[Music] welcome to wireless communication lecture Series in this video you will learn about introduction to multiple access techniques now before we start this topic I will like to give you one simple example that will help you to understand about the multiple access technique let us consider that we have a true measurement scale and there are two persons who want to use this scale now this problem is really simple as we have a two scale we can allocate a one scale to first person and another scale to second person so in this way this both person will be able to use the both the skill but let's say we have two more person over here and they also want to use the same skill now here the problem will arise and we need to come up with a some good idea to share this limited resources among the multiple persons now same situation will happen in the cellular system also so here we will have a base station it will have some limited frequency band and with this limited frequency band it need to serve the multiple Mobile station within the cell now for this we need to provide some proper Solutions so that multiple Mobile station can access this base station and they can communicate effectively so for that we required a multiple access technique so in order to implement the multiple access technique usually we will require a duplexing so duplexing will allow a simultaneous transmission as well as the reception of the information and that is very important we can achieve a multiple access Tech technique in a two domain one is a frequency domain another is a Time domain so in frequency domain what we will do we will divide the frequency that means that technique will be called as a frequency division duplexing FDD and in time domain we will divide the resources in terms of a time that means the time division duplexing so here in this example if you want to apply a frequency division duplexing then we may need to divide this scale into four different parts right and we can allocate the same amount of part to all these four person and if you want to apply the time division duplexing then we can allocate this full scale to one person for some amount of time then we will allocate the same scale to next person for the again some amount of time and so on we will switch so whole resources used by the person for a limited amount of time and that we will call as a Time division so let us first start with frequency division duplexing FDD so it consist of two Simplex Channel we will required a two channel for the FDD one for the forward link and another for the reverse link so forward link means the signal coming from base station to the Mobile station it is also called as a down link and the reverse link is a Mobile station to base station it is also called as a up link here both the up link and down link or the forward link and reverse link will use a separate frequency so and for that we will require a duplexer so duplexer is required to allow allow simultaneous bir directional transmission so at a time the mobile will mobile can also transmit the signal as well as receive the signal and frequency separation between two channel is constant through the system we need to separate out the both the frequency so to understand this concept in a better manner let us consider this diagram so here we have a transmitter it has a band pass filter and the frequency we are using for transmission is let's say F1 so this will this Mobile station will transmit information with the F1 frequency and when this base station will receive that frequency it will again use the bend pass filter for the F1 frequency and it will receive it simultaneously if B base station want to transmit something it will use a different frequency that is F2 frequency it will transmit to the Mobile station and Mobile station will again use a band pass filter for the F2 frequency and it will receive the signal so in this way both Mobile station and the base station can receive the signal simultaneously at a different frequency so we have divide the resources in terms of a frequency now let us consider the example of time division duplexing so in time division duplexing instead of using different Channel at a different frequency we will have a Channel at the same frequency so here forward link and the reverse link both will have a same frequency so it used time for a forward and the reverse link instead of a division of frequency we will divide in terms of a time let's say first there will be a reception of the forward link and then we will receive the reverse link so multiple users share a single Channel by taking turn in the time domain so it will receive a signal one by one each user assign a time slot each duplex channel has both forward and the r RSE time slot to provide a bir directional transmission so here you can also receive the signal in one time slot in another time slot you can send the signal the separation between the forward and reverse time slot is important otherwise it will overlap each other so the one important advantage of time division duplexing is we do not require the duplexer because it communicate at the single frequency same frequency so here you can see the diagram of time division duplexing the Mobile station will have a same frequency as the base station and both are need to synchronize for a reception as well as the transmission of the message so whenever Mobile station want to transmit something it will send with F1 frequency and at the same time the Bas station will receive that signal and when base station want to transmit some information it will again use the this F1 frequency but at a different time slot and then this will receive that signal so overall we need a synchronization between the transmitter and the reception so overall if we summarize FDD and tdd the FDD will required a duplexer because it consist of two Simplex Channel at a different frequency and here we will have only single channel in the tdd and it doesn't require a duplexer but FDD is a full duplex communication at a time you can transmit as well as receive the signal and the transmission in the tdd is not full duplex right because we are transmitting or receiving the signal in terms of a time slot so here in FDD frequency separation must be controlled carefully so that there will be no interference and here we need to take care about the time slot because if time slot will overlap each other that can also cause an issue now this FDD can be used for the long distance transmission because they both are working at a different frequency so there will be no issue of delay due to the long distance but here if we have a long distance the delay will be added into signal and that may create an issue of interference now this FDD user provide a separate frequency and and the inity we will have a time slot separate time slot for the each user so this is between the FTD and tdd and in the end uh in this chapter we will going to discuss about the frequency division multiple access method time division multiple access method and code division multiple access method so all these three topic will be covered in the next lecture thank you so much for watching this video