 Hello friends, myself, Darshan Pandit, Assistant Professor, Department of Computer Science and Engineering from Walchand Institute of Technology in Solarpur. So today we are going to discuss about CDMA scheme in medium access control. So the learning outcome is at the end of this session student will be able to describe CDMA mechanism that regulates user access to the medium. So in this we are going to see what is code division multiple access and spread aloha multiple access. So in CDMA scheme all terminal send on same frequency at same time using bandwidth of transmission channel. So in this each sender has a unique random number and this sender exhausts the signal with the random number. Remember that receiver can tune into the signal if he knows the pseudo random number. So this is simple concept of CDMA. So the advantage of CDMA is higher complexity of receiver as receiver cannot just listen into medium and start receiving if there is a signal. So receiver has to know the pseudo random number. So another disadvantage is all signal should have same strength at receiver. So advantage of this scheme is all terminal can use same frequency and no planning is required. So CDMA provides huge code space when compared to frequency space and in this interference is not coded and also forward and error correction and encryption techniques can be easily integrated in the CDMA scheme. So as an example you can see so sender A and sender B we are having two senders. So sender A is sending data equal to 1, sender B is sending data equal to 0. So both signal get superimposed in the space. So to identify the data from A and data from B we require a unique code that is pseudo random number. So let us see an example where two senders A and B want to send data then CDMA assigns orthogonal key sequence AK equal to 0 1 0 0 1 1 to sender A and BK that is key BK equal to 1 1 0 1 0 1 for sender B. Now sender A wants to send data that is AD equal to 1 and sender B that is BD equal to 0. So in this let us assume that binary 0 as minus 1 and binary 1 as plus 1. So now both sender spread their signal using their key as a chipping sequence. So here chipping sequence are applied to single bit for spreading. So now sender A then sends the signal as AS, AS equal to AD into AK. So the resulting data will be minus 1 plus 1 minus 1 minus 1 plus 1 plus 1. After that sender B does the same with the data to spread the signal. So it will be like BS equal to BD into BK. So resulting data will be minus 1 minus 1 plus 1 minus 1 plus 1 and minus 1. So here both signals are transmitted that is sender A and sender B both signal then transmitted at same time using same frequency. So this signal get superimposed in the space. So we get signal C, C equal to AS plus BS. So the resulting data will be minus 2 0 0 minus 2 plus 2 0. So now if receiver want to know data from sender A, so it has to tune into code of A that is C into AK. So C is minus 2 0 0 minus 2 plus 2 0. So this is multiplied with AK that is minus 1 plus 1 minus 1 minus 1 plus 1 and plus 1. So the resulting signal will be 2 plus 0 plus 0 plus 2 plus 2 plus 0. So the resulting data will be 6. So as the result is much larger than 0 the receiver detects as binary 1 that is send by sender A. Now tuning into sender B that is if you want to know data from sender B then B code is applied that is BK C into BK that gives you result of 2 plus 0 plus 0 minus 2 minus 2 plus 0. So the resulting data will be minus 6. So here the result is negative, so 0 has been detected. So that is this is the original data sent by sender B that is 0 and same is detected at the receiver. So in this way we can apply code division multiple access to identify the data received from sender A and sender B which was superimposed in the space. So let us see the next scheme that is spread Aloha multiple access. So here CDMA sender and receiver are not really simple device. So communicating with N devices require programming of receiver that it should be able to decode N different code that is while communicating with N device CDMA scheme require N different pseudo random number that is N unique random numbers. So the wireless and mobile devices communicate with base station only. So here Aloha was very simple scheme but could only provide a relatively low bandwidth due to collision. So spread Aloha multiple access it is a combination of spreading of CDMA plus medium access of Aloha. So in this we use CDMA with only a single code that is without collision detection. So in this diagram you can see spread Aloha multiple access which is a combination of CDMA and TDMA that is code division multiple access and time division multiple access. So in this Aloha has only low efficiency so it has very low efficiency and CDMA needs complex receiver to be able to receive different sender with individual code at a same time. So in this spread Aloha multiple access works as so in this you can see each sender uses same spreading code that is sender A and sender B access medium at a same time in their narrowband spectrum. So all three bits shown as a collision so in the diagram you can see. So to avoid this collision same data could be also sent with higher power and for a shorter period. Now spread spectrum is used to spread shorter signal that is to increase bandwidth that is spreading factor S equal to 6 in this example. So in this way we can send data using spread Aloha multiple access that is without collision. So advantage of this scheme is finding good chipping sequence so that is the disadvantage and so that is for each sender has to find a good chipping sequence. So that is one of the disadvantage and advantage is like this scheme is very robust against narrowband interference and simple coexistence with other system in the same frequency band. So you can pause the video here and answer the question how are guard space realized between users in CDMA. So the answer is like guard space between user in CDMA system is the orthogonality that is single data bit is spread over a longer sequence of transmitted bit between spreading codes. So the lower the correlation is the better is the user separation. So in this orthogonality bit that is pseudo random number are used as a guard space. So these are the references which are used to create this video. Thank you.