1. Improving Wireless Access Technologies
Adam Wolisz
Professor of EE&CS, Technische Universität Berlin, TKN Adjunct Professor, EE&CS Dept, UC Berkeley, BWRC
Sept.26,2007

2. Overview Short introduction of the TU Berlin research environment
Towards Dynamic OFDM (OFDMA)
Dynamic spectrum usage with Cognitive Radios

3. Acknowledgements:
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.

4. Main-Campus
Berlin University of Technology

5. TU Berlin: The Faculty of EE&CS
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.

6. Communications Research relevant environment
The Institutes in 2004
total budget of € 165m, (€110m in external grants)
1800 employees, incl. 380 employees with a PhD 2 1 2 1 3
Fraunhofer-Gesellschaft
COOPERATION:
21 joint faculty appointments;
incl.17 fully funded by partners 2 TU
Berlin 4 2 1 1 1 1

7. 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 2000
9 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

8. 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

9. 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

10. 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…

11. OFDM as transmission scheme!
Basic Scenario
Access Point
Backbone
Terminals
Downlink
Data Queues at
Access Point
OFDM as transmission scheme!

12. Link Adaptation
IEEE 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

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

14. 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]
BPSK
Sub-Carrier 1
16-QAM
Sub-Carrier 2
Sub-Carrier 3
Sub-Carrier 4
64-QAM
Sub-Carrier 43
QPSK
Sub-Carrier 44
Sub-Carrier 45
Sub-Carrier 46
Sub-Carrier 47
Sub-Carrier 48
STA 1
Assignments:
NO MOD.  SNRx ≤ Xi
BPSK  Xi < SNRx < Xj
QPSK  Xj ≤ SNRx < Xk …

15. 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 2
STA 1
STA 3
BPSK
Sub-Carrier 1
16-QAM
Sub-Carrier 2
Sub-Carrier 3
Sub-Carrier 4
64-QAM
Sub-Carrier 43
QPSK
Sub-Carrier 44
Sub-Carrier 45
Sub-Carrier 46
Sub-Carrier 47
Sub-Carrier 48
MULTI-USER DIVERSITY

16. 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)

17. 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

18. 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:
Each subcarrier only assigned once
Maximize capacity
Number of
sub-carriers per
terminal fixed
out of
Allocation!

19. 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 500µs Performance loss:  10%

20. A heurisitc for the Assignment...
Sub-Carrier
Weight: 1 3 3 2 2 3 3 3 2 1 2 1
Sub-Carriers
Single Sub-Carrier
States towards: 1 2 3 4 5 6 7 8 9 10 11 12
User 1 G G G G B G G G G G B B
User 2 B G G G G G G G G B G G
User 3 B G G B G G G G B B G B
Assigned
Sub-Carrier Set of
User 1
Assigned
Sub-Carrier Set of
User 2
Assigned
Sub-Carrier Set of
User 3
Dynamic FDM assignment of sets of sub-carriers to individual terminals

21. 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 … as backward compatible solution.
- Technical Report: TKN
- Submissions to IEEE , VHT SG

22. 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-carrier for 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.

23. Schemata of the changes
Single user
Multi-user

24. 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
– a/g with RTS/CTS and Round Robin Scheduling
• Metrics:
– Goodput, PER and PHY Efficiency

25. This approach has potential…
Goodput - 8 STAs - Large Packets (1564 Byte) a/g with RTS/CTS
Goodput - 4 STAs - Large Packets (1564 Byte) a/g with RTS/CTS

26. 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.

27. The underlying philosophy: Using the best opportunity.
Why not use the same philosophy for grabbing more spectrum…
This is Cognitive Radio based spectrum utilization…

28. Nutrition facts: The bad news...
The amount of non-licensed spectrum (in interesting frequencies) is limited 
GHz
Allocation in the 3GHz-6GHz

29. 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…

30. Re-Usage of sectrum: What are the options?
The licensed users – called PRIMARY USERS – could be screened, and
Be 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”

31. 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!)

32. Cognitive radio for “Secondary Users”

33. Sensing while sending??? (a system view)
A fundamental challenge … three possible answers
Interrupted sending… (see e.g. IEEE 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…

34. Interleaved frequency usage (see CORVUS)
Primary user frequency band (F-Band)
Active primary user Hz
Sub-channel
Secondary user link (SUL)
Bandwidth B [Hz]
Divided into N sub-channels of bandwidth b=B/N [Hz]
PU F-Band covers multiple sub-channels
Sub-channels of active Primary users (Pus) can't be used by SUs
SUs compose SU-Links out of free sub-channels
(Re-)appearance of PU
All affected Sub-Channels have to be cleared
New sub-channels should be acquired

35. 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 Spacing
Robust to channel positioning (offset) and bandwidth changes
Interesting options for usage for the secondary devices in the “Interleaved transmission” modus.

36. Sensing while sending??? (a system view) cont.
A fundamental challenge … three possible answers
Interrupted sending… (see e.g. IEEE 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 DFH mode)
Changing frequency bands regular event (collision in 802.3)
“Relaxed” sensing in free(?) band – assuming coordination

37. IEEE 802.22 (DFH) Worldwide first draft of a Cognitive Radio standard
Provide wireless broadband Internet access using TV-bands
Ensure non interference with incumbents (grace period 2s) through spectrum sensing -
Philosophy following WiMax (IEEE )
Basic Mode: Assure respecting the grace period by sensing during TRANSMISSION INTERRUPTION
DFH: Perform data transmission and sensing in parallel
Transmit 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 cell hops through a set of working channels

38. 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 discovery
One-hop broadcast of used frequencies and current interference situation by all cells
Used to create and maintain communities
It is possible to support N Cells with (N+1) frequencies…

39. 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 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
WRAN2
WRAN1
WRAN3

40. Protocol sketch for schedule maintenance
Hopping patterns can change
Due 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

41. 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

42. 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

43. In any case: Detection of (possibly) Weak Signals
Cognitive Radio users must guarantee non-interference requirement
Primary User Tx
CR(Tx) Rx
CR(RX)
Decoding SNR
Sensing SNR
distance
Distance and channel not known
Cognitive radio observes (senses) primary system signals
Those might be strongly attenuated
While the transmission of the CR(Tx) towards Rx is not…
In the order of … This is a noise dominated regime, that we did not address so far in communication systems

44. Solution: Network Spectrum Sensing [BWRC,Cabric]
If spacing >> λ/2 a few cooperative radios give big improvements
Prob. of detection
5 radios
1 radio
Prob. of false alarm

45. 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??

46. 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…

47. 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?
UWB
Not interfering
Tradeoff: distance vs. bit-rate might be very useful
Cost? Deployment?
One of the “available channels”
Convention for selection needed
IEEE considers this variant….

48. 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
Handle multiple radios on one board.
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

49. Reconfigurable Wireless Radio Modem [BWRC]
Suitable for sensing and transmission in TDD mode
A/D 12b/64MHz
D/A 14b/128MHz
2.4 GHz radio
(85 MHz)
ISM band
Sensing radio
processor
10 Gbps infiniband connection
supports fiber optic cables
Antenna

50. Why 2.4GHz? Very crowded spectrum with unlicensed devices.
IEEE 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

51. First Experimental setup at 2.4GHz [BWRC/TKN]
BEE2 radios perform sensing
Laptops perform transmission
Laptops connect to the BEE2 via standard TCP/IP

52. What about control channel ??
In first set-up:
“Internal communication” between sensing boards
Ethernet communication between the laptops…
One selected wireless channel, ot IEEE 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 a 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….

53. 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 steps
Finding 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

54. 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 schemata
Approach: Use OFDMA  (see )

55. Looks like a revolution? Not entirely…
Usage of specific frequency bands ?
GSM: 900/1800/1900 ….
IEEE : 2.4GHz, 5.4 GHz, 5.9 GHZ (DSRC)…
WiMAX…
Specialized transmission schemata?
General trend towards OFDM /OFDMA!!!
Exclusive usage of frequency bands? Take ….
Coexistence with radars (in 5 GHZ/Europe), microwave ovens, Bluetooth, , etc
Persistent usage of assigned channel?
Some frequency hopping during the communication ( FH, Bluetooth, Some GSM)
Channel change in (e.g. Mobility…)

56. 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”

57. Thank You !

58. 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, kbps
Power/sensitivity
Tx: up o dBm (plus ext. amplifier for long range)
Rx: 250kbps; BER= 10-3 (with FEC)
Symmetrical double-sided two way ranging
128 bit hardware encryption.
RS 232 interface (USB expected…)

59. 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 = 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 = 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)

60. Communication Stack PHY Layer Link UCC GCC Data Transfer Channel
Secondary users create communicating GROUPS (SUGs)
A universal control channel as well as group control channels are assumed...
PHY
Layer
Link
UCC
GCC
Data Transfer Channel
Spectrum
Sensing
Channel
Estimation
Data Transmission
MAC
Group Management
Link Management
Data
Transmission
Control channels....