increasing cellular capacity using cooperative networks

8/2/2019 Increasing Cellular Capacity Using Cooperative Networks

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WWIRELESSIRELESSIINTERNETNTERNETCCENTERENTERFORFORAADVANCEDDVANCEDTTECHNOLOGYECHNOLOGY

NNSFSF IINDUSTRY/NDUSTRY/UUNIVERSITYNIVERSITY CCOOPERATIVEOOPERATIVE RRESEARCHESEARCH CCENTERENTER

Increasing cellular capacityIncreasing cellular capacity

using cooperative networksusing cooperative networksShivendra S. Panwar

Joint work with Elza Erkip, Pei Liu, Sundeep Rangan, Yao Wang

Polytechnic Institute of NYU


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OutlineOutline

Motivation for Cooperation Robust Cooperative MIMO Design

Randomized Space Time Coding Randomized Spatial Multiplexing

Cooperation in Heterogeneous Network Cooperative Handover

Cooperative Interference Coordination

Combating Macrocell Backhaul Bandwidth Shortage

Implementation Efforts Conclusions


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OutlineOutline








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Cellular Networks are becoming heterogeneousCellular Networks are becoming heterogeneous

Macrocell based network architecture isexpensive and cannot keep up with user demand(Ciscos 66X traffic increase prediction)

Heterogeneous networks enable

flexible and low-cost deployments andprovide a uniform broadband experience The network becomes a mix of macro, pico, femto base

stations and operator deployed relay stations

The dense deployment greatly improves network capacity,

and provides richer user experience and in-buildingcoverage

Reduces operating cost, such as backbone cost, siteacquisition cost, and utility cost for operators


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Emerging trends and our researchEmerging trends and our research

The future network architecture is heterogeneous, with macro-,pico- and femto-cells, along with WiFi and (some) ad hoc nodes

A large part of the 66x increase predicted by Cisco will bedrained by increased deployment of WiFi, femto/picocells forstationary or slow moving users

Femtocells, in particular, are the carriers Trojan Horses!

Macrocell bandwidth is precious and should be used only whenthere is no alternative (like satellite networks are today)

Cooperative networking can be used in such emergingenvironments by using user end devices, femtocells, WiFiaccess points, picocells, and macrocell infrastructure as thedevices that constitute the cooperating nodes


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Cooperation and HeterogeneityCooperation and Heterogeneity

Cooperation performs much better if the number ofrelays is large In a macrocell based deployment, the number of operator

deployed relay stations will be limited

In traditional networks, the performance gain for cooperation

is limited unless user (MS) cooperation is enabled But user cooperation gives rise to the following problems:

battery consumption, synchronization, security and incentive

The proliferation of pico/femto base stations will

provide enough relays (femtorelays) They do not have the battery consumption problem They are easier to synchronize:

stationary, backbone connection and better radio design

They are more secure because they are part of the

operators network


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Motivation for CooperationMotivation for Cooperation

Wireless channel by nature is a broadcast one. The broadcast channel can be fully exploited for broadcast traffic.

But it is considered more as a foe than a friend, when it comes tounicast.

Cooperative communications allow the overheard information

be treated as useful signal, instead of interference. Relays process this overheard information and forward to

destination.

Network performance improved because edge nodes transmit athigher rate thus improving spectral efficiency.

Candidate relays?Mobile user, macro/pico-cell BS, fixed relays, femtocell BS, etc.

What are the incentives? Throughput, power, interference.

A cross-layer design encompassing physical, MAC, network andapplication layers is required to address this problem.


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Relaying in commercial systemsRelaying in commercial systems

Cooperative / multihop communications have beenadopted in the next generation wireless systems.

IEEE 802.11sEnables multihop and relays at MAC layer, does not providefor joint PHY-layer combining.

IEEE 802.16jExpands previous single-hop 802.16 standards to includemultihop capability. Integrated into IEEE 802.16m draft.

3GPP LTECooperative multipoint is supported with joint transmissionsand receptions to enable cost-effective throughputenhancement and coverage extension.


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OutlineOutline








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Robust Cooperative MIMO DesignRobust Cooperative MIMO Design

Limitations of previous cooperative methods: Single relay: low spatial diversity gain

Multiple relays: consume more bandwidth resource when severalrelays sequentially forward signal

Any alternative?

Distributed Space-Time Coding (DSTC) How does DSTC work?

Recruit multiple relays to form a virtual MIMO

Each relayemulates an indexed antenna

Each relaytransmits encoded signal corresponding to its antenna index

Pros: Spatial diversity gains

Cons: Tight synchronization required

Relays need to be indexed, leading to considerable signaling cost

Global channel state information needed

Good DSTC might not exist for an arbitrary number of relays

Unselected relays cannot forward, sacrificing diversity gain


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Robust Cooperative MIMORobust Cooperative MIMO

Randomized cooperation strategies provide powerful PHY layercoding techniques that alleviate the previous problems and allow robust and realistic

cooperative transmission with multiple relays.

randomize distributed space-time coding (R-DSTC) for diversity.

randomized distributed spatial multiplexing (R-DSM) for spatialmultiplexing.

Highlights of randomized cooperation: Relays are not chosen a-priori to mimic particular antennas

Multiple relays can be recruited on-the-fly

Relays are used opportunistically according to instantaneousfading levels

Signaling overheads and channel feedback greatly reduced

Performance comparable to centralized MIMO is attained


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R-DSTC: A New SolutionR-DSTC: A New Solution

Randomized Distributed Space-Time Coding (R-DSTC) How does R-DSTC work in PHY?

Two-hop network: source station, relays, destination station.

Relays re-encode the first-hop signals and forward over the second hop

Unlike DSTC, R-DSTC relay does NOT transmit the signal from a specific indexedantenna

Instead, each relay transmits a weighted linear combination of all streams of anunderlying STC codeword of size L x K.

As long as the number of relays N>L-1, a diversity order ofL is achieved.


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R-DSTC AdvantagesR-DSTC Advantages

DSTC R-DSTC

Only selected relays forward.Low diversity gain.

All relays that overhear first hop signal canrelay.High diversity gain.

Global and latest channel information

REQUIRED for rate selection.

Detailed channel information NOT

REQUIRED; outdated estimates can beused.

STC codeword allocation REQUIRED. STC codeword allocation NOTREQUIRED; transmissions can simply berandomized.

Tight synchronization among relays

REQUIRED.

Tight synchronization among relays NOT

REQUIRED.

Received power unbalanced. Average received power from all relaysbalanced.

Performance degrades whenever anyselected relay fails to relay.

Full diversity order of L is reached whenN>L .

Performance Comparison


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Underlying orthogonal STBC codeword size: 2, 3, 4.

PHY layer rates: 6, 9, 12, 18, 24, 36, 48, 54 BPSK, QPSK, 16-QAM, 64-QAM; Convolutional code 1/2, 2/3, 3/4 20 MHz bandwidth Contention window: 15 -1023 Transmit power: 100mW

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Number of Subscriber Stations

Throughput(Mbps)

Single-hop

Two-hop Single-helper (CoopMAC)

Two-hop R-DSTC Channel Statistics

Two-hop R-DSTC User Count

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Number of Subscriber Stations

Delay(seconds)

Single-hop

Two-hop Single-helper (CoopMAC)Two-hop R-DSTC Channel Statistics

Two-hop R-DSTC User Count

R-DSTC Performance (WiFi)R-DSTC Performance (WiFi)


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CoopMAX: A Cooperative Relaying Protocol inCoopMAX: A Cooperative Relaying Protocol in

Mobile WiMAX NetworkMobile WiMAX Network

CoopMAX enables robust cooperation in a mobile environmentwith low signaling overheads.

It is robust to mobility and imperfect knowledge of channel state.

Simulation shows 1.8x throughput gain for a single cell withmobility, and 2x throughput gain for multicell deployment.

Single cell

deployment

Multicell deployment

W I C A T


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R-DSM for spatial multiplexingR-DSM for spatial multiplexing

Mismatch in the number of antennas on BS and MS Assuming each mobile station has only one antenna and the base station

has L antennas

Randomized Distributed Spatial Multiplexing (R-DSM) is basedBLAST scheme

The channel capacity between the relays and the destinationsscales linearly with min(N,L), where N is the number of relays

How does R-DSM work in PHY? Two-hop network: SISO transmission from source to relays first, followed by

relays transmitting together to the destination using R-DSM. Each relay independently generates a random coefficient and then

transmits a weighted sum of the signals for each antenna in BLAST scheme

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PerformancePerformance

Our results demonstrate that R-DSM scheme delivers MIMOsystem performance Average data rate for the second hop (relays-destination link) scales with

the number of relays

For direct transmissions, the peak data rate is supported at a short range

R-DSM can increase the number of stations that can transmit near the peak

data rate

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Cooperative Video MulticastCooperative Video Multicast

Performance of conventional video multicastschemes in an access network is limited

Source transmits atthe lowesttransmission rate

Receivers with

good channel qualityunnecessarily suffer

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Cooperative Video Multicast with R-DSTCCooperative Video Multicast with R-DSTC

Source station transmitsa packet

Nodes who receivethe packets become relayswhich re-encodethe first-hop signals andforward over the second hop

Each relay transmits a

weighted linear combinationof all streams ofan underlying STCwith a dimension of

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Results: Single Layer SchemesResults: Single Layer Schemes

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OutlineOutline

Motivation for Cooperation

Robust Cooperative MIMO Design Randomized Space Time Coding

Randomized Spatial Multiplexing




Implementation Efforts

Conclusions



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Cooperative MIMO for Heterogeneous NetworksCooperative MIMO for Heterogeneous Networks

For high mobility MSs or MSs that are covered by any femtocell,cooperative MIMO enables fully opportunisticuse of all available surrounding radios.

increases network capacity and helps to reduce coverage holes.



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Cooperative handoff for Pico/FemtocellsCooperative handoff for Pico/Femtocells

Handoffs happen much more frequentlyfor MSs in a heterogeneous network Smaller BS coverage area

Loosely planned or unplanned deployment

Higher signaling overheads and more dropped calls Cooperative handoffs in Heterogeneous Networks

Separate signaling and data paths Macrocell BS orchestrates handoff and allocates radio

resources for data transmissions

User data goes through surroundingpico/femtocell BSs either through their backhaul or bycooperative relaying



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Cooperative Handoff for delay tolerantCooperative Handoff for delay tolerant

applicationsapplications

Macrocell BS tracks the locations of the MS and makes handoffpredictions based on which pico/femtocell BSs the MS ismoving to.

In the downlink

Macrocell BS pre-fetches user data packets to a

cluster of pico/femtocell BSs via their backhauls Macrocell BS allocates frequency/time slots for the

downlink data transmission

Pico/femtocell BSs cooperatively transmit to the MSusing R-DSTC

In the uplink

Macrocell BS broadcasts the allocated frequency/time slotsfor the MSs

A pico/femtocell BS that successfully decodes an uplink user

packet forwards it to the Macrocell BS via its backhaul



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Cooperative Interference CoordinationCooperative Interference Coordination

Pico/femtocell BS deployments are unplanned with vastlydifferent power levels compared to macrocell BSdeployments

The interference patterns are significantly different

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Crossover gain (dB)

Ra

te(bps/Hz)

Reuse 1

Orthog

HK

Han-Kobayashi

Orthogonalization

Treat interferenceas noise

Current cellular systemstreat interference as noise,which is not effective for highinterference levels

Dynamic orthogonalization orHan-Kobayashi is needed



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Very bad links(restricted

assoc)

Macro cell - planned

Short-range model

Very goodlinks

(SNR>10 dB)

Changing Interference ConditionsChanging Interference Conditions

Macro - unplannedLoss fromrandomness

(~2dB)



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Belief Propagation SolutionBelief Propagation Solution

Iterative message passing algorithm

Widely used in coding, non-Gaussian estimation,machine learning

Pass beliefs along edges of graphs representingestimates of the marginal distribution

Natural distributed implementation for wireless.

Similar methods used in many approximate BP algorithmsfor CDMA multiuser detection & non-Gaussian estimation:

Caire, Boutros (02), Guo-Wang (06), Tanaka-Okada (05), Neirroti-Saad (05), Kabashima (05), Donoho, Maleki, Montanari (09),Bayati-Montanari (10), Rangan (10)



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BP Multi-Round ProtocolBP Multi-Round Protocol

TX1RX1RX2

TX2

Desired linkInterference Interference

TX vector x2(0)

Sensitivity D2(0)

TX vector x1(0)

Interference z1(0) and sensitivity D1(0)

TX vector x2(1)

Sensitivity D2(1)

TX vector x1(1)

Interference z1(1) and sensitivity D1(1)

Data scheduledalong TX vector

x1

Round 0

Round 1

Datatransmission



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Interference Coordination with RelaysInterference Coordination with Relays

Still an open problem What are the optimal strategies for transmitters, relays and

receivers to maximize spectrum efficiency? What is the best strategy for relays -

Forwarding signal or forwarding interference?

Preliminary information theoretical results show both signalrelaying and/or interference forwarding could be optimal undercertain regimes (Elza Erkip)

Missing Components: Practical coding and signal processing schemes for

cooperative interference coordination MAC design that handles the signaling between different

entities participating in the cooperative interferencecoordination



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OutlineOutline

Motivation for Cooperation

Robust Cooperative MIMO Design Randomized Space Time Coding






Conclusions



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The Second-Last Mile ProblemThe Second-Last Mile Problem

Explosively growing traffic demand More than 5 billion cell phones by 2010 Increasing number of data intensive applications 3G/4G standards are pushing up the macrocell data rates

(~100 Mbps)

Poor cellular infrastructure Most of the BS backhauls use four to six T1/E1 lines (~8 Mbps) Adding BSs or updating data lines is expensive

(more than $10,000 per line and $50,000 per site annually)

Macrocell backhaulhas become thebottleneck!



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Solution: FemtoHaulSolution: FemtoHaul

System Architecture for FemtoHaul FemtoHaul is a novel solution to the macrocell backhaul problem.

In FemtoHaul, the femtocellbackhaul is used to carry non-

femto user traffic by forwardingthrough a relay.

Detailed Design Channel allocation

mechanism based on

OFDMA WiMAX; Policy for base stations to

schedule user transmissions.



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FemtoHaul Performance EvaluationFemtoHaul Performance Evaluation

Backhaul SupplyRate Comparison Average Download Ratein Stationary Scenario

Simulations demonstrate that our solution can significantly reducethe macrocell backhaul traffic while still guaranteeinga high rate to the subscribers



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OutlineOutline


Randomized Space Time Coding






Conclusions

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Goal: Build a large scale experimental,deployable and scalable cooperative network(Erkip, Korakis, Panwar, Liu, Wang, Bertoni) Funding from NSF (MRI, CRI), WICAT, NYU-Poly

We have taken two approaches PHY layer: Software Defined Radio (SDR) platform MAC layer: Open Source Driver Platform on Linux

Cooperative Networking TestbedsCooperative Networking Testbeds



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Implementing Cooperative PHYImplementing Cooperative PHY

Cooperative protocols require changes inthe PHY layer Commercial devices do not give access to PHY

Use Wireless Access Research Platform (WARP), aSDR by Rice University

We have a basic three node system operating,consisting of one source, one relay and one

receiver Cooperative coding using convolutional codes and

soft decision decoding implemented

We also have basic R-DSTC implemented



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WARP SystemWARP System



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Implementing Cooperative MAC forImplementing Cooperative MAC for

IEEE 802.11IEEE 802.11

Use open source drivers and commercialWiFi cards

Advantages Backward compatible with 802.11

Can be used in large testbeds such as ORBIT

Disadvantages:

No access to PHY(but still gains from Cooperative MAC)



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OutlineOutline


Randomized Space Time Coding






Conclusions



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ConclusionsConclusions

Cooperation is a perfect match for the emergingheterogeneity in wireless communications

Robust cooperative schemes (R-DSTC, R-DSM) requirelittle overhead and well suited even for MSs with highmobility

Heterogeneous networks provide many capable relays forcooperation Cooperative handoff

Cooperative interference coordination

FemtoHaul: Offload traffic from constrained macrocellbackhaul to abundant femtocell backhaul



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TThankhank YYou!ou!

Our Cooperative Research website:

http://coop.poly.edu



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BackupBackup



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Synchronization IssuesSynchronization Issues

Nodes cooperating without central control will encounter thepractical problem of synchronizing their access to the channel. Distributed relays have no access to a global clock.

Relays need to be synchronized both in time and frequency.

Synchronization accuracy affects physical layer performance ofcooperative MIMO system.

How to achieve synchronization? 4G systems (LTE and WiMAX) synchronize the transmissions from

UE both in time and frequency via close-loop control.

In a wireless LAN, relays can be synchronized by letting relays lockto a common reference signal. For example, the source cancontinuously transmit a reference carrier.

R-DSTC performs well under residual synchronization errors1.

1. M. Sharp, A. Scaglione and B. Sirkeci-Mergen, Randomized cooperation in asynchronous dispersivelinks, IEEE Transactions on Communications, vol. 57, no. 1, pp. 64-68, January 2009.



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Incentives for cooperationIncentives for cooperation

Cooperative relaying improves network capacity and reducesdelay. In a wireless LAN, throughput for each individual node can be

improved.

In a cellular network, the BS can provide incentive for relays byallocating more time/frequency resources to relays.

Battery consumption Average Joule/Bit performance is improved.

Energy consumption for nodes acting as relays (CoopMAC) is alsoreduced in wireless LANs2.

By employing several relays, the energy consumption for eachindividual relay is just 1/L of the case of employing one relay.

It is possible that a nodes battery drains faster because it acts as arelay for multiple sources, possibly as a result of its position.

Not an issue for dedicated fixed relays, or femtocells acting as relays.2. S. Narayanan and S. Panwar, To Forward or Not to Forward - That is the Question, Wireless PersonalCommunications Special issue on cooperation in wireless networks Vol 43 No 1 pp 65-87 2007

increasing cellular capacity using cooperative networks

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