 As we discussed just now that it is important for us to seek highest data rates which can be offered in wireless networks, 100 megabits per second for highly mobile user and up to 1 gigabits per second for a relatively static user. So it is now important for us to exactly understand what do we mean by these high data rates in terms of the spectrum usage and how can this be provided because overall the spectrum is limited, the transmitted power from the mobile station is limited and the base station is limited in addressing multiple mobile stations at the same time. So we'll discuss the data rates and then we'd look at various smart approaches that can be adopted to increase these data rates. First let's quickly look at in a very generalized way how can we possibly increase the data rate for any wireless technology. The first one is increase the spectrum availability. It means the higher or the broader the spectrum the more frequencies would be available and these frequencies would be able to carry more data. That's one aspect. The second aspect is we can increase the utilization of the existing spectrum in the most efficient manner. In technical terms we can say we can transmit more number of bits against a certain board rate while providing this service to multiple users in a wireless competing environment. The data rates which are promised in 4G networks actually vary from the deployment aspect, the size, the number of users which are present in that particular cell. So we'd look at these in their descending order. Let's start with the bit rate requirement as a general concept. So we say that in 4G the spectral efficiency or the spectrum utilization has to be that it is going to be measured in one bit per second per hertz in a single cell. It means we are just focusing on the utilization of the spectrum within a single cell. So we are looking at how many bits can be transmitted in a single board or in a single wavelength in a single in per unit time. It usually is advised that for 4G networks it can be from 1 to 3 to begin with. For instance it is 1.1 bits per second per hertz in downlink and it is 0.7 in uplink. Now one may argue that why is it more in the downlink and why is it less in the uplink. Though we are going to discuss it over the course of our lecture in more detail but just to make things clear in such a short time we'll say that it is because of the capacity and capability of the device which is going to transmit. For instance in downlink it's going to be from the base station to the mobile station. In the uplink it's from the mobile station which is battery operated, limited transmit power, it is uplink. So that is why we see that the spectral efficiency expected out of an uplink is less than that of the downlink. Now if we take these two relatively urban areas that is areas with high population density but with clear line of sight we can expect the downlink to increase to 2.2 and 1.4 respectively. When we reduce the overall size of the cell let's say we can call it microcell, a pico or a femtocell. Since we are reducing the overall distance and the number of obstructions through which fading and other channel impairments are triggered we can expect up to 3 bits per second per hertz in a single cell and 2.25 in the uplink. Now as I just had said there are certain approaches which allow us to maximize the spectral efficiency. The most obvious way is to use something known as the MIMO system. MIMO means multiple input multiple output. Just compare it to SISO or CISO, single input single output. So MIMO actually is the usage of multiple antennas, multiple frequencies which can be utilized to distribute a given user stream onto multiple frequencies or multiple carriers. Each carrier is transmitted through an independent antenna. This is expected to increase the spectral efficiency by certain orders. This allows the user traffic to be sent in parallel and to receive traffic from the other end in parallel. Assume an example in which we have 4x4 MIMO. That is we have four signals which are transmitted in parallel as input to the system and at the same time there are four signals which can be received as an output from the system. It means simultaneously four duplex connections are established. Now of course with MIMO over SISO we would see four orders of improvement straight away in the spectral efficiency. It is as simple as that for now. The other approach is actually to think about better utilization of the wavelength or the board. It can be realized through something known as the quadrature amplitude modulation. It allows the spectral efficiency by taking an advanced modulation scheme which can carry more number of bits per board or a symbol. The well-known example is 64 QAM but it can be any number. It can go up to 256 QAM or usually it is known as 16 QAM. So the modulation works simply by using different amplitudes and different phases for representing more than one bit onto a single symbol. So this allows a symbol to be utilized more efficiently in terms of bits. Different QAM types can be compared to each other in terms of the spectral efficiency which they provide. Since 64 QAM is related to the two to the power six so it means each symbol can carry up to six bits. 16 QAM is only carrying four bits. So if we just compare these two there is an improvement of the order of 1.5 times when once we move from 16 QAM to 64 QAM.