1. CDMA Based MAC Protocol for Wireless Ad Hoc Networks Alaa Mouquatash & Marwan Krunz Presentation by: Moheeb Abu-Rajab

2. Agenda Motivation Protocol Design Protocol Description Simulation Results & Evaluation Conclusion and Future Work

3. Motivation Fundamental problem in MANETS is low throughput  Contention-Based MAC protocols  Harsh Characteristics of the radio channel Challenge:  How to increase throughput while maintaining low energy consumption? This paper focuses on improving the overall network throughput by means of a CDMA-based design of the MAC-protocol

4. “CDMA” & Spread Spectrum Communication Bandwidth is one of the most scarce resources in wireless communication systems. Traditionally signals were transmitted using the least possible bandwidth  M-ary Communication systems (eg: QAM, 16-QAM, 32-QAM… etc.) TDMA and FDMA were the most prevalent schemes for sharing the bandwidth between multiple users.  Attributed by low spectrum utilization.  Low noise immunity. Wireless Communication researchers strive for a paradigm that can provide better spectrum utilization and lower energy requirements

5. “CDMA” & SS (continued) Spread Spectrum: follows a completely different approach:  Instead of limiting the Tx. Signal bandwidth; SS spreads the bandwidth on a wider band.  How it achieves better spectrum utilization, and better noise immunity? SS uses low correlated Pseudo Random codes (PN codes) to spread the original digital bit stream. Multiple users can share the same spectral band if they were spread using different codes The quality of the received signal is judged by  Power constraints  Correlation between spread codes

6. CDMA System PN-Sequence Generator QPSK Modulator QPSK De- modulator PN-Sequence Generator Digital Input Stream Spread Digital Stream Output RF Spread Digital Stream Digital Output Stream

7. CDMA/MAC How can we exploit this feature of CDMA to enhance the throughput of traditional MAC protocols for MANETs?  802.11 uses CDMA as a modulation scheme to achieve better noise immunity  Only uses a common code to spread the tx. Bit stream The basic approach to enable a CDMA/MAC is uses multiple codes rather than one code, thus allowing more than one user to share the comm. channel

8. CDMA/MAC, Challenges Code Assignment:  We need to assign a unique and orthogonal code for each node to avoid interference  Complicated for large networks with large number of nodes  Not feasible for MANETs “time-asynchronous systems”  Even unique Codes exhibit non-zero correlation creating Multi-Access Interference “MAI” at the Receiver Spreading Code Protocol  We need to decide which codes will be used for packet transmission and which codes to use for monitoring the anticipation of signal

9. CDMA/MAC, Challenges (continued) Spreading Code protocol  Three different Approaches: Receiver Based Transmitter based Hybrid of both

10. Spreading Code Protocols Receiver Based  Sender uses the code of the intended Rx. To spread the signal  Idle node will monitor its own code only.  Advantages: Simple Rx.  Disadvantage: Primary collision can happen

11. Primary Collision ABC

12. Code Spreading Protocols (continued) Transmitter Based  A different code is assigned to each node  But, the receiving node must listen to all codes  Advantages: Avoids Primary Collision Simplified Broadcast  Disadvantage: Increased complexity of the Rx stage.

13. Code Spreading Protocols (continued) Hybrid Protocols:  Prevalent Approach Fields of the packet are spread using a common code. Other fields are spread by a receiver or a transmitter based mechanism  In the reservation based schemes: a code is used for RTS/CTS Another code for data exchange  Receivers will listen to the common code If a receiver was intended by the tx. Switch to own (or tx.) code to receive the signal  Example: RA-CDMA

14. RA-CDMA/ MAC Reservation based. Uses hybrid code assignment scheme Based on Randomly accessing the channel “starting with RTS and CTS” With proper code assignment, it can avoid primary collision However, the non-zero cross-correlation between codes  Increased MAI at Rx.  Can lead to Secondary collision -> Near Far Problem

15. Near-Far Problem (Secondary Collision) AB C D Near Far High-MAI F G Secondary Collision High-MAI Secondary Collision

16. Effect of Near-Far Problem Probability of Transmission Expected Progress per Hop N= 5 N= 10 N= 20

17. The Proposed Protocol: CA-CDMA Main Goals:  To provide a CDMA-based MAC solution that addresses the near-far problem  A Protocol that can achieve enhanced throughput keeping the same energy requirement Basic idea  a distributed admission and feedback among nodes

18. CA-CDMA AB C D Near Far High-MAI Secondary Collision

19. Design Goals Asynchronous, distributed, scalable solution for large Networks (Matches MANET environment) Receiver stage shouldn’t be overly complex (Rx. Based spreading code) Adapt to channel characteristics and mobility patterns Able to coupe with incorrect code assignment “code assignment is left to the upper layers”

20. Design Architecture Two Separate Frequency channels (FDM-like partitioning) - one for the RTS/CTS and the other for data exchange Common Spreading Code for the control channel Receiver Based spreading codes for the data channel Codes are not assumed to be orthogonal Control and data channels are completely orthogonal

21. Design Architecture (Continued)

22. Assumptions Control and data channels are completely orthogonal Channel gain is stationary for the duration of the control and data packet Tx. Period. Gain is same in both directions Data packets between pair of terminals observe similar gain The radio stage can provide a feedback to the upper MAC layer (about the interference level) “both ways”

23. Protocol Description Contention based Uses a variant of RTS/CTS reservation scheme RTS and CTS are spread using a common code and transmitted over the control channel using fixed power P max RTS and CTS are heard by potentially interfering nodes, however, these nodes are allowed to transmit based on some constraints For the Data channel, Receive and Transmitter should agree on:  Spreading Code: “code assignment is dealt with at upper layers”  Transmit Power Choice of power is critical and represents a trade-off between link quality and (Max. Allowable Interference)

24. Protocol Description (continued) In addition, the protocol incorporates an Interference Margin into the power computation This margin Allows nodes at some distance from a Rx. to start new transmissions in the future. Nodes exploit the knowledge of the power level of the overheard RTS and CTS transmissions to compute this Margin Distributed Admission Control: A transmitter can decide when and at what power it can transmit without disturbing ongoing transmissions in its surrounding and at the same time ensuring enough power at the receiver given the current “MAI at the receiver”. Distribute Feedback to neighbors, through the CTS messages.

25. The Interference Margin Allows nodes at some distance from a Rx. to start new transmissions in the future. Highly dependent on the network load which is Expressed in terms of the noise rise:

26. Interference Margin (continued) Min Power required at the receiver:  Is the min required power level required at the Rx level to achieve the min (Eb/N)–  Higher load -> Higher noise rise -> need more Tx. Power to achieve required Eb/N Increasing the Transmit power is constrained by two factors:  Energy Consumptions restrictions in MANETs  FCC regulations

27. Interference Margin (continued) To coupe with the first constraint:  Choose the noise rise to the so that the needed Tx. Power to cover the same max distance covered by the 802.11. To coupe with the second constraint:  Choose the noise rise so that the required energy per bit is the same as for 802.11  Given that the rate used by CA-CDMA for data is less than 802.11 then for a a control channel that occupies 20% of the total available BW. -> 6dB max. allowable noise rise !!

28. Distributed Admission Control How the channel is accessed? Goal:  Allow only Transmissions that cause neither primary nor secondary collisions RTS and CTS packets are used to:  Allow nodes to estimate the channel gains between the Tx. and the Rx.  A receiver uses CTS to notify its neighbors of the additional interference noise “allowable noise rise” it can accept without impacting its current reception  Each node keeps listening to the control channel regardless of the signal destination in order to keep track of the number of active nodes and the interference levels

29. Admission Control (continued) A B RTS, P max RTS, P (A) map G (AB) = P (A) Rx. /P max. P (AB) min = M*(P th + MAI)/G (AB). P (AB) allowed = NR M*P th /G (AB). If (P (AB) min. > P (AB) allowed. ) : MAI >>> Suspend A’s Tx. Else: respond with +ve CTS If (P (AB) allowed. < P (AB) Max. allowed. ) : transmit data at P (AB) Max. allowed P (AB) allowed F G K L K L Allowable Noise Rise

30. Simulation: Event Based CSIM simulation tool

31. Simulation Results Packet Generation Rate Network Throughput CA-CDMA 802.11

32. Conclusion & Future Work Conclusion:  CA-CDMA is a distributed power control CDMA based MAC protocol.  CA-CDMA provides an enhancement for the throughput in MANETs through addressing the near far problem Future Work:  Combine CA-CDMA with other capacity optimization schemes -> directional antennas for example  Multi-rate support is also another opportunity for capacity optimization  Devise better schemes for access control over the control channel

33. THANKS !!