How can multiple users operate on the same frequency? Radio is like speaking in a meeting. If everyone speaks at the same time, no one understands anything. With radio, it’s the same thing. When several pieces of equipment transmit at the same time, interference is likely. All radio systems have to find a solution to share the resource between users. In this video, we will see how L. T. E. does this. And, at the end, we will get a global view of the transmission chain. Radio resources are rare and expensive. And users don’t need to communicate continuously. So, it’s preferable to allocate resources to users only when they need them. L. T. E. operates on a band of the radio spectrum between a minimum and a maximum frequency. L. T. E. ’s band-witdth can vary from 1.4 to 20 Megahertz. It is divided into sub-carriers and each sub-carriers can be allocated to a different user. These frequencies are periodically re-allocated, based on the evolution of users needs. So, L. T. E. shares the resources both in frequency and in time. In the previous video, we saw that the smallest piece of information produced by a modulation is a symbol. Each symbol constitutes a “Resource Element”. A Resource Element occupies 15 kHz of frequency space and lasts 66 microseconds. Because it is so small, it is complicated to allocate these resources one by one. Therefore, they are grouped in what is called “Resource Blocks”. A Resource Block lasts half a millisecond. It consists of 84 Resource Elements spread over 12 sub-carriers. One resource block occupies 180 kHz. The number of available Resource Blocks varies from 6 when operating on a 1.4 MHz wide band to one hundred when working with a 20 MHz large band. Every millisecond, L. T. E. re-allocates the Resource Blocks to the users that need to communicate. Since a Resource Block lasts half a millisecond, this means that Resource Blocks are allocated by pairs. The pattern formed by the Resource Blocks during a 1ms period of time is called a “sub-frame”. It’s important to THAT 1 millisecond. In a way, it’s the pulse of L. T. E, its heartbeat. And all allocation mechanisms of L. T. E. are based on this 1ms period or on a multiple of it. To sum up: the available spectrum is cut into Resource Blocks that are allocated every millisecond. These Resource Blocks ares allocated to this phone in yellow. The pattern formed every millisecond by all the resource blocks is called a sub-frame. Each resource block contains 84 Resource Elements. Each one transport one symbol. Howeve, in the resource blocks, some resource elements are reserved for L. T. E. internal control. On this graphic, we’ve colored them in purple or red. We’ll come back to that when we talk about physical channels. For the moment, just remember that only resource elements in white can be used to transmit messages. When a device needs to transmit data, it is provided with one or several of Resource Blocks for the next 1ms. Should this resource not be enough, additional resources will be allocated on next sub-frames. This allocation is managed by the eNode B. We will see how in the next videos. The amount of data a device can send during a given sub-frame is called a “Transport Bloc”. Its size depends on the number of resource blocks available for this user. It also depends on the modulation, or more precisely, on the MCS used with this user. Obviously, the more efficient the modulation, the bigger the transport block. This table gives the size of transport blocks in bits, depending on the MCS in use and the number of resource blocks allocated. It's a lot of numbers, but don't panic. We'll just make a few observations. And come back to it during the exercises. Firstly, it is easy to transpose the transport block size into throughput. Indeed, there are one thousand sub-frames per second. So, all we need to do is multiply the figures of this table by one thousand to get the throughput. Here, we can see that the maximum achievable throughput is 75 Megabits per second. Secondly: The size of transport blocks varies widely, ranging from 16 bits when using only one Resource Block and an MCS of 0 to just over 75 kilobits when using all the Resource Blocks with an MCS of 28. Lastly: the same transport block size can be found in several cells. For example, with this value of 256 that we’ve colored green. This property is interesting if the modulation changes after a transport block was generated. For example, imagine that we were allocated 2 Resource Blocks to transmit a transport block of 256 bits using an MCS of 8. If conditions get worse and we now have to use an MCS of 4. We can keep the same transport block and we just have ask for 4 Resource Blocks instead of 2. In conclusion: L. T. E. shares resources by both time and frequency. A Resource Element corresponds to one symbol. To facilitate allocation, L. T. E. defines Resource Blocks which are groups of 84 Resource Elements. At every millisecond, the eNodeB defines the size of the transport block for each device on the current sub-frame. As promised, we now have a global view of the transmission chain. First, we fill a transport block of the size that was allocated to us. We did not discuss that yet, but we add a cyclic redundancy check, or CRC, to let the receiver check whether received data are correct. The packet then passes through the coding stage which adds redundancy to enable error correction. The flow of bits is then modulated by a digital modulation to produce symbols. These symbols will be inserted in one the the Resource Blocks that have been allocated to us on the current sub-frame. This process repeats every millisecond. In the next video, we will see how theses resources are allocated between different users.
