable

1,2,...,k

1 2[ , ]f f

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Resource Allocation

Channel assignment matrix :

: channel is assigned to link

: otherwise

The constraints of channel assignment:

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Resource Allocation

Data streaming matrix :

where

: number of data streams transmitted over on channel

The constraints of data stream:

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Resource Allocation

Relay matrix :

: node works as a relay for node

: otherwise

The constraints for relays:

1)Channel availability:

2)Half duplex:

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Resource Allocation

Data transmission rate matrix :

where

: data transmission rate of link

on channel

: constant transmission rate for each data stream

Data transmission rate for link :

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Resource Allocation

Throughput for SU nodes:

If node receives data from on channel directly,

the average throughput of link on channel is

If node acts as a relay , then

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Resource Allocation

1) direct-receiving node:

We divide nodes into three categories: direct-receiving nodes,

relay nodes and relay-assisted nodes, and calculate their throughputs.

2) Relay node:

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Resource Allocation

3) relay-assisted node:

We combine three categories together:

Our final problem:

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C. Heuristic Solution for Resource Allocation


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

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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.

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Resource Allocation

Max flow problem:

AP

k channel nodes N SU nodes

Sink node

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Resource Allocation

After solving the max-flow problem, we can get the direct

transmission throughput of each node

If , add to potential relay set

If , add to potential relay-assisted set

In , we first calculate relay potential

Where , if , is removed from

In , we calculate data gap

Where

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Resource Allocation

Another Max flow problem:

Source node

Nodes in set

Channel nodes

Satisfying:

Sink node

Nodes in set

1p

2p

mp

1g

2g

ng

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Outline

Introduction



Resource Allocation

MAC Layer Design


Conclusion


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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.


MAC Layer Design

The whole network is in global

synchronization The transmission process is based

on frames

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Outline

Introduction



Resource Allocation

MAC Layer Design


Conclusion


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We implement cooperative MIMO relays system based on the existing

codes of MIMO technology introduced in [17] on USRP—LABVIEW

platform.


Testbed Description

NI USRP 2920 series

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Transmitting process


Testbed Description

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Receiving process


Testbed Description

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Experiment parameters


Experiments

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

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Experiments of two end users


Experiments

Setup 1:

Traffic demand for D : 25 Kbps

Traffic demand for R : 10 Kbps

Setup 2:

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Experimental results of two end users


Experiments

Set up 1:

Theoretical throughput gain: 50%

Average throughput gain: 34.74%

Set up 2:



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Experiments of three end users


Experiments

Traffic demand for D1: 15 Kbps

Traffic demand for D2: 15 Kbps

Traffic demand for R: 0 Kbps

Experimental results:



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Evaluation of Experimental results


Experiments

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

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Outline

Introduction



Resource Allocation

MAC Layer Design


Conclusion



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.

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Thank you!


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