COMM 907: Spread Spectrum Communications Lecture 2 Mobile Evolution Introduction to Spread Spectrum Systems
Evolution of Mobile Telecommunications
Evolution of Mobile Telecommunications
Evolution of Mobile Telecommunications
Evolution of Mobile Telecommunications
Evolution of Mobile Telecommunications
Evolution of Mobile Telecommunications 2G were commercially launched on the GSM standard in Finland in 1991
Evolution of Mobile Telecommunications 2G: (Second generation): Second generation 2G cellular telecom networks were commercially launched on the GSM standard in Finland in 1991. Radio signals on 2G networks are digital. The primary benefits of 2G networks are: - 2G systems were significantly more efficient on the spectrum (since it is digital, compression techniques can be used to decrease the bandwidth). - 2G introduced data services for mobile, starting with SMS text messages. 2G cannot support multi-media service.
Evolution of Mobile Telecommunications (Cont.) 2G technologies can be divided into TDMA-based and CDMAbased standards. Depending on the type of multiplexing used. The main 2G standards are: - GSM (Global Systems for Mobile communications) it is TDMA-based. - IS-95 (Interim Standard 95): It is the first CDMA based digital cellular standard. 2.5 G services enable high-speed data transfer over upgraded existing 2G systems.
History of GSM 1982 - Conférence Européenne des Postes et Télécommunications (CEPT) began specifying a European digital telecommunications standard. This standard later became known as Global System for Mobile communication (GSM). 1987 - A combination of TDMA and FDMA was selected as the transmission technology for GSM. - Operators from 12 countries signed 1991 - The GSM 1800 standard was released. 1994 - The GSM had over 100 signatories covering 60 countries. - The total number of GSM subscribers exceeded 3 million. 1998 - A total of 253 members in over 100 countries and there are over 70 million GSM subscribers world-wide. GSM subscribers account for 31% of the world s mobile market.
Evolution of Mobile Telecommunications (Cont.) 3G: (Third generation): - Application services of 3G include: mobile internet access, video calls, and mobile TV, all in a mobile environment. - It has high data rate, large Bandwidth - Compared to the 2G and 2.5 G standards, a 3G system must allow simultaneous use of speech and data services, and provide peak data rates of at least 200 kbit/s according to the IMT-2000 (international mobile telecommunications-2000) specifications. - Recent 3G releases, often denoted 3.5G and 3.75G, also provide mobile broadband access of several Mbit/s to laptop computers,.. - Some 3G systems and radio interfaces are based on spread spectrum radio transmission technology such as the CDMA2000 system.
Evolution of Mobile Telecommunications (Cont.) 4G: (4th generation): 4G accommodates services such as: - multimedia messaging services (MMS), - mobile broadband access (high speed internet access), - video chat, mobile TV, but also new services like HDTV. - 4G may allow roaming with wireless local area networks, and may interact with digital video broadcasting systems. For mobile use, including smartphones and tablets, connection speeds need to have a peak of at least 100 megabits per second, and for more stationary uses such as mobile hotspots (internet access point), at least 1 gigabit per second. LTE stands for Long Term Evolution, and isn t as much a technology as it is the path followed to achieve 4G speeds.
Concept of Spread Spectrum Spread spectrum systems is a class of wireless digital communication systems that offer three main advantages: (1) Spread-spectrum transmissions can share a frequency band with many types of conventional transmissions with minimal interference. i.e: The spread-spectrum signals add minimal noise to the narrow-band frequency communications. As a result, bandwidth can be utilized more efficiently.
Concept of Spread Spectrum (2) Spread-spectrum signals are difficult to intercept. An interception receiver would only be able to intercept the transmission if the pseudorandom sequence was known. (3) Spread-spectrum signals are resistant to narrowband interference. i.e: The process of re-collecting a spread signal spreads out the interfering signal
Applications of Spread Spectrum (SS) Communications 1. CDMA radios: SS is useful in multiple access mobile communications where many users communicate over a shared channel. Ex: in cellular phones IS-95 standard and CDMA2000 system 2. The United States GPS, Russian Glonass, and European Galileo satellite navigation systems. 3. WLAN: Wireless LAN (Local Area Networks) widely use spread spectrum communications. IEEE 802.11 is Wi-Fi a standard that is developed for mobile communication, and widely implemented throughout the world.
Applications of Spread Spectrum (SS) Communications 4. Cellular base stations interconnection. 5. Cordless Phones: Several manufacturers implement Spread Spectrum in Cordless phones. The advantages of using SS in cordless phone include the following: a. Security: Inherently, a SS communication is coded. b. Immunity to Noise: SS modulation is immune to noise when compared with other modulation schemes such as AM and FM. c. Longer Range: Due to noise immunity, it is possible to achieve a longer range of communications, for a very small transmitted power.
General Model of Spread Spectrum System
Types of Spread Spectrum Systems Direct Sequence Frequency Hopping Hybrid DS/FH system
2-Direct Sequence Spread Spectrum System
Direct Sequence Spread Spectrum System with BPSK Modulation Direct sequence spread spectrum system (DSSSS), achieves bandwidth spreading through the use of a high rate symbol sequence (termed a chip sequence) that directly multiplies the information symbol stream. Since the chip sequence has a rate much higher than the data rate, the bandwidth is increased. The simplest form of DSSSS uses binary phase shift keying (BPSK) modulation and is illustrated in Fig. 1.1.
Transmitter Block Diagram of DSSSS with BPSK Modulation Transmitter Fig. 1.1 s( t) 2Pb( t) c( t) cos 2 f t c
The transmitted signal can be represented by s( t) 2Pb( t) c( t) cos 2 f t c i where b ( t) bi pb ( t itb ) is the information signal 1, 1 each bit has duration T b b i (t) p b is the unit energy pulse shape used for the information waveform (assumed to be rectangular) i c ( t) ci pc ( t itc ) is the spreading sequence where each symbol (usually called a chip) has duration T T N c b / N is the number of spreading symbols (chips); it is called also. Processing gain. Bandwidth expansion factor. N T b / T c
Data and Chip sequences C(t)
Definitions baseband Spreading Factor
Power Spectral Density
Power Spectral Density
Transmitter Block Diagram of DSSSS with BPSK Modulation Transmitter Fig. 1.1 s( t) 2Pb( t) c( t) cos 2 f t c
Receiver Block Diagram of DSSSS with BPSK Modulation Sampler every Tb Receiver
Receiver Analysis
Receiver analysis (Cont.) b o 1, 1
Performance of DSSSS with BPSK Modulation
Performance of the DSSSS with BPSK Mod.
Appendix A: Noise Variance in DSSSS The variance of noise term is given by: Since the noise is wide sense stationary random process, then Thus the autocorrelation is given by: Then: