1. Colorado School of Mines
ALLow Improved Access in the Network via Cooperation and Energy Savings (ALLIANCES)
Athina Petropulu
Drexel University
&
Tracy Camp
Colorado School of Mines
Supported by NSF, CNS

2. SUMMARY
ALLIANCES is a novel high-throughput medium access scheme for wireless networks that is suitable for multimedia sources.
Exploit the broadcasting nature of the wireless channel. View the network as a spatially distributed antenna; elements linked via the wireless channel.
Propose an ALOHA-type approach, that resolves collisions by processing the collided packets and cooperative retransmissions of the them, forwarded by a relay set of nodes.
The spatial diversity introduced by the cooperative relaying enables us to deal with the wireless channel without bandwidth expansion nor additional antenna hardware.
ALLIANCES enables efficient use of network power.

3. MOTIVATION
Multimedia sources have diverse bandwidth requirements (bursty)
Fixed resource allocation schemes are inefficient for variable-bit-rate traffic.
Simple medium access schemes for bursty sources include random access methods (ALOHA).
The throughput performance of slotted ALOHA is limited.
MAC design cannot handle multi-packet reception without declaring a collision.
MAC assumes that collision is the only cause of packet loss. BUT….channel fading can result in lost packets.

4. MOTIVATION (2)
For a long time, the focus of multi-user research has been on preventing collisions.
On the other hand, in wireless communications literature, multi-user detection and separation is possible via advanced signal processing at the receiver.
The key to achieving multi-user separation and overcoming the effect of the wireless channel is diversity (temporal diversity, spatial diversity).
In a recent method, the diversity was provided by the network (NDMA approach). In a K-fold collision, the packets involved in the collision are not discarded but rather stored in memory and later combined with retransmissions during the subsequent K-1 slots.

5. MOTIVATION (3)
The NDMA Method
3-fold Collision at slot n

6. MOTIVATION (3)
The NDMA Method
slot n+1

7. MOTIVATION (3)
The NDMA Method
slot n+2

8. MOTIVATION (3)
The NDMA Method
Y=AX+W

9. MOTIVATION (3) The NDMA Method Y=AX+W
Estimation of X from Y is a typical MIMO problem

10. MOTIVATION (4) The NDMA Method
NDMA exploits time diversity introduced by retransmissions.
Diversity exists if fading channels have coherence time in the order of the packet slot.
In practice, the channel coefficients are usually correlated over adjacent slots.
If the channel during a certain slot is in deep fade, it will probably continue to be in deep fade over subsequent slots, thus compromising the rank of the mixing matrix A.

11. ALLIANCES Small-scale slotted multi-access system
Assumptions
Small-scale slotted multi-access system
All transmitted packets have the same length, each packet requires one time unit/slot for transmission
Transmitters are synchronized and all nodes operate in half duplex mode
Only non-regenerative relays are used
Every node is equipped with only one antenna.
Each node in the system obtains feedback from the base station/access point specifying whether the packet was transmitted successfully.

12. ALLIANCES
Once the collision has been detected, the system enters a cooperative transmission epoch (CTE).
One control bit is broadcasted to all nodes indicating the start of a CTE.
Initially the CTE consists of K-1 slots. The BS will increase this number if necessary.
During slots in the CTE, one node is selected to act as relay and bounce off the signal mixture that it received during the n-th slot (collision slot).

13. ALLIANCES 1 5 4 2 6 7 3 3-fold Collision at slot n Signal at BS:
Signal at the i-th relay node: 3

14. ALLIANCES 1 5 4 2 6 7 3 3-fold Collision at slot n
Signal at BS at slot n+1: 6 7 3

15. ALLIANCES 1 5 4 2 6 7 3 3-fold Collision at slot n
Signal at BS at slot n+1: 6
Signal at BS at slot n+2: 7 3

16. ALLIANCES 1 5 4 2 6 7 3 3-fold Collision at slot n
Signal at BS at slot n+1: 6
Signal at BS at slot n+2: 7
Signal at BS at slot n+3: 3

17. ALLIANCES
Again we can set up a MIMO system Y=AX+W and solve for X (collided packets)
ALLIANCES offers cooperation diversity in addition to time diversity
If nodes in the system are well separated, their transmissions experience independent attenuation. The correlation between channel coefficients of consecutive transmissions is greatly reduced.
The packets initially transmitted by source nodes are forced to follow different paths, thus the probability that all these paths will be in deep fade is small.

18. Simulation of ALLIANCES
We tested the ability of the network to handle a certain aggregated traffic load (newly generated traffic and retransmitted traffic)
There were 32 users in the network.
A user ID was included in the packet, which was selected based on the rows of an M-th order Hadamard matrix. Packet length N =424 bits. Each packet contained 4-QAM symbols.
The fading channel was simulated according to Jake’s model with 12 Mbps rate and Doppler freq. shift 52 Hz. At 5.2 GHz carrier this corresponds to relative user speed of v=3m/s.
The channel was estimated based on the user ID potion of the packet.
A Zero Forcing equalizer was used for symbol recovery
The transmission was considered to successful when all the active users were correctly detected and received packets contained bit errors at a rate <= 0.02.

19. Simulation of ALLIANCES

20. Simulation of ALLIANCES

21. ISSUES Work at the PHY Layer Adaptive arrays Work at the MAC Layer
Effects of channel fading on the length of CTE
Effect on bit-error-rate as the order of collisions increases.
Channel estimation (training based, blind)
Frequency selective fading
Adaptive arrays
An multiple beam adaptive array (MBAA) can be used to form several antenna patterns simultaneously. Each beam pattern captures different packet groups, by automatically pointing its pattern towards one group while nulling other contending groups.
Subsequently, only packets that fall with the same beam need to be resolved.
Exploiting MBAA ideas can reduce the CTE duration and the complexity of packet recovery.
Work at the MAC Layer
Selection of good relay nodes
Allow fairness and prioritized access
Extension of ALLIANCES to a multi-hop environment