Hi, it's very nice to see you again. I'm Kwang Soon Kim professor in Yonsei University. So far, you learned about the principles of wireless communication theory in the second week, including how to transfer a large amount of information reliably in practice, even in case of simultaneous access by sharing radio resources among users. In the third week, you learned about the principles of resource management, including how to manage such radio resources to handle the interference from other uses and improve the capacity of the cellular system. And, in the last week you learned about the principles of multi-antenna communication theory, including how to use multiple antennas for further improvement in both the information transfer rate and the reliability. Do you remember the lecture about the history of advances in cellular communication systems in the first week? Then you know that you're served by LTE network as the 4th generation now. Then it is time to learn how the principles of wireless communication theory, wireless resource management and multi-antenna communication theory are applied in the LTE network, and I’m sure you are ready. The second generation cellular system, such as the GSM comprised of each cellular core network and it's radio access network connecting mobile phones to the core network via base stations. Also, the cellular network is connected to the public switches telephone network. So, it can provide phone calls between mobile phones as well as phone calls between mobile phones and home phones. Also, a short message service is available. However, only very limited data service up to a few tens of kilo VPS can be supported. As the need for data communications through the cellular network increases, the third generation cellular systems, such as the WCDMA is connected to the public Internet. Also, the advances in wireless communication technology allows the third generation cellular system to support the use of smart phones, such as web surfing with data rate up to a few mega bps. Now, this is the LTE network you are being served by, and you can enjoy real time multimedia services with data rate up to a few hundred mega bps. Actually this is amazing that everyone can enjoy real-time high definition wireless video services on his or her own personal devices, and I'm sure that you are curious of how it can be realized. Do you remember that the resources we need to pay for transferring information are power and bandwidth? And the upper limit on the achievable rate given by the Shannon capacity in the equation at the top of this slide. In the second generation cellular system, the bandwidth is as narrow as only 1.25 megahertz. In the third generation cellular system, it is increased to 5 megahertz so that it can support the use of smartphones. However, in LTE, it is further increased to 20 megahertz in such a wide band is the first enabler for the real-time, high definition wireless video service capability of the LTE. However, there are technical challenges for using wide band communications. Larger bandwidth means shorter symbol period, which means that the electrical signal pulse responding to each symbol becomes more narrow and the digital circuits in modem need to run faster and process more amount of data. Although the latter can be overcome by the advances in electronic devices, the former can pose a fundamental problem. Consider a practical cellular environment where multiple reflected rays are combined at the receiver with different propagation delays and phase rotations. When the bandwidth is narrow, each signal pulse corresponding to each symbol is wide, and it is much wider than the propagation delay difference at the receiver. In this case, although the received signal is slightly distorted it can be handled by using an equalizer and a receiver which was the case of the GSM or WCDMA. However, as bandwidth increases more and more the signal pulse corresponding to each symbol becomes more narrow and it become quite less than the propagation delayed difference at the receiver. In this case, the receptor signal is quite distorted so that we cannot deliver as many amount of information as expected in theory, even if a powerful equalizer is employed. So, in order to solve this problem, LTE employs a parallel transmission method, using multiples of carriers called orthogonal frequency division multiplexing, an acronym, OFDM. Here, carrier signal means the sinusoid's used for upconverting. If only one carrier is used, a single base-band information-bearing signal with a large boundaries and short signal period is to be upconverted as shown above. However, if multiples of carriers with different frequencies are used, multiple base-band information bearing signals within that narrow bandwidth and a long single period are to be upconverted together by using their own subcarrier frequencies as shown in below. In addition, if the subcarrier spacing the frequency difference between adjacent subcarriers is the inverse of the symbol period. The subcarriers are orthogonal so that multiple information bearing signals can be simultaneously extracted at the receiver without any interference among them. In LTE, 1,200 subcarriers with 15 kilohertz of carrier spacing are used. Since the symbol period should be the inverse of the subcarrier spacing it is 66.67 microseconds. Note that, if a single carrier is used, the symbol length would be the inverse of 20 megahertz, that is 15 nanoseconds. One more thing is that a cyclic preface, the blue one, which is a copy of the tail part of the OFDM symbol, the orange one, is added. Such a cyclic prefix guarantees the orthogonal among the multiple information-bearing signals at the receiver, even in practical cellular environments. So LTE can successfully utilize such a wide band by employing OFDM. By considering the cyclic prefix, there are seven OFDM symbols with their own cyclic purposes in 0.5 milliseconds. That means we have 14,000 OFDM symbols per second. Then for each of 1,200 subcarries in each OFDM symbol, QPSK, 16 QAM and 64 QAM signal constellations, each can deliver 2, 4, and 6 bits per symbol are used in LTE. Considering about 11% of overhead in LTE frame structure, 89% of 2 bits times 1,200 subcarriers times 14,000 symbols per second. About 29.9 megabits can be delivered in a second by using QPSK. Also when 16 QAM or 64 QAM is used the bit rate is double to be 59.8 mega bps or triple to be 89.7 mega bps Note that such bit rates are those after the channel coding. So, we can get the information bit rate by multiplying the code rate to them. For example, 64 QAM and a code with rate of 5 over 6 resulting 74.7 information bits per second. Now, recall the modem architecture at the transmitter and receiver you learned in the second week and compare them to the LTE modem that architecture. Can you see the difference? Here, the coded bits are allocated to subcarriers for the constellation mapping, and an OFTM modulation is performed on the constellation signals of all subcarriers together, including the cyclic prefix insertion At the receiver, the OFDM demodulation is performed on each OFDM symbol to get all the subcarrier symbols and the detection is performed on each subcarrier symbol. In LTE, various modulation and coding sets can be used and those in this slide is an example. Here, constellations from BPSK to 64 QAM with code rate from 1 over 2 to 5 over 6 are available. And each modulation and coding set is mapped to its own channel quality information index. In LTE, downlink delivers data from base station to users, and uplink delivers data from users to base station and they are separated either in time or frequency. In each subframe of 1 millisecond, each base station selects users for transferring information in downlink to users, and uplink from users. In each subframe there is some overhead such as reference signal or control signal, so that about 89% can be used for the information transfer. Then, what are those reference signals and control signals for? Each user monitors the reference signal to estimate the channel quality and delivers the channel quality information as one of the control signals for selecting and adaptive modulation and coding at the base station. The schedule information about which users are selected, and transmit or receive in each subframe. And the corresponding modulation and coding set is delivered as another control signal. Also, acknowledge and no-acknowledge signals are delivered as another control signal too. So, with the aid of such control channels, the adapting modulation and coding, AMC and the hybrid ARQ are employed in the LTE system to deliver large amount of information reliably in practical cellular environments. So, in each subframe, more than one cyclical users access simultaneously and it can be done by using an appropriate multiple access scheme. In LTE, orthogonal frequency division multiple access an acronym OFDMA is used for the downlink, and the single carrier, frequency division multiple access, an acronym, SC-FDMA, is used for the uplink. For the downlink, for each subframe of one millisecond, 12 subcarriers in frequency forms the basic resource unit, which is orthogonal to each other. And they are partitioned with the same power to the selected users. So it is called OFDMA. Note that users close to the base station have high SNR, but users far from the base station have low SNR. Although the achievable data rate may be different. All of them enjoy as many amount of information transfer as possible by virtue of the orthogonal multiple access, adapted modulation and coding, and hybrid ARQ. SC-FDMA is a kind of variation of OFDMA to make it suitable for uplink by reducing the amplitude variations of an OFDM modulated signal. Also, consecutive resource units are allocated to each user. In addition, unlike the downlink, the transmission power of each user may be controlled for battery saving and interference reduction. Although the multiple access scheme is slightly different from that in the downlink, it is same to all our users enjoy as many amount of information transfer as possible by virtue of the orthogonal multiple assets, adaptive modulation and coding and hybrid ARQ. How are the principles of wireless communication theory applied in the LTE network to realize real time high-definition wireless video services? Now, you have the answers.
