Contents
Application of CDMA
CDMA is much more efficient as compared to FDMA or TDMA as it increases network capacity five to six times compared to the other two. Along this the power controlled CDMA gives enhanced system throughput as compared to power controlled protocols in non-spreading systems. This is due to the fact that in a non-spreading case, the W/R ratio is unity (see eqn. 1) (as R equals W here) and Eb/I0 is equal to the ratio of the total signal power received divided by the total interference power received due to the other users in the network and the background thermal noise while in the CDMA, the W/R ratio is much greater than unity because of the spreading of signal . So from Eqn. 1, we can see that the received signal-to-interference ratio is multiplied by the factor of W/R. Since minimum required Eb/I0 ratio for correct reception is typically a fixed number depending on the basis of receiver design, the tolerance level of the receiver increases because the signal power is multiplied by the processing gain. Hence, the receiver is able to decode packets which otherwise would not have been received correctly. This allows more number of simultaneous transmissions in single hop area for adhoc network. Thus power controlled CDMA MAC for adhoc networks combines the advantages of both variable transmission range protocols as well as spread spectrum multi-access communication.
Design aspects
1) Excess bandwidth requirements – Are We Through???CDMA has additional requirement of larger bandwidth because of signal-spreading. So, CDMA-based network requires more resources in terms of bandwidth as compared to non-spread spectrum based network. Hence, they are beneficial only when the gain offered by CDMA compensates the excess bandwidth requirement. 2) Code Assignment – Needs Mental Perspicacity!!!
Lack of centralized control makes the code assignment challenging. If the network is small then a fixed code can be assigned to every node. But for large networks there should be some way to assign codes dynamically (ensuring reuse of the codes ) such that immediate neighbors get unique codes. Spreading Protocols
In spread spectrum adhoc networks, the code assignment scheme determines the code to be used for transmission. There are four basic type of code assignment.
a) Common code: Here all nodes are assigned a common code, and hence all transmissions are done on a single spreading code. In this case, multiple simultaneous transmissions are not feasible.
b) Receiver based: In this scheme, each node is assigned a code. The transmitter uses the code of the intended receiver to spread the packet, while an idle terminal constantly monitors its own code. This approach simplifies the receiver’s circuitry because the receiver does not have to monitor the whole code set. Collisions may occur at the receiver, when two transmitters transmit at the same time for the same receiver using the receiver’s code.
c) Transmitter based: In a transmitter-based spreading protocol, a transmission code is assigned to each terminal, and receivers must be able to monitor the activity on the whole set of PN codes. The advantage of this approach is that collisions cannot happen at the receiver. In addition, broadcast is inherently supported. However, the drawback is that the receiver circuitry is very complex and expensive.
d) Hybrid Schemes: Various hybrids of the above two approaches are also possible where headers of data packets containing destination and source address can be sent in common code or in receiver code and the data in sender’s code. This helps receiver to tune to the particular sender code in next time slot. One of the issues, which can be observed in aforementioned schemes, is that receivers and transmitters required to know the spreading code associated with an another node and this association can be known statically or dynamically. In adhoc network’s self-configuring mode dynamic approach is suitable as new nodes are expected to join and leave arbitrarily.
4)orthogonal OR non-orthogonal codes??? Orthogonal codes require perfect synchronization among the nodes- something difficult to realize in a perfectly non-centralized domain. Also, multipath effect in cellular network introduces interference that is not orthogonal to the transmitted signal. So, it is wiser to use non-orthogonal codes with a small cross-correlation factor which would eliminate the need for perfect synchronization. But then non-zero cross correlation of the codes will generate certain amount of the MAI in adjacent receivers.
USE OF ADHOC NETWORKS IN REAL LIFE
Military environments:-
Infrastructure typically break down in military areas. In such areas Adhoc networks are the only rescue. In addition to this adhoc networks provide good networking facilities in case of soldiers, tanks, planes for example.Civilian environments
There are cases where infrastructure needed is either too small or no infrastructure is needed at all. Eg:-Taxi cab network work with the help of adhoc networks without any infrastructure. Unplanned meetings not relying on pre-infrastructural work, depend on adhoc networks.Personal area networking
The availability of several mobile computing devices and handheld devices have increased the use of WLANs which have typical fearture of adhoc networks. Egs:-cell phone, laptop, ear phone, small aircraft.Emergency operations:-
In such cases, due to no time for infrastructure setup, adhoc networks prove to be a great help. Egs.:-search-and-rescue, policing and fire fighting
