 Hello. Welcome to the session on multiplexing. This is Dhyanand Patil, a faculty member of Computer Science and Engineering Department, Valchan Institute of Technology, Solapur. At the end of this session, you will be able to understand multiplexing or generalization in wireless communication. What is multiplexing? It is a multiple access method in which the available bandwidth of link is shared in a time frequency or through code. That is, the available spectrum is divided into the different channels. The different channels can be used by the different stations with the different characteristics like time, frequency or through code assigned with each station transmitting the data on the spectrum. Multiplexing is a technique in which multiple simultaneous analog or digital signals are transmitted across a single data link. When multiple senders try to send over a single medium, a device called multiplexer is used at the sender side that the physical channel and allocates one to each, that the available spectrum is divided and the divided physical channels are allocated to the each stations. On the other end of the communication, at the receiver side, the de-multiplexer that receives data from a single medium identifies each and sends to the different receivers. The multiplexer will divide the channel and allocates to each station and the de-multiplexer will receive the data from different stations that transmitted on a single medium and identified and sends to the appropriate stations. The multiplexing techniques, there are four basic multiplexing techniques FDMA, TDM, CDM, SDM or it is also called as FDMA, TDMA, CDMA, SDMA. That FDM stands for frequency division multiplexing. Sometimes it is referred as frequency division multiple axis. Similarly for time division multiplexing or time division multiple axis, these are the four techniques used for the multiplexing or channelization. The first one is the frequency division multiplexing or FDM. In FDM, available bandwidth is divided into frequency bands. Each station is allocated a band to the send its data and it belongs to the station all the time. That available spectrum or the entire bandwidth is divided into the different channels that each channel is assigned to a different stations that station can independently use that particular channel or that particular channel at all the time. This is the best example is radio stations that each station is assigned with some frequency at that frequency that particular station will broadcast the programs. FDM is used because of the fact that number of frequency band can work simultaneously without any time constraint. If we are dividing the frequency and assigning channel to the different stations, all the stations can simultaneously work without any time constraint. At any time those particular stations can send the data on that frequency band at particular range at any time. The FDM can be used for radio stations in a particular region as every radio station will have their own frequency and can work simultaneously without having any constraint of time. Already we are familiar with the radios and tuning the radio buttons. So, this is the best example for frequency division multiplexing. Here we can visualize from this diagram. So, that is the available frequency is divided into the different frequency band. Assume that there are 5 stations C1, C2, C3, C4, C5. So, each station is assigned with that particular frequency range. At this frequency range that station C1 can transmit the data, C2 can transmit the next range assigned. So, these are the different that same that available frequency is divided into the different region and assigned to the particular stations. So, these stations can use at all time the available frequency. This is time division multiplexing. So, in this method the stations share the bandwidth of a channel in time that each station is allocated a time slot during which it can send data. So, instead of in the previous technique we were divided the frequency and assigned to each station. But here the entire frequency is assigned to each station during some time slot. At that time only that station can use the entire available frequency. So, there is no division in a frequency in during that particular time slot assigned for that station only that station can send data over the medium. So, here the time is divided in the previous one the frequency was divided in this technique the time is divided. So, during this time slot the station C5 can transmit data on the frequency. So, that is single channel other any stations cannot send data during this time slot. The next time slot that C4 can send data during this time slot. So, assuming that there are 5 stations the time slot is divided in these 5 stations and at that time that particular station can use the entire frequency band. In TDMA the bandwidth is just one channel that is time shared between the different stations. In the previous one the bandwidth was divided into different channels and shared by all the stations at a time at particular frequency. In this technique the bandwidth is just a single channel that frequency is single channel that single channel entire channel can be used by particular stations at different time slot assigned for those channels. Next one is the code division multiplexing. In the code division multiplexing that each station will be assigned with a different code. Here the frequency is acting as a single channel that is the shared media that media is acting as a single, but it is shared between the different stations while any station is transmitting data on that particular media that particular media it is transmitting data with its own code, but all can simultaneously transmit data on the single channel only with each but each station is sending its data with its code that will be helpful at the receiver. So, it is used in cell phone spectrum technology. So, this is here we can visualize at any time all the stations can send data on the entire available frequency, but the each of these station will be having its own unique code that code will be sent through its data on a single medium. You can see this in detail. So, in the code division multiplexing in this method one channel carries all transmission simultaneously because there is a single channel, but the each channel that but all the stations are sending data on the single channel only with its own code. Each channel transmit data with different code while transmitting the data there are two properties to recognize proper code. These two properties helps the receiver to identify the data sent by that particular station. If codes are multiplied with each other then the answer is zero. If codes are multiplied by itself we will get the four, but here the four stands for the number of stations. If there are five stations we can use it is five so it depends on. So, these two properties helps to recognize the code at the receiver side. So, assume that there are four stations the station one, station two, station three and station four. So, each of these station is having the code C1, C2, C3, C4. So, according to the properties if the codes are multiplied with each other we will get zero. The codes are multiplied by itself we will get four. So, assume that the stations one sending the code C1. If the C1 is multiplied with each other means other stations we will get zero C1 into C2 equal to zero C1 into C1 is four because if the codes are multiplied by itself we will get four. So, if C2 is multiplied with any other code that is C4 it will get zero. If the C2 is multiplied by itself we will get the four. The code is used in such a way that if codes are multiplied with each other we will get zero. If codes are multiplied by itself we will get one. So, we can form different cases. So, these are the some examples. So, here we can see the example the station one is transmitting a data D1 with a code C1 that C1 into D1. So, that is the data is multiplied with a code and transmitted on a single media. The station two is sending a data two on the media that is multiplied with a code C2 that C2 into D2 is transmitted. Similarly, for other station the station three is transmitting the data C3 into D3 the station four is transmitting C4 into D4. So, all the station can send the data on a single channel simultaneously. So, this is the users with a different code user one with a code one user with a code two. So, all can send on the single channel. So, and this is received at the particular side. We can see the how it is recognized at the receiver side. So, now how stations two can recognize the station one. So, by using the given properties you think and write the answer for the given question. The question is how S2 can recognize the S2 Sd1 data transmitted on the media. Now, you pause the video think and write the answer. The answer is if stations two wants to listen what Sd1 says then that according to the properties. So, first that all station can send data on the media. We can see from the previous slide all the stations are sending on the single channel. So, this is to receive the data from that particular channel that S2 S2 two will multiply the code with one because the S2 want to listen what Sd1 says. So, it is going to multiply the code of the station one that is C1. So, that entire data transmitted on the channel and multiplied with a C1. So, this is the simple mathematics. So, now it is multiplied the C1 into C1 is 4 because we are multiplying itself it is 4 it is multiplied with the other code. So, that is 0 into d2 0 into d3. So, we are obtaining 4d1. So, then S2 two divides the this obtained data by 4. So, after dividing we will get the d1. So, d1 is issued at the station 2. So, this is the comparison between the different techniques. These are the references. Thank you.