1. 1 CDMA-Based MAC Protocol for Wireless Ad Hoc Networks Alaa Muqattash and Marwan Krunz Department of Electrical and Computer Engineering The Unniversity of Arizona Tucson, Arizona 85719 alaa,krunz@ece.arizona.edu

2. 2 Goal Propose a CDMA-based power controlled MAC protocol for mobile ad hoc networks Improving the network throughput of a MANET Maintaining low energy consumption

3. 3 Outline Introduction Near-far Problem In RA-CDMA The Proposed CA-CDMA Protocol Simulation Conclusions

4. 4 Introduction Challenges in current MANETs What is CDMA? Why apply CDMA technology to MANET? Preparation for using CDMA-based solutions

5. 5 Challenges in Current MANETs Increase the overall network throughput Maintaining low energy consumption for packet processing and communications

6. 6 What is CDMA? A spread spectrum technology Each user occupies the entire available bandwidth The transmitter’s signal is multiplied by a Pseudo-Random noise(PN) code. The receiver despreads the received signal using a locally generated PN code The PN code is distinct for each signal

7. 7 Why CDMA? Advantages of CDMA – Achieve much higher channel bandwidth efficiency for a given wireless spectrum allocation – Overcome strong intentional interference – Has been widely adopted in popular cellular systems(for example, 3G systems)

8. 8 Concurrent transmission Problem in IEEE802.11 IEEE 802.11 uses SS technology at physical level Since all signals are spread using a common PN code, concurrent transmissions are rejected in the a vicinity of a receiver

9. 9 Concurrent transmission Problem in IEEE802.11 Example — Figure 1 A → B and C → D cannot take place at the same time Figure 1

10. 10 Introduce CDMA to MAC Protocol To increase network throughput, we try to apply CDMA technology to MAC protocol

11. 11 Preparation Designing a code assignment protocol Assign distinct codes to different terminals Meet the requirement that all neighbor nodes of a node have different PN codes Deciding a spreading-code protocol Decide codes used for transmission and for monitoring the channel in packet reception Can be receiver based, transmitter-based, or a hybrid

12. 12 Near-far Problem In RA- CDMA (random access CDMA) Limitation of previously proposed CDMA- based MAC protocols Imperfect Orthogonality of CDMA Codes Impact of the MAI Problem on network throughput

13. 13 Previously Proposed CDMA- based MAC Protocols Based on random channel access – A terminal can transmit a packet immediately disregarding the state of the channel – Called Random access CDMA (RA-CDMA) Limitation: Near-far problem – Although RA-CDMA are free of primary collisions, multi-access interference (MAI) can lead to secondary collisions at a receiver

14. 14 Near-far problem When all transmission powers are equal, if the receiver is much closer in distance to transmitter STA1 than STA2, the signal of STA1 will arrive at the receiver with a sufficiently larger power than that of the STA2, causing incorrect decoding of the transmission STA2(i.e., a secondary collision).

15. 15 Near-far problem(A Example) Figure 2

16. 16 Imperfect Orthogonality of CDMA Codes Reasons for near-far problem – Cross correlations between CDMA codes are nonzero, which can induce multi-access interference MANETs are time-asynchronous – Signals originate from multiple transmitters – It is generally not feasible to have a common time reference for all the transmissions that arrive at a receiver

17. 17 Imperfect Orthogonality of CDMA Codes MANETs are time-asynchronous – Transmissions propagate through different paths, so they have different time delays In an asynchronous system, it is not possible to design spreading codes that are completely orthogonal for all time offsets

18. 18 Impact of the MAI Problem The near-far problem can severely affect packet reception, and consequently, network throughput. A measure of network throughput – EFP : the expected forward progress per transmission, defined as the product of the local throughput of a terminal and the distance between the transmitter and the receiver

19. 19 Impact of the MAI Problem – P: the probability that a terminal is transmitting a packet in a given time slot – L : the number of nodes that are within a circle centered at the transmitter and of radius that equals the transmitter-receiver separation distance. Example: Figure 3: Throughput performance versus load in RA-CDMA networks

20. 20 Impact of the MAI Problem

21. 21 Impact of the MAI Problem EFP starts to decrease rapidly when the load exceed P* Our objective – Designing a CDMA-based MAC protocol that prevents this rapid degradation in network throughput

22. 22 The Proposed CA-CDMA Protocol Designing Principles Architecture Channel Model Controlled Access CDMA Protocol Interference Margin Channel Access Mechanism Protocol Recovery

23. 23 Designing Principles In CDMA Cellular Systems – Open- loop and closed-loop power control are employed to have the signals of all STAs arrive at the base stations with the same power The same solution cannot be used in MANETs. – In some cases multiple transmissions cannot take place simultaneously – Example: Figure 4

24. 24 Designing Principles Figure 4 If A increases its power to combat the MAI at B, then this increased power will destroy the reception at D Power control alone is not enough to combat the near-far problem in MANETs.

25. 25 Solve Two Problems Medium access problem – It may not be possible for two transmissions that use two different spreading codes to occur simultaneously Power control problem – Solution: The two transmission can occur simultaneously if the terminals adjust their signal powers so that the interference caused by one transmission is not large enough to destroy packet reception at other terminals

26. 26 Architecture Two frequency channels – One control channel – One data channel Spreading code – A common spreading code is used by all nodes over the control channel – Several terminal-specific codes can be used over the data channel Signal over the control channel is completely orthogonal to any signal over the data channel

27. 27 Architecture Figure 5: Data and control codes in the proposed protocol

28. 28 Protocol Assumptions The channel gain is stationary for the duration of the control and the ensuing data packet transmission periods The gain between two terminals is the same in both directions Data and control packets between a pair of terminals observe similar channel gains. Each terminal is equipped with two transceivers and a carrier-sense hardware that senses the control channel for any carrier signal

29. 29 Protocol Description RTS and CTS packets are transmitted over the control channel (on the common code) at a fixed (maximum) power Pmax Interfering nodes may be allowed to transmit concurrently The receiver and the transmitter must agree on two parameters: the spreading code and the transmission power Interference margin allows terminals at some interfering distance from the intended receiver to start new transmissions in the future

30. 30 Protocol Description The power level is critical and represents a tradeoff between link quality and MAI Apply a distributed admission control strategy that decides when terminals at some distance can transmit concurrently

31. 31 Compute the Interference Margin Minimum required received power (P 0 (i) ) min – To achieve the target error rate, we have P 0 (i) /(P thermal + P MAI (i) ) ≥ μ *, (1) μ * —effective bit energy-to-noise spectral density ratio E b /N 0eff,that is needed to achieve the target error rate P 0 (i) — the average received power of the desired signal at the ith terminal; P thermal — the thermal noise power P MAI (i) — the total MAI at receiver i (P 0 (i) ) min = μ * (P thermal + P MAI (i) ) (2)

32. 32 Noise Rise The interference margin depends on the network load, which itself can be conveyed in terms of the noise rise (ξ (i) ) ξ (i) = ( E b /N 0 ) unloaded / ( E b /N 0 ) loaded ) (3) = (P thermal + P MAI (i) )/ P thermal Thus (P 0 (i) ) min = ξ (i) μ * P thermal (4) The maximum planned noise rise is set as ξ (max),

33. 33 Interference Margin Assume that the transmission power attenuates with the distance d as k/d n (k is a constant and n ≥ 2 is the loss factor). The minimum required transmit power in CA- CDMA P CA-CDMA = ξ (max) μ * P thermal d n /k (5) Assuming that d is uniformly distributed from 0 to d max,we have the expectation of PCA-CDMA E[P CA-CDMA ] = ξ (max) μ * P thermal d n max /k(n+1) (6)

34. 34 Interference Margin As for the 802.11 protocol, its corresponding transmission P 802.11 = μ * P thermal d n max /k(7) Therefore, to achieve equal average energy per bit consumption,we must have: E[P CA-CDMA ] / R CA-CDMA = P 802.11 /R 802.11,(8) R CA-CDMA and R 802.11 are the bit rates for the transmitted data packets in the CA-CDMA and 802.11 protocols, respectively.

35. 35 Interference Margin From (6)(7)(8), we have the interference margin ξ (max) =(n+1)R CA-CDMA /R 802.11,(9)

36. 36 Channel Access Mechanism The admission scheme allows only transmissions that will not cause either primary or secondary collisions to proceed concurrently. – RTS/CTS packets allow nodes to estimate the channel gains between transmitter-receiver pairs. – A receiver i uses the CTS packet to notify its neighbors of the additional noise power(denoted by P (i) noise ) that each of the neighbors can add to terminal i without impacting i’s current reception – Each terminal keeps listening to the control channel regardless of the signal destination in order to keep track of the average number of active users in their neighborhoods.

37. 37 Channel Access Mechanism Step 1 If terminal j has a packet to transmit, it sends a RTS packet over the control channel at P max, and includes in this packet the maximum allowable power level (P (j) map ) that terminal j can use that will not disturb any on going reception in j’s neighborhood. The format of the RTS packet is similar to that of the IEEE 802.11, except for an additional two-byte field that contains the P (j) map value.

38. 38 Channel Access Mechanism Step 2 -The intended receiver i receives the RTS packet, and uses the predetermined P max,value and the power of the received signal P (ji) received to estimate the channel gain G ji = P (ji) received / P max between terminals i and j at that time. -Terminal i will be able to correctly decode the data packet if transmitted at a power P (ji) min : P (ji) min = μ * (P thermal + P MAI -current (i) )/ G ji, (10) P MAI -current (i) -- the effective current MAI from all already ongoing (interfering) transmissions.

39. 39 Step 2 All neighbors of terminal i will have to defer their transmissions during terminal i’s ongoing reception According to link budget calculations(4)(5), the power that terminal j is allowed to use to send to i is P (ji) allowed = ξ max μ * P thermal / G ji, (11)

40. 40 Step 2 If P (ji) allowed < P (ji) min --MAI in the vicinity of terminal i is greater than the one allowed – i responds with a negative CTS to inform j that i cannot proceed with j’s transmission If P (ji) allowed > P (ji) min and P (ji) allowed < P (j) map n – i calculates the interference power tolerance P MAI -future (i) that it can endure from future unintended transmitters P MAI -future (i) =3W G ji (P (ji) allowed - P (ji) min )/2 μ *, (12) W---Processing gain

41. 41 Step 3  i equitably distributes this power tolerance among future potentially interfering users in the vicinity of i(to prevent one neighbor from consuming the entire P MAI -future (i) ) – Calculate the number of terminals in the vicinity of i that are to share P MAI -future (i) : K (i), K (i) = β ( K avg (i) － K inst (i) ), (if K avg (i) > K inst (i) ) K (i) = β, otherwise(13) K inst (i) － the number of simultaneous transmissions in i ’ s neighborhood K avg (i) － average of K inst (i), β> 1 is a safety margin

42. 42 Step 3 The MAI at terminal i can be split into two components: – terminals that are within the range of i ( P (i) MAI -within ), – terminals outside the range of i (P (i) MAI -other )) i cannot influence P (i) MAI -other Let P (i) noise = P (i) MAI -other, Assume that P (i) MAI -other = α ( P (i) MAI -within ), The interference tolerance P (i) noise that each future neighbor can add to terminal i is P (i) noise = P (i) MAI –future /(1+ α ) K (i), (14)

43. 43 Step 3 When responding to j’s RTS, – terminal i indicates in its CTS the power level P (ji) allowed that j must use. – i inserts P (i) noise in the CTS packet and sends this packet back to terminal j at P max over the control channel using the common code.

44. 44 Step 4 Compute P (s) map (Used in RTS) Potentially interfering terminal s hears the CTS message from i, then – compute the channel gain G si between s and i – compute the maximum power P (s) map that s can use in its future transmissions P (s) map =min( P (i) noise /G sk ) for all neighbors k of s

45. 45 Step 5,6 Step 5 – j send data to i Step 6 – If transmission is successful, receiver i responds j with an ACK packet over the data channel using the same power level that would have been used if i were to send a data packet to j.

46. 46 Protocol Recovery while receiving a data packet, terminal i hears a RTS message (destined to any terminal) that contains an allowable power P (.) map value that if used could cause an unacceptable interference with i’s ongoing reception. Then terminal i shall respond immediately with a special CTS packet over the control channel, preventing the RTS sender from commencing its transmission.

47. 47 Protocol Evaluation Evaluate both the network throughput and the energy consumption of the CA-CDMA protocol and contrast it with the IEEE 802.11 scheme Results are based on simulation experiments conducted using CSIM programs Each node generates packets accordingto a Poisson process with rate λ The routing overhead is ignored the maximum transmission range under the CA- CDMA and 802.11 protocols is the same

48. 48 Protocol Evaluation

49. 49 Simulation Results Consider two types of topologies: random grid and clustered In the random grid topology, M mobile hosts are placed across a square area of length 3000 meters. The square is split into M smaller squares. Part (a) of the figure 6 depicts the network throughput. CA-CDMA achieves up to 280% increase over the throughput of the IEEE 802.11 scheme.

50. 50 Simulation Results- random grid Part (b) of Figure 6 depicts the energy consumption versusλ. CA-CDMA requires less than 50% of the energy required under the 802.11 scheme. Part (c) of Figure 6 investigate the effect of varying the number of nodes. The throughput enhancement due to CA-CDMA increases with node density

51. 51 Simulation Results- random grid Figure 6: Performance of the CA-CDMA and the 802.11 protocols (random grid topologies)

52. 52 Simulation Results - random grid Figure 6: Performance of the CA-CDMA and the 802.11 protocols (random grid topologies)

53. 53 Simulation Results- random grid Figure 6: Performance of the CA-CDMA and the 802.11 protocols (random grid topologies)

54. 54 Simulation Results - clustered topology To generate a clustered topology, consider an area of dimensions 1000 × 1000 (in meters). Let M = 24 nodes, which are split into 4 equal groups, each occupying a 100 × 100 square in one of the corners of the complete area. For a given source node, the destination is selected from the same cluster with probability 1 − p or from a different cluster with probability p

55. 55 Simulation Results - clustered topology Part(a) of Figure 9 depicts the network throughput versusλ for p = 0.25. CA-CDMA makes three to four transmissions proceed simultaneously, results in a significant improvement in network throughput. Part (b) of the figure investigate the locality of the traffic by fixing λ and varying p. As the traffic locality p increases the enhancement of CA-CDMA increases.

56. 56 Simulation Results - Clustered Topology Figure 7: Performance of the CA-CDMA and the 802.11 protocols as a function of λ (clustered topologies)

57. 57 Figure 7: Performance of the CA-CDMA and the 802.11 protocols as a function of λ (clustered topologies) Simulation Results - clustered topology

58. 58 Conclusions CA-CDMA accounts for the multiple access interference, thereby solving the near-far problem that undermines the throughput performance in MANETs. CA-CDMA uses channel-gain information obtained from overheard RTS and CTS packets over an out-of-band control channel to dynamically bound the transmission power of mobile terminals in the vicinity of a receiver.

59. 59 Conclusions Adjusts the required transmission power for data packets to allow for interference-limited simultaneous transmissions to take place in the neighborhood of a receiving terminal Simulation results showed that CA-CDMA can improve the network throughput by up to 280% and, achieve 50% reduction in the energy consumed

60. 60 Future Work Focus on other capacity optimizations such as the use of directional antennas in CDMA-based protocols