Now all these users were users like your phones or your tablets. But what about the cell tower or the base station as we have talked about earlier, does it utilize the same resources as your phone, or does it utilize different resources from your phone? Well, the answer is, it depends. That leads us to two distinct variants of wireless communication called FDD and TDD, which respectively stand for frequency division duplexing and time division duplexing. At its heart, they are simply two ways to share the limited and expensive spectrum. But in this particular context, they aren't necessarily the way to share the spectrum among other users on the phone. Rather, they are other ways to share spectrum between the cell tower or the base station and the rest of the phones. Let's look at what those are one-by-one at a high level. FDD, once again, it stands for frequency division duplexing, entails and requires rather a pair of radio channels for simultaneous two-way communication. For example, if you have abundant frequency spectrum, you could allocate one channel on which your cell tower would transmit. You could allocate a different independent frequency channel on which your phone would transmit. In technical parlance, now the transmissions that are sent by the base station or the cell tower are called downlink and the transmissions that are sent by your phone are called uplink. It is called so because traditionally base station is located at a larger height, it is taller than normal users, and hence, the signal from the base station comes down to you. Whereas, when your phone transmits a signal to a base station because base station is located at a higher level, the phone signal has to travel upwards, so to speak. That's why the name downlink, whereas this is uplink. In FDD, your system is rich enough to buy two separate channels. One channel, wherein only the base station will transmit and the phone will only receive on the channel. Whereas on the second uplink channel, it is only the phone that is allowed to transmit, and the base station will only need to receive on that channel. There is separation of responsibilities in the frequency domain, so to speak. There are, as always, pros and cons. The pros are that, as the first point mentions here, because those two are independent channels, it allows for simultaneous two-way communications. In that the base station can transmit something to the phone on the downlink. At the same time while the phone is listening, it can also try to send some information to the same base station on the uplink, thus allow for two-way communication. However, the basic disadvantage of this FDD mechanism is that you have to have enough budget in order to buy two separate frequency channels. Not only that, your local regulatory authorities have to have abundance spectrum available in order to allocate two independent channels for your system. One for downlink, one for uplink. But regardless of those pros and cons, one fundamental factor about FDD is what attribute of the frequency channel it leverages. Keep in mind, I mentioned that multiple access is about leveraging certain attributes of the wireless channel. In case of FDD, it goes without saying that it leverages the frequency attribute of the channel in that it separates the uplink and downlink transmissions in frequency by making those go on two different channels. That is the fundamental premise of FDD. The other side of the coin would be TDD, also known as time division duplexing. As you may have guessed, it is called so because it leverages the time aspect of the wireless channel. The hallmark of a TDD system is that, unlike an FDD system, a TDD system has access to only one frequency channel. There are no two frequency channels anymore. Both the base station and your phones have to operate on the same frequency channel. Single radio channel is the basic characteristic of a TDD system. But if you have to accommodate transmissions in two different directions on the same channel, how can you make that happen? Well, that is where leveraging the time attribute of the channel comes into picture. For example, on the same channel, you might have the base station transmits from, let's say, time t_1 to t_2. From time t_2 to t_3, you will open that channel up for the other direction of transmission, that is uplink. Once again, if the operating data demand has been satisfied, you can open the same channel back to the base station from t_3 to t_4. Once again, from t_4 to t_5, you can let the phone transmit. Because you have just one frequency channel, you effectively have to alternate between the downlink and uplink on the base station transmissions and the transmission by your phone. The pros and cons of TDD are the exact counterparts of those for FDD. In that because you have to have only one channel, a TDD system, given everything else equal, will be cheaper to implement as compared to an FDD system. But because both downlink and uplink are looking to use the same channel, the peak throughput or data rate that you can achieve in a TDD system is by definition, limited as compared to an FDD system. Once again, given everything else is equal. As with any other engineering problems, FDD and TDD are two ways to leverage that trade-off between the cost of deploying the system versus the performance that the system will achieve. There is a simpler way to understand FDD and TDD. Imagine a typical airport. That airport has a different arrival terminal versus a different departure terminal. People who are trying to fly out go to the departure terminal and people who are flying in come to the arrival terminal. In that sense, that airport is the equivalent of an FDD system because you have different resources all together for two different directions; input versus output or inbound versus outbound. But TDD system, on the other hand, is like a common building, wherein it has just one entrance no matter whether you are trying to enter or exit. In a typical building, and maybe there are some people trying to exit up the building, and if there is a large group of people trying to exit the building, you will wait outside for them to come out. Once they have exited the building, that is when you will enter the building, which would be, let's say, uplink. On the other hand, if there is a large group of people trying to enter the building, and while at the same time you are trying to exit the building, most likely you will let them all enter the building. Once they have entered the building, that is when you will utilize the same entrance as an exit and you will exit the building yourself. Because there is just one door to the melody, that will have to be used for both the directions; entrance as well as exit. That is a rough equivalent of a TDD system. Now that we have learned about some of the basic building blocks of wireless systems and some of the practical considerations related to it, such as multiple access, FDD and TDD, let's try to correlate all those factors together and try to understand some of the fundamental factors that affect the data rate of the speed or the two-port that your phone ultimately gets.
