1 Lecture Notes 1 Interference Limited System, Cellular. Systems Introduction, Power and Path Loss

Transcription

1 ECE 5325/6325: Wireless Communication Systems Lecture Notes, Spring Lecture Notes 1 Interference Limited System, Cellular Systems Introduction, Power and Path Loss Reading: Mol 1, 2, 3.3, Patwari s 2013 lecture notes 1,2 Homework: HW Course introduction What is this course about and not about? Five aspects of wireless communication systems: Requirements Data rate Range Mobility Number of users Error rate Wireless propagation Path loss and link budget Antennas Multipath fading, Rayleigh fading, Ricean fading Doppler fading Limited frequency resources Channel reuse, frequency reuse Interference, power limit Cellular system Sectoring Trunking Hardware constraints Cost Portability, small, powered by battery Computing capability Battery life Safety issue Available Techniques (to overcome wireless propagation issues and take full use of resources, under hardware constraints)

2 Modulation, PSK, QAM, FSK Multicarrier modulation and OFDM Spread spectrum Coding Multiple antenna, MIMO MAC layer, random access, Aloha, CSMA/CA wireless communication system When all these meets, you have a wireless communication system. Wireless communication system design: find a solution to satisfy these constraints: wireless propagation, limited frequency resources, hardware, available technologies, and requirements. We will learn all these aspects. However, we are not going into very details. For example, we are going to design a particular receiver or the algorithms therein. Big picture The most important capability you will gain from this class is that you will have a big picture of what a wireless communication system is and understand why an existing communication system is designed as they are, and you will have the basic skills of designing a communication system. Equations and formulas A good news is that the equations and formulas are simple in this course. But that doesn t make the class easy. So be sure to understand every formula and its position in the big picture of wireless communication system. OK, let s start our course by examine an example: For example, let s consider the situation that you are managing a cellular network, and one custom

3 complains that he is not able to make calls sometimes. You know that he is at the edge of the cell, and the interference from other cells is the reason. Then, what are you going to do about it? 1. Raise the transmit power of serving basestation 2. Lower the power of interference basestation 3. Get rid of the interference (use a unique channel) My answer is to do nothing. I will explain why. 1.2 Interference Limited Systems and Cellular Systems Intro Signal, noise and interference, the receiver receives Noise: thermal noise, man-made noise, receiver noise Interference: co-channel communication signal Receive power SINR = Interference + Noise = Noise limited system: A system where P N is the major limiting factor for SINR. P r i 0 i=1 I i + P N An example: Voyager (ˈvɔɪɪdʒər) 1, the spacecraft, that has left the solar system. In this example, it is ensured that there is no interference. Then, the signal quality is only determined by the transmit power, distance and noise. Sometimes, it is difficult to tell between interference and noise. One difference between interference and noise lies in the fact that interference suffers from fading, while the noise power is typically constant (averaged over a short time interval). You can vary location, frequency and time to avoid interference, while it s not possible for noise. Interference limited system: A system where i 0 i=1 I i is the major limiting factor for SINR. This is the case when the channel is reused at different locations. User s throughput is higher with higher SINR. However, we are more concerned with the system throughput, which is the sum of all users throughput. Clearly, if we raise the power of this user for higher SINR, other users SINR is lowered. A cellular system is about channel reuse, it is designed to separate co-channel users away from each other, to mitigate the co-channel interference. 1. One user per channel in each cell (the channel could be shared in time), a user will not be interfered by other users of the same cell 2. Reuse the channel in neighboring cells, while avoiding generating too much interference among cells. One way is not to use the same channel among direct neighbor cells. It s a simple and effective solution.

4 We will explain the cellular system in detail later. Answer the previous question: 1. To raise the transmit power, you will also raise the interference to other cells. 2. To lower the interference power, you will also lower the signal quality of other cells. 3. To use a unique channel, neighbor cells can t use the same channel, it would be a big loss in system throughput. It s a system design issue. 1. On one hand, you want to reuse as many channels as possible, so that each users have maximum number to channels to serve its users. 2. On the other hand, you don t want to reuse too much, to ensure that all users have acceptable SINR, especially the edge users. 3. It s an optimization. Your long term revenue relies on a good optimization. You have to design the network carefully in the first place, so that all users have acceptable SINR and at the same time each cell has as many channels as possible. However, there may be users with signal quality lower than a threshold, due to random fading. In this case, hardly anything you can do, if a single edge user is complaining. One exception is that, if there are considerable number of users have the same problem, you could add hotspot basestation (relay) to increase their signal quality, which incurs certain cost and complexity. 1.3 Power and Path Loss Power How we measure the power: Decibel units Convert from linear power P in Watts to dbw power using: P [dbw] = 10 log10 P [W] Convert from linear power P in mw to dbm power using: P [dbm] = 10 log10 P [mw] dbx is the ratio to x, given in db. Why use decibel to describe power? The power we are looking at vary from transmit powers of 500W and receive powers of W. The scale varies greatly. And, we do more multiplications than additions on power. Multiplication is equivalent to addition in decibels. Therefore, we like decibel. Multiplying a linear power by 2 always adds 3 db to the db power Multiplying a linear power by 10 always adds 10 db to the db power Multiplying a linear power by 1.25 always adds 1 db to the db power For example: multiplying a linear power by 20 adds 13 db to the db power. Convert back from dbm and dbw power with

5 P [mw/w] = 10 P[dBm/W]/10 Why we should not raise the power to be greater than the limit set by the government. Interference: (1) The channel we are using is being reused by other users at some other places. (2) The adjacent channels are used by other users, we don t want too much spectrum leakage to the adjacent channels. There are also safety issues: Mobile phone radiation and health Power of some systems: Bluetooth Power Class 1: 100 mw; Power Class 2: 2.5 mw; 4.0: 10mW Wifi ISM band, FCC 1W Cell phone GSM peak power 2W, WCDMA 200mW, Basestation GSM 10W Large-scale Path Loss In average, the receive power is decayed proportional to 1/d n, where n is called the path loss exponent. P r = (P 0 d n 0 ) 1 d n = P 0 ( d n 0 d ) = P 0 ( d n ) d 0 The reference power P 0 at reference distance d 0. You only use this to obtain the average receiver power. The actual receive power fluctuates around this average, the fluctuation is related to other random factors such as shadowing, multipath fading, etc. In db units: P r [dbm] = P 0 [dbm] 10nlog 10 d d 0 [db] Question: How much does your average received power change when you double your path length? n = 3 Basically, if you raise the power by 9dB, you doubles your distance of coverage. 1.4 Some Other Key Terms duplex Simplex: paging, broadcasting, one channel and one direction Half duplex: push-to-talk, one channel and two directions, Wifi, you may think it as full dupex, however actually it is not, only one channel, each time one party transmitting signal. Full duplex: two channels required, cell phone a typical full duplex, downlink and uplink channel

6 1.4.2 multiple access Multiple access a physical channel shared by multiple users One typical solution is: users signals are orthogonal to each other. FDMA: very basic, different user/system works on different frequency, guard band and spectral mask (limited out-of-band emission) to prevent interference. TDMA: very basic, usually much cheaper than FDMA, (digital circuits, clocks are cheap, however for FDMA, analog circuits, filters are expensive). For multiple users in one system, it is more likely to employ TDMA. CDMA: each user s signal is multiplied by a different code, orthogonal codes