Improving Wireless Access Technologies Adam Wolisz Professor of EE&CS, Technische Universität Berlin, TKN Adjunct Professor, EE&CS Dept, UC Berkeley, BWRC

Improving

Wireless Access Technologies

Adam WoliszProfessor of EE&CS, Technische Universität Berlin,

TKN Adjunct Professor, EE&CS Dept, UC Berkeley,

BWRC http://www.tkn.tu-berlin.de/~wolisz/wolisz.html

Sept.26,2007

TKN Telecommunication Networks Group 2

Overview

Short introduction of the TU Berlin research environment

Towards Dynamic OFDM (OFDMA)

Dynamic spectrum usage with Cognitive Radios


The contribution of my collaborators/students notably

TUBerlin: Dr. Gross, Mathias Bohge, Oscar Punal, Daniel Willkomm, Murad

Abusbeih

UCBerkeley: Prof. Brodersen, Dr. Cabric, Mubaraq Mishra

ST Microelectronics: Wendong HU, Dr. George Vlantis

Is gratefully acknowledged.

Acknowledgements:


Main-CampusBerlin University of Technology


Established in April 2001 as a pilot fusion of EE and CS.

43 associate or full professors (German C4 or C3) + a numerous “Professors in Residence”

3 curricula (Number of beginners/Year, trend)EE (260+), CE (150+), CS (300+-)

Communication Technology is one of the major focus areas – for the whole TUB.

TU Berlin: The Faculty of EE&CS


Communications Research relevant environment

COOPERATION: 21 joint faculty appointments;incl.17 fully funded by partners

The Institutes in 2004 total budget of € 165m,

(€110m in external grants) 1800 employees, incl.

380 employees with a PhD

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2

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TUBerlin


Telecommunication Networks Group (TKN) People: Full professor (Chair) A. Wolisz + (2-4) Post-Docs / Assistant Professors + (20-25) Research Assistants/PhD students + 3 Technical Staff + 2 administrative assistants

Supported by grants from: EU, BMBF, DFG, DAAD, Siemens AG, Ericsson, DoCoMo, ....

External Funding – approx. 1.5 Mio €/Year

Som-2006e numbers for the time 20009 post-doc research associates/Phd Graduates/adjunct lectureres have been appointed professors. Degrees granted: Doctorate: 20 , Diplomae: Over 90

Papers: ca. 50 in journals, magazines or book chapters ca. 150 in refereed conferences/workshops

see www.tkn.tu-berlin.de/publications


Research Topics (selection)General Direction:

Architectures and Protocols for Networks

Optical backbone/ Optical Metro Networks...

Wireless access...(QoS + Capacity).

Mobility incl. Group mobility and High speed mobility

Sensor networks

Cognitive Radio


The Fundamental Problem

Wireless dominates last hop (s?) Because cable is always a constraint...

Two fundamental features of wireless communication:

Interference; i.e. influence of any transmission on other ones calling for proper Separation (space, Frequency, time, code)

Dynamic change of the received signal strength in spite of constant transmission power even without interference.

These features result in challenges:

Limited Capacity, given frequency spectrum and space.

Difficulties in proper QoS support even on a single link


Introduction: OFDM

Orthogonal Frequency Division Multiplexing (OFDM) splits the bandwidth into narrowband sub-carriers

Parallel symbol transmission (reduces intersymbol interference)

Orthogonal sub-carriers (no intercarrier interference) Fading produces a strong variation in the sub-carrier gains: always some sub-carriers in a “bad state”

Orthogonal Frequency Division Multiplexing

This changes in the time domain as well…


Basic Scenario

Terminals

Access Point

Backbone

Downlink

Data Queues at Access Point

OFDM as transmission scheme!


Link Adaptation

IEEE 802.11 a/g : Time division Multiple Access and adaptive modulation/coding - the same over all sub-carriers.

Average channel gain adaptation: the few sub-carriers with the lowest gain dominate the BER & PER [Awoniyi06] • Same Modulation and

Coding Scheme on all Sub-carriers, which are assigned to one station


Adaptive Modulation Scheme

For each individual sub-carrier the selected modulation schema assures the highest bit-rate for upper-bounded BER.


Dynamic OFDM: Adaptive Modulation

Adapts the modulation type to the current gain of each sub-carrier subject to a bit error probability target, performing adaptation on a per-packet base.

Theoretically this procedure has been shown to outperform Link Adaptation [Czylwik98]

Assignments:

NO MOD. SNRx ≤ Xi

BPSK Xi < SNRx < Xj

QPSK Xj ≤ SNRx < Xk

…

BPSKSub-Carrier 1

16-QAMSub-Carrier 2

BPSKSub-Carrier 3

BPSKSub-Carrier 4

64-QAMSub-Carrier 43

QPSKSub-Carrier 44


BPSKSub-Carrier 46


QPSKSub-Carrier 48

ST

A 1


Dynamic OFDMA

Performance Improvement [Wong99]

The whole set of sub-carriers is split into sub-groups, which are then assigned to different stations in a FDM fashion.

STA

2ST

A 2

STA

1ST

A 3

MULTI-USER DIVERSITY

BPSKSub-Carrier 1

16-QAMSub-Carrier 2

BPSKSub-Carrier 3

BPSKSub-Carrier 4


QPSKSub-Carrier 44


BPSKSub-Carrier 46


QPSKSub-Carrier 48


Now: Optimization approach Open question: How to generate the subsets of

sub-carriers serving individual flows?

Two-step approach (Yin et al. 2000) per time SLOT.

First: sub-carrier allocation

- Determine the number of sub-carriers for each subset

(meeting the demand)

- IDEA: Utilize packet queue…

Second: sub-carrier assignment

- Choose sub-carriers according to the allocated number for each subset

- IDEA: Utilize channel-related information for the assignment

(goal: increase the capacity, assure fairness)


Allocation: satisfying the demand...

Let us assume ONE Queue per terminal (i.e. flow)The queue will built up if bad transmission ...

Packets should be dropped if waiting excessively. Observation: Not all frames in an MPEG video are equally important (I, B, P frames)

Drop packets based on importance: I frames on deadline, P frames 25% earlier, B frames 50% earlier.

Allocate the bit-rates (simplified: NUMBER of sub-carriers) based on weighted lengths of queues

The size of important packets is given a larger weight ...

Queue length is the sum of these weights


Assignment: Integer Programming Formulation

More flexibility by formulating assignment as Integer Program

{1,...,J} : Set of wireless terminals,

{1,...,S} : Set of sub-carriers

gj,s: CNR of terminal j on sub-carrier s

ps : power assignment for sub-carrier s

F(): Mapping of subcarrier SNR to applied modulation type

cj,s:(= 0,1): Assignment of sub-carrier s to terminal j

zj: Subcarrier allocation for terminal j

Basic formulation:

sjsj

jsj

sj zcj

cscCtosubject

,

,

, :

1::

sj

sjssjcC

gpFcsj ,

,,)( ,

maxMaximize capacity

Each subcarrier

only assigned

once

Number of sub-carriers perterminal fixed

out ofAllocation!


Optimal solution (constant power assignments)

Assignment problem maps to a graph theoretical problem

Maximum weight bipartite matching problem

An optimal algorithm for this problem exists – the Hungarian algorithm with complexity of O(S3)

Measured run times of the algorithm: ~2ms are too long compared to the Allocation/Assignment

TIME SLOTS < coherence time of wireless channels in our test data

Good heuristics exist for this problem, solving the problem with average parameter setting within 500µsPerformance loss: 10%


AssignedSub-Carrier Set of

User 3


User 2


User 1

A heurisitc for the Assignment...

1 2 3 4 5 6 7 8 9 10 11 12

G

B

B

G

G

G

G

G

G

G

G

B

B

G

G

G

G

G

G

G

G

G

G

G

G

G

B

G

B

B

B

G

G

B

G

B

Sub-Carriers

User 1

User 2

User 3

Single Sub-CarrierStates towards:

Sub-CarrierWeight:

1 3 3 2 2 3 3 3 2 1 2 1

Dynamic FDM assignment of sets of sub-carriers to individual terminals


Numerous results addressing:

Joint optimization of power (loading) and assignment. Including the overhead for signaling (how should the

receiver now which sub-carriers have been assigned?) Allocation including priorities See our URL…==============================================================

=====

But: does this require completely new systems? We suggest the Adaptive Modulation in the up-link

and Dynamic OFDMA in the down-link… A proposal for including this to IEEE 802.11… as

backward compatible solution.

- Technical Report: TKN-07-002

- Submissions to IEEE 802.11, VHT SG


Required changes to use Dynamic OFDM

In order to choose an (optimal) modulation/coding per sub-carrier, we need to

- - estimate the channel gain per sub-carrierfor each transmission Obligatory RTS/CTS

- inform the receiver about modulation/coding used per sub-carrier extended header in data packets

- adjust the NAV settings after the transmission For the multi-user case (parallel transmission of packets), we

have to add Multiple CTS per RTS and ACKS per data packet

- multtiple CTS per RTS, and ACKS per data packet; signal the assignment of sub-carrier sets,

Higher Performance but also higher processing power.


Schemata of the changes

Single user

Multi-user


Some performance data (simulation!)

• Simulation Settings:

- Saturation mode,

- 4 and 8 stations,

- downlink traffic

• Scenarios: – Multi-user mode

– Single-user mode with Round Robin Scheduling

– 802.11 a/g with RTS/CTS and Round Robin Scheduling

• Metrics:

– Goodput, PER and PHY Efficiency


This approach has potential…

Goodput - 8 STAs - Large Packets (1564 Byte) - 802.11 a/g with RTS/CTS

Goodput - 4 STAs - Large Packets (1564 Byte) - 802.11 a/g with RTS/CTS


Ongoing work …

We have proposed recently a proposal for the interface definition and API between MAC-PHY

3FPP LTE simulator (Ericsson) – used for sensitivity of the system efficiency on the Control Channel error due to interference from neighbor cells…

The real overhead, inaccuracies of sub-channel estimations, etc. require experimental investigation.

Several PHY developments (FPGA based) in preparation.

Execution of the algorithms on FPGA platforms is to be considered.


The underlying philosophy: Using the best opportunity.

Why not use the same philosophy for grabbing more spectrum…

This is Cognitive Radio based spectrum utilization…


Nutrition facts: The bad news...

The amount of non-licensed spectrum (in interesting frequencies) is limited

3 4 5 6 GHz Allocation in the 3GHz-6GHz


Nutrition facts: The good news...

Licensed users do not REALLY use ALL THEIR spectrum ALL the time in EACH place the license holds

The estimates are like 80%? 90%? 99%? unused...

A snapshot from Berkeley: real measurements…


Re-Usage of sectrum: What are the options? The licensed users – called PRIMARY USERS – could be

screened, andBe permitted to “sell” or “lease” their licenses….

Loose licenses (for some area) if spectrum not used properly

Have licenses limited to some times of the day“recycled” assignments could be opened as ISM bands

The primary user (or a “broker” on their behalf) could be obliged to “announce” unused frequencies (again in space and time domain); usage as ISM bands is possible.

A new category of users - SECONDARY USERS - possess the ability to assess autonomously the temporarily unused spectrum and grab it for “specific” usage without primary users being aware of this “kidnapping”


A Primary User X … legally owns some frequency band

… can tolerate a maximal interference time tx each time he resumes channel usage

A secondary user system has tx time units to detect a primary user and clear the corresponding Sub-Channels (time domain!)

tx is dependent on the primary user system and may vary from system to system

… is not (cognitive) secondary radio aware (i.e. does not provide specially signaling of activity – especially no preparation to re-gain his frequency band)

NOTE: IF primary user would use a carrier sensing protocol, neglecting the (unknown!) secondary user MUST be assured (operation below the carrier sense sensitivity!)


Cognitive radio for “Secondary Users”


Sensing while sending??? (a system view)

A fundamental challenge … three possible answers

Interrupted sending… (see e.g. IEEE 802.22 basic…)

Quick sensing needed.

Bad for QoS of the secondary's

Part of the “primary” band not re-used (see Corvus)

Coordination of frequency usage required

Band only partially available for sensing…


Interleaved frequency usage (see CORVUS) Active primary user

Hz

Sub-channel

Primary user frequency band (F-Band)

Secondary user link (SUL)

Bandwidth B [Hz]

SUs compose SU-Links out of free sub-channels

Sub-channels of active Primary users (Pus) can't be used by SUs

PU F-Band covers multiple sub-channels

New sub-channels should be acquired

Divided into N sub-channels of bandwidth b=B/N [Hz]

(Re-)appearance of PU All affected Sub-Channels have to be cleared


This resembles OFDMA… sure:

Efficiency: Potential for usage of the spatial diversity (anyway in downlink)Available actual bandwidth of each sub-carrier (depending on the channel conditions) fully utilized.

Constant Sub-carrier SpacingRobust to channel positioning (offset) and bandwidth changes

Interesting options for usage for the secondary devices in the “Interleaved transmission” modus.


Sensing while sending??? (a system view) cont.

A fundamental challenge … three possible answers

Interrupted sending… (see e.g. IEEE 802.22 basic…)Quick sensing needed.

Bad for QoS of the secondaries

Part of the “primary” band not re-used (see Corvus) Coordination of fre-quency usage required.

Band only partially available for sensing…

Regular hopping (see e.g. IEEE 802.22 DFH mode)

Changing frequency bands regular event (collision in 802.3)

“Relaxed” sensing in free(?) band – assuming coordination


IEEE 802.22 (DFH)

Worldwide first draft of a Cognitive Radio standardProvide wireless broadband Internet access using TV-bands

Ensure non interference with incumbents (grace period 2s) through spectrum sensing -

Philosophy following WiMax (IEEE 802.16)

Basic Mode: Assure respecting the grace period by sensing during TRANSMISSION INTERRUPTION

DFH: Perform data transmission and sensing in parallelTransmit data on channel X and perform sensing on channel Y

After 2 seconds channel Y is used for data transmission and the next channel is sensed. Might be X again...2 channels/cell

Thus an 802.22 cell hops through a set of working channels


DFHC (DFH Communities) – some ideas

Each community has a community leader Leader is selected through leader election

Leader calculates a hopping pattern for each member of the community (i.e. cell)

Leader is responsible for accepting / rejecting new members

Neighborhood discoveryOne-hop broadcast of used frequencies and current interference situation by all 802.22 cells

Used to create and maintain communities

It is possible to support N Cells with (N+1) frequencies…


DFHC hopping pattern

W. Hu, D. Willkomm, L. Chu, M. Abusubaih, J. Gross, G. Vlantis, M. Gerla, and A. Wolisz, "Dynamic Frequency Hopping Communities for Efficient IEEE 802.22 Operation", IEEE Communications Magazine, Special Issue: "Cognitive Radios for Dynamic Spectrum Access", May 2007

Cells need to shift their operation periods by one quiet time

Quiet time is the minimum time needed to sense a channel

Ch A

Ch B

Ch C

WRAN 1

WRAN 1

WRAN 1

WRAN 1

WRAN 1

WRAN 2WRAN 2

WRAN 2

WRAN 2

WRAN 2

Quiet Time Operation Periond

Ch D WRAN 1

WRAN 1

WRAN 2 WRAN 2 WRAN 1

WRAN 1WRAN 3

WRAN 3

WRAN 2

WRAN 2

WRAN 3

WRAN 3

WRAN 3

WRAN 3

WRAN 3

WRAN 3

WRAN2 WRAN1

WRAN3


Protocol sketch for schedule maintenance

Hopping patterns can changeDue to incumbent appearance on a used channel

Due to a member leaving / joining a community

Community leader needs to calculate new hopping pattern and distribute it in the community

Consistency issue: How to assure that all members receive the new hopping information

If not all members switch to the new hopping pattern simultaneously there might be collisions between the old and the new hopping pattern

Solution: Hopping pattern lifetime and sequential switching


Hopping pattern lifetime

Hopping patterns have a specific lifetime After expiration of the lifetime the hopping pattern

cannot be used anymore The leader thus has to periodically renew the

hopping pattern Upon renewal the pattern can be changed, new

members can be added, etc. This ensures consistency: even if some members do

not receive the new pattern, they cannot use the old one anymore

But what if a hopping pattern needs to be changed in the middle of a lifetime (i.e. due to appearance of an incumbent)?

Solution: Sequential Switching


Sequential Switching The leader sequentially switches all members one

by one to the new hopping pattern New hopping pattern is collision free with the

pattern of all members not switched yet Implicit acknowledgement: sensing on the newly

assigned channel (implicit confirmation by acting) Even if somebody fails to follow - all members

already switched can use the new pattern without any collisions


In any case: Detection of (possibly) Weak Signals

Cognitive radio observes (senses) primary system signals

Those might be strongly attenuated

While the transmission of the CR(Tx) towards Rx is not…

Primary User

Cognitive Radio users must guarantee non-interference requirement

distance

Distance and channel not known

Tx

Rx

CR(Tx)

CR(RX)

Decoding SNR Sensing SNR


Solution: Network Spectrum Sensing [BWRC,Cabric]

Prob. of false alarm

Prob

. of d

etec

tion

1 radio

5 radios

If spacing >> λ/2 a few cooperative radios give big improvements


We need a signaling channel:

Cooperation is needed… at least in “proximity”For assuring that “no other secondary is transmitting during the sensing”

For assuring network spectrum sensing.

Who should be subject to coordination?

How to organize the exchange of information for coordination??


But: Control Channel needed (logically) for: Universal Control Channel (UCC)

Globally unique Used to get necessary information for creation of new

groups and to announce them Used by new users to choose and join a specific

Group Group Control Channel (GCC)

Each SUG has own control channel Used for exchange of sensing information – recognition

of primary users. Used for data channel establishment (out of

temporarily available resources) and its maintenance in spite of re-appearing primary users.

Numerous claims in favor of a specific Control Channel are recently being made…


Options for the control channel implementation

An Universally/regionally pre-assigned frequency….

Globally unique Will be difficult….

ISM band…Globally available…What about possible (strong??) interference?

UWBNot interfering Tradeoff: distance vs. bit-rate might be very useful Cost? Deployment?

One of the “available channels”Convention for selection neededIEEE 802.22 considers this variant….


BWRC Platform [BWRC: Cabric, Tkaczenko]

Sensing PHY real-time processor:

• 4 FPGAs ~ 10M gates ASIC at 250 MHz

• On-chip memory: Soft+Hard > 10 Mbits

• Dynamic Partial Reconfiguration

• Dedicated DSP blocks: 18b mult + MAC

• Architecture optimization for ASIC

- Parallelism/Pipelining/Interleaving

- Bitwidth optimization

- Area estimate: 10,000 slice = 1mm2

Sensing MAC embedded processor:• Central FPGA: Linux + Full IP

• Embedded processors: PPC+ARM

• On-chip Ethernet MAC

• Bus connection to 4 other FPGAs

Radio interfaces:• 16 high speed radio links (10 Gbps)

• 4 interfaces per FPGA

• Fiber optic cable compatible


Reconfigurable Wireless Radio Modem [BWRC]

10 Gbps infiniband connectionsupports fiber optic cables

Sensing radioprocessor

Antenna

A/D 12b/64MHzD/A 14b/128MHz 2.4 GHz radio

(85 MHz) ISM band

Suitable for sensing and transmission in TDD mode


Why 2.4GHz? Very crowded spectrum with unlicensed devices.

IEEE 802.11 b/g cards within laptops, are quite programmable and allow users to control their transmission parameters.

Easy to implement protocols on these cards

Hardware and software support for the 2.4GHz bands is already developed within BWRC

BWRC cards can be programmed to the complete 80MHz band


First Experimental setup at 2.4GHz [BWRC/TKN]

BEE2 radios perform sensing

Laptops perform transmission

Laptops connect to the BEE2 via standard TCP/IP


What about control channel ?? In first set-up:

“Internal communication” between sensing boards

Ethernet communication between the laptops…

One selected wireless channel, ot IEEE 802.11.a could also have been used.

This might be enough for SOME of the experiments…

==================================

Let us also keep in mind some (available) other option, the IEEE 802.15.4a CHIRP solution (Nanotron, Berlin)

Promises: A pretty robust transmission with range sufficient for WLAN type deployment…. + inherent (precise) LOCATION

Samples being in possession of TUBerlin/TKN for evaluation….


Functions of „Classical Wireless Systems“

We will consider here: Cellular, infrastructure WLANs, ad-hoc networks…

What has been typical for all of them? (history)Usage of specific frequency bands optimized transmission

Exclusive usage of frequency bands by regulation

Channel structure within this bands - using of a selected /negotiated channels for the whole transmission

assigned at the beginning of the data exchange

Basic stepsFinding the partner mostly by beaconing (notably base stations!)

Exchanging set-up data separate signaling channel or in-band

Selecting a channel to work on ditto


What does change in CR ? (potentially) Usage of arbitrary frequency ranges (for much

better spectrum utilization) Approach: Consulting data basis + distributed sensing

No fixed channel structure (for flexible adaptation to the traffic needs…)

Approach: Negotiating variable channel structure

Imposed high dynamics in changing the used frequencies (if the primary user pops-up!)

Approach: sense while communicating (how?)

Usage of arbitrary transmission schemataApproach: Use OFDMA (see 802.22)


Looks like a revolution? Not entirely…

Usage of specific frequency bands ?GSM: 900/1800/1900 ….

IEEE 802.11: 2.4GHz, 5.4 GHz, 5.9 GHZ (DSRC)…

WiMAX…

Specialized transmission schemata? General trend towards OFDM /OFDMA!!!

Exclusive usage of frequency bands? Take 802.11 ….

Coexistence with radars (in 5 GHZ/Europe), microwave ovens, Bluetooth, 802.15.4, etc

Persistent usage of assigned channel?Some frequency hopping during the communication (802.11 FH, Bluetooth, Some GSM)

Channel change in 802.11 (e.g. Mobility…)


Lessons learned:

There is a trend to increase the dynamics of adaptive resource usage on each time scale and granularity.

OFDMA seems to be especially attractive transmission schema

Available systems/set-ups allow for investigation of multiple features for the future “Cognitive Radio”


Thank You !


IEEE 802.15.4a Chirp … some facts [www.nanotron.com]

Chirp technology operating in 2.4 GHz ISM band, 20 MHz, 7 channels (3 non overlapping)

Data rates 2, 1 Mbps; 500, 250, 125, 62.5, 31.25 kbps

Power/sensitivityTx: up o dBm (plus ext. amplifier for long range)

Rx: -97dBm @ 250kbps; BER= 10-3 (with FEC)

Symmetrical double-sided two way ranging

128 bit hardware encryption.

RS 232 interface (USB expected…)


IEEE 802.15.4a Chirp … early data [IEEE Documents]

Indoor (European office building) measurement results for a data rate of 1 Mbps (over air interface) and BER = 0.001 are as follows:

Output power (EIRP) = -30 dBm (1 µW), distance = 5 m, 1 wall

Output power (EIRP) = -15 dBm (32 µW), distance = 23 m, 4 walls

Outdoor measurement results for a data rate of 1 Mbps (over air interface) and BER = 0.001 are as follows:

Output power (EIRP) = +7 dBm (5 mW), distance = 739 m (+/-10 m)

Both transceivers use equivalent isotropic antenna (gain = 0 dBi)

For long ranges the transmit power may be allowed to rise to each country’s regulatory limit

For example the US would allow 30 dBm of output power with up to a 6 dB gain antenna

The European ETS limits would specify 20 dBm of output power with a 0 dB gain antenna

Ranging: 2m indoors – 1 m outdoors…. (data sheet)


Communication Stack

Secondary users create communicating GROUPS (SUGs)

A universal control channel as well as group control channels are assumed...

PHYLayer

LinkLayer

UCC GCC Data Transfer Channel

SpectrumSensing

ChannelEstimation

Data Transmission

MAC

Group ManagementLink Management

DataTransmission

DataTransmission

MACMAC

Control channels....