Power Controlled Medium Access Control in Wireless Networks – The story continues

1. Power Controlled Medium Access Control in Wireless Networks – The story continues

2. [9] Power Controlled Dual Channel (PCDC) Medium Access Protocol for Wireless Ad Hoc Networks , by Alaa Muqattash and Marwan Krunz, IEEE INFOCOM 2003.

3. Objective • In previous schemes nodes compute the right power to communicate when using the DCF function of 802.11 – power adaptive extensions. • Earlier work suggests the use of maximum power to transmit RTS/CTS messages – provides energy savings but not frequency re-use. • The objective then is to achieve both – somehow nodes need to know how much power to use in order to communicate. • A new dual-channel (control channel + data channel) based scheme to do just this.

4. Key Ideas • Choose a connectivity set among the neighbors – ensure that routing updates flow only through the set – limits the next hop and hence the maximum power used. • Indicate the required power and the interference margin in the RTS/CTS packets so that nodes might decide on whether or not to initiate communications. • RTS/CTS messages are transmitted at full power but on a separate control channel so as not to interfere with data communications.

5. Inefficiency of prior schemes • High power RTS CTS messages exchanged between nodes A and B preclude the communications between nodes C and D. Figure from [9]

6. Problems with Asymmetry Figure from [9] • Using the low power RTS/CTS on the other hand leads to asymmetry – C and D initiate a high power communication without hearing the low power RTS/CTS between A and B.

7. Determination of the Connectivity Set • Each node determines within its neighborhood a sub-set of its neighbors to form the connectivity set. • The connectivity set (Csi) of a node i contains those nodes to whom direct communication requires less power than going via a two hop path. • Continuously cache estimated channel gain for every signal that it receives on the control channel.

8. Connectivity Set (Continued) Figure from [9]

9. Some Properties • It is sufficient only to ensure that a two hop lower cost path exists in order to exclude a node from the CS (Read paper for proof). • If the initial graph is connected, the new graph created by reducing the power so as to reach only those nodes in the connectivity set is connected.

10. Routing • Each node i uses the computed connectivity power P(i)conn to transmit its RREQ packet. • Of course, an on-demand routing scheme such as AODV or DSR is used. • The broadcasts are power efficient. • Reduction in contention – increases the power efficiency of the broadcast while ensuring the same coverage. • Scheme is similar to our paper in ICNP 2003 (to be presented by one of you.)

11. Channel Access Mechanism • RTS/CTS provide the following three additional functionalities: • Estimate the channel gains between transmitter-receiver pairs. • Notify neighbors of the additional interference power (denoted by P(i)noise that each of the neighbors can add to node i without impacting node i’s current reception. • They allow nodes to keep track of the connectivity set. • RTS packet is transmitted at power Pmax over the control channel. • Upon receiving this packet, the receiver estimates the channel gain : ratio of received power to transmitted power.

12. Computing the Right Power Level • Given the gain, the receiver can now estimate the minimum transmit power that the sender can use. • SNRth is the minimum signal to noise ratio that is needed to achieve the desired bit error rate. • PMAI-current accounts for current multiple access interference.

13. Prevention is better than cure! • The previous expression does not take into account future interfering transmissions that may be initiated in the vicinity of the communication. • In order to allow for this, the requested transmit power is > the minimum required. • The minimum required is scaled up by a factor a(i)min where a(i)min >= 1. • Therefore :

14. What is next ? • If the maximum allowable power level for the transmitter that the sender can use P(j)map (indicated in the RTS) is smaller than that to be requested the receiver does not send a CTS message. • Else, a CTS is sent and the requested power (lower than the allowable power) is indicated in the CTS message.

15. Tolerating additional interference • Scaling up the requested transmission power implies that additional multiple access interference can be tolerated by the node. For achieving the threshold SNR, the noise factor was h(i). • Now, since the signal power is higher, a larger noise can be tolerated – how much more ? – what you can tolerate minus what is already the noise:

16. How many more interferers can I tolerate? • Node i keeps track of the number of interferers at each sampling instant K(i)inst. • It also keeps a moving average K(i)avg. • Then, it calculates the number of interferers that it can allow by:

17. What does this mean? • Prior to the instant the first term gives an estimate of the the change in the number of interferers from the previous instance – if this number went down, we can have more. • If the number of instantaneous interferers is more than the average then, it looks like most of the neighbors are in the process of transmitting. We should not have many more interferers. • They limit the additional number in this case to a small constant b.

18. Computing the Interference Margin and using it. • Once K(i) is known, the interference tolerance is given by: • This is then inserted along with requested power in CTS packet Figure from [9]

19. Using this information • Any neighbor that hear this CTS message, compute the associated gain and then can compute the maximum allowable power that it can use for future communications without interfering with “that neighbor”. • It then computes the minimum of such computed P(k)map for each of its neighbors k and chooses the minimum of these powers as its maximum allowable power.

20. Other Titbits • The authors provide a implementation version. • How is a(i) computed based on battery life ? – again look at paper – uses a battery model.

21. Reported Performance Results • They show that the scheme greatly outperforms the IEEE 802.11 MAC. • Lower Power, higher channel utilization, lower delay due to reduced contention.

22. What else ? • How much bandwidth to allocate for the control channel ? Unclear. • The estimation of future multiple access interference can be tricky – it is really an estimation at best! • The model tries to take all communications into account when computing the transmit power range – if this range is different for communicating with different nodes, it is unclear what problems will arise. • Macroscopic performance metrics – we need greater insight.

23. Bottomline: A new scheme should emerge 