Code Division Multiple Access
- 1. Code Division Multiple Access D.Gopinath AP/ECE Ramco Institute of technology Academic year (2017-18 Even)
- 2. Spread Spectrum Multiple Access (SSMA) Transmission bandwidth is several orders of magnitude greater than the minimum required RF bandwidth. Pseudo-noise (PN) sequence converts a narrowband signal to a wideband noise-like signal. Provides immunity to multipath interference and robust multiple access capability. Bandwidth efficient in a multi user environment. Two main types SSMA: Frequency hopped multiple access (FH) Direct sequence multiple access (DS) Direct sequence multiple access is also called code division multiple access (CDMA).
- 3. Frequency Hopped Multiple Access (FHMA) • The carrier frequencies of the individual users are varied in a pseudorandom fashion within a wideband channel. • The pseudorandom change of the carrier frequencies of the user randomizes the occupancy of a specific channel at any given time, multiple access allowed. • At any given point in time, a frequency hopped signal only occupies a single, relatively narrow channel.
- 4. Frequency Hopped Multiple Access (FHMA) Difference between FHMA and FDMA: • In FHMA, The frequency hopped signal changes channels at rapid intervals. Fast hopping and slow hopping: • fast frequency hopping ---- the rate of change of the carrier frequency is greater than the symbol rate • slow frequency hopping ---- the channel changes at a rate less than or equal to the symbol rate
- 5. Code Division Multiple Access (CDMA) • In CDMA, the narrowband message signal is multiplied by a very large bandwidth signal called the spreading signal. • The spreading signal is a pseudo-noise code sequence that has a chip rate which is orders of magnitudes greater than the data rate of the message. • All users use the same carrier frequency and may transmit simultaneously. • Each user has its own pseudorandom code word which is approximately orthogonal to all other code words.
- 7. Code Division Multiple Access (CDMA) • The receiver performs a time correlation operation to detect only the specific desired codeword. All other codewords appear as noise due to decorrelation. • The receiver needs to know the codeword used by the transmitter. • Each user operates independently with no knowledge of the other users.
- 8. Near-far problem in CDMA • The near-far problem occurs when many mobile users share the same channel. • In general, the strongest received mobile signal will capture the • demodulator at a base station. • In CDMA, stronger received signal levels raise the noise floor at the base station demodulators for the weaker signals, thereby decreasing the probability that weaker signals will be received. • the power of multiple users at a receiver determines the noise floor after decorrelation.
- 9. Power control • Provided by each base station in a cellular system and assures that each mobile within the base station coverage area provides the same signal level to the base station receiver. • This solves the problem of a nearby subscriber overpowering the base station receiver and drowning out the signals of far away subscribers. • Power control is implemented at the base station by rapidly sampling the radio signal strength indicator (RSSI) levels of each mobile and then sending a power change command over the forward radio link.
- 10. Features of CDMA: • Many users of a CDMA system share the same frequency. Either TDD or FDD may be used. • Unlike TDMA or FDMA, CDMA has a soft capacity limit. Increasing the number of users in a CDMA system raises the noise floor in a linear manner. • Multipath fading may be substantially reduced because the signal is spread over a large spectrum. • Channel data rates are very high in CDMA systems.
- 11. Features of CDMA (continued): • Soft handoff is performed by the MSC, which can simultaneously monitor a particular user from two or more base stations. The MSC may chose the best version of the signal at any time without switching frequencies. • The near-far problem occurs at a CDMA receiver if an undesired user has a high detected power as compared to the desired user.
- 12. Space Division Multiple Access (SDMA) • SDMA serves different users by using spot beam antennas to control the radiated energy for each user in space. • These different areas covered by the antenna beam may be served by the same frequency (in a TDMA or CDMA systern) or different frequencies (in an FDMA system). • Sectorized antennas ---- primitive application of SDMA. • Adaptive antennas ---- simultaneously steer energy in the direction of many users at once.
- 13. Channel Capacity
- 15. TEXTBOOKS: • 1. Rappaport,T.S., “Wireless communications”, Second Edition, Pearson Education, 2010. • 2. Andreas.F. Molisch, “Wireless Communications”, John Wiley – India, 2006. REFERENCES: • 1. David Tse and Pramod Viswanath, “Fundamentals of Wireless Communication”, Cambridge University Press, 2005. • 2. Upena Dalal, “ Wireless Communication”, Oxford University Press, 2009. • 3. Van Nee, R. and Ramji Prasad, “OFDM for wireless multimedia communications”, Artech House, 2000.
- 16. Thank You