Random Packet-CDMA: Reducing Delay and IncreasingThroughput of WLAN Systems

Roland Kempter and Behrouz Farhang-Boroujeny

kempter@ece.utah.edu

farhang@ece.utah.edu

Organization

1) Introduction: Scheduled vs. Random Channel Access

2) Random Packet-CDMA (RP-CDMA)

3) RP-CDMA: Header Detection

4) RP-CDMA: Payload Detection

5) Conclusions

Introduction: scheduling-based access

Packet in queue

Transmitter (TX) Handshake w/ Base

Ask for resources

Base

Thinking about it!

TX

Handshakew/ TX

Tell about resources

AdvantagesBase knows everything:• packet collisions can be avoided• service can be guaranteed (great for voice)

DisadvantagesResources need to be negotiated:• OK in circuit switched systems• large overhead (or low efficiency) with packetized traffic

Data (IP) traffic is bursty!

Introduction: random channel access

Packet in queue

Transmitter (TX) TX

Check medium(sometimes)

TX Base

Send Acknowledgement

Advantages• self regulating (good for bursty traffic)• load adaptive• simplifies infrastructure

Disadvantages• Collisions, low stable throughput• degrades heavily as load increases • delay variations

Introduction: background on random channel access

1970

Aloha

1976

Ethernet

1980

IEEE 802.3CSMA

1990

Spread Aloha

1997

IEEE 802.11

2000+

CDMA:ConcurrentTransmissions

Advances in (Random) Channel Access

RP-CDMA: combination of Spread Aloha with CDMA technology

Max. throughput of Spread Aloha

(18% of a fully coordinated system)

No collisions

Know SS for detection handshakingNot Random Protocol

S = G

RP-CDMA packet structure [KemAmiFar06, SchKemKot06]

[KemAmiFar06] R. Kempter, P. Amini and B. Farhang-Boroujeny, “Throughput and Stability of RP-CDMA and Spread Aloha in Multipacket Capture Channels", submitted to IEEE/ACM Transactions on Networking, October 2006

[SchKemKot06] C. Schlegel, R. Kempter and P. Kota, “A Novel Random Wireless Packet Multiple Access Method Using CDMA”, IEEE Trans. Wireless Communications, p. 1362-1370, Vol. 5, No. 6, June 2006

Header Channel: low-traffic Spread ALOHA channel

Data Channel: CDMA, possibly large number of TX

System Characteristics

RP-CDMA Performance: interference in the header channel

System EffectPacket format separates TX into a virtual header and virtual data channel

If header detection limited by (h/d) interference: CSMA/CA on the header unlikely to improve performance

Note

Lh<Ld and concurrent TX: feature of RP-CDMA header detection likely (h/d) interference limited

RP-CDMA Performance: collisions in the header channel

Time

Collision at the base station? - a packet level point of view

Payload portionwith randomspreading

Header portionwith commonspreading

Packet from node x

Packet from node y

Collisions: not necessarily

As long as no overlap at chip level: still separated by Spreading Gain N.

Collision? -on the chip level

RP-CDMA Header Detection: results

• Header detected successfully iff after receiver, SNIR > header detection threshold

• No chip-level collisions happened

• Equal power scenario

Assume

• Solve:

• Check for chip-level collisions

Matched Filter [TseHan99]

Packet timing established during header recovery:Matched Filter for header detection

RP-CDMA: header detection, spread aloha, equal powers

(b) Nh=Nd=20 (a) SNRh/SNRd=5 dB

(c) Ld / Lh=25

RP-CDMA: performance

Conclusions from the header process

• After a certain point which is f(N,SNR), increasing Ld / Lh does not improve performance

Interference limitation

Collision limitation

Small systems Interference can be suppressed

Large systems Interference can onlybe suppressed up to a certain load

Pointf (Nh, Nd, K, Ph/Pd, pathloss, , header technology, data technology)

RP-CDMA: data channel performance

Data frame is detected successfully iff after receiver, SNIR > threshold .

SNIR is multi-user receiver specific

Assume

RP-CDMA Data Detection

• Matched Filter [TseHan99]

• Decorrelator [TseHan99]

• MMSE [TseHan99]

• Partitioned Spreading [KemSch05, ShiSchKem06]: iterative demodulation, resolves (virtually) all multiuser interference

[KemSch05] R. Kempter and C. Schlegel, “Packet Random Access in CDMA Radio Networks”,in Proceedings of Allerton Conference 2005.

[ShiSchKem06] Z. Shi, C. Schlegel and R. Kempter, “On the Performance of Partitioned-Spreading CDMA”, Conference on Information Sciences and Systems, CISS 2006, Princeton, 2006

RP-CDMA: network simulations

IP packet sizes are trimodally distributed [Inet2]:

• Pr(L=50 bytes) = 0.5

• Pr(L=500 bytes) = 0.4

• Pr(L=1500 bytes) = 0.1

Also

• RP-CDMA header size: Lh= 50 bits E[Ld / Lh] = 60

• Payload and header spreading gains: N=32• Header/Payload SNR: 5 dB• Header and data detection thresholds: 3 dB • Power control• Nodes transmit to a central base station

RP-CDMA: equal power, central base station

RP-CDMA receiver: SOFTWARE DEFINED RADIO

Laid out formax. # users

Software processes,payload decoding technologycan be adopted f(load)

Conclusions 1/2

• We demonstrated the performance in the RP-CDMAheader channel by modeling it as a Spread Aloha packet.

• After a point, RP-CDMA becomes interference rather than

collision limited: f (Nh, Nd, K, Ph/Pd, pathloss, , h. technology, d. technology)• For the data channel, we compared:

- Matched Filter - Decorrelator - MMSE- Partitioned Spreading

in base station centric networks w/ power control

Conclusions 2/2

• RP-CDMA is determined by header process:improving header directly is hard (non-linear in parameters):

BUT: with Partitioned Spreading, for a targeted data rate, Pd can be low, increasing Ph / Pd pushing the interference limit

• We presented the general block-diagram of an RP-CDMAsoftware defined receiver

• RP-CDMA allows to adopt to different load situations via software changes low load: power save w/ matched filter high load: partitioned spreading

The EndAdditional References

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[FLUX] David Johnson, Tim Stack, Russ Fish, Dan Flickinger, Rob Ricci, Jay Lepreau, “TrueMobile: A Mobile Robotic Wireless and Sensor Network Testbed,” University of Utah Flux Group Technical Note 2005-02, April 2005. Revised version to appear in INFOCOMM 2006

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