Cellular and Mobile Communication-lecture 3.pdf
- 1. DIT Dar es Salaam institute of Technology (DIT) ETU 08122 Cellular and Mobile Communication Ally, J ICT Professional (Radio Frequency Communication) Huawei Certified Academy Instructor (Routing and Switching, Cloud Computing, Storage, and 5G) jumannea@gmail.com
- 2. DIT Multiple Access Technique for Wireless Communications
- 3. DIT Contents ◼ Overview ◼ Frequency Division Multiple Access (FDMA) ◼ Time Division Multiple Access (TDMA) ◼ Code Division Multiple Access (CDMA) ◼ Space Division Multiple Access (SDMA) ◼ Capacity of Cellular Systems
- 4. DIT Overview ◼ Multiple access schemes are used to allow many mobile users to share simultaneously a finite amount of radio spectrum. ◼ The sharing of spectrum is required to achieve high capacity by simultaneously allocating the available bandwidth to multiple users. ◼ For high quality communications, this must be done without severe degradation in the performance of the system. ◼ In wireless communications systems, it is desirable to allow the subscriber to send simultaneously information to the BS while receiving information from the BS. ◼ In conventional telephone systems, it is possible to talk and listening simultaneously, and this effect, called duplexing, is generally required in wireless telephone systems.
- 5. DIT Duplexing ◼ Duplexing may be done using frequency or time domain techniques. ◼ Frequency Division Duplexing (FDD) provides two distinct bands of frequencies for every user. ◼ In FDD, consists of two simplex channels, and a duplexer which used inside each MS and BS to allow simultaneous radio transmission and reception on the duplex channel pair. ◼ Time Division Duplexing (TDD) uses time instead of frequency to provide both a forward and reverse link. ◼ In TDD, if the time split between the forward and reverse channel time slot is small, then the transmission and reception of data appears simultaneous to the user. TDD FDD
- 6. DIT Introduction to Multiple Access ◼ FDMA, TDMA, and CDMA are the three major access techniques used to share the available bandwidth in a wireless communication systems. ◼ These techniques can be grouped as narrowband and wideband systems. ◼ Narrowband Systems ➢ The term narrowband is used to relate the bandwidth of a single channel to the expected coherence bandwidth of the channel. ➢ In a narrowband multiple access system, the available radio spectrum is divided into a large number of narrowband channels. ➢ The channels are usually operated using FDD. ◼ Wideband Systems ➢ In wideband systems, the transmission bandwidth of a single channel is much larger than the coherence bandwidth of the channel. ➢ Multipath fading does not greatly vary the received signal power within a wideband channel, and frequency selective fades occur in only a small fraction of the signal bandwidth at any instance of time. ➢ In wideband multiple access systems a large number of transmitters are allowed to transmit on the same channel. ◼ In addition to FDMA, TDMA, and CDMA, two other multiple access schemes are used for wireless communications, which are Packet Radio (PR) and Space Division Multiple Access (SDMA).
- 7. DIT Multiple Access Techniques in Modern Wireless Communication System
- 8. DIT FDMA ◼ FDMA system assigns individual channels to individual users. ◼ These channels are assigned on demand to users who request service. ◼ During the period of the call, no other user can share the same frequency band.
- 9. DIT The features of FDMA ◼ The FDMA channel carries only one phone circuit at a time. ◼ If an FDMA channel is not in use, then it sits idle and cannot be used by other users to increase or share capacity. ◼ After the assignment of a voice channel, the BS and the mobile transmit simultaneously and continuously. ◼ FDMA is usually implemented in narrowband systems (channel bandwidth is about 30 kHz). ◼ The symbol time is large as compared to average delay spread. ISI is low and thus, little or no equalization is required. ◼ The complexity of FDMA mobile systems is lower when compared to TDMA systems, though this is changing as digital signal processing method improve for TDMA. ◼ The FDMA mobile unit uses duplexers since both the transmitter and receiver operate at the same time. It increase the cost of MS and BS. ◼ FDMA requires tight RF filtering to minimize adjacent channel interference.
- 10. DIT Nonlinear Effects in FDMA ◼ In a FDMA system, many channels share the same antenna at the base station. ◼ The power amplifiers or the power combiner, which at or near saturation for maximum power efficiency, are nonlinear. ◼ The nonlinearities cause signal spreading in the frequency domain and generate intermodulation (IM) frequencies. ◼ IM is undesired RF radiation which can interfere with other channels in the FDMA systems. ◼ Spread spectrum results in adjacent channel interference ◼ IM is the generation of undesirable harmonics. ◼ Harmonics generated outside the mobile radio band cause interference to adjacent services, while those present inside the band cause interference to other users in the mobile system.
- 11. DIT Number of Channels in FDMA ◼ Number of channels that can be simultaneously supported in FDMA system is given by ◼ Where Bt is the total spectrum allocation, Bguard is the guard band allocated at the edge of the allocated spectrum band, and Bc is the channel bandwidth. t guard c B 2B B N − =
- 12. DIT TDMA ◼ TDMA system divide the radio spectrum into time slots, and in each slot only one user is allowed to either transmit or receive. ◼ TDMA systems transmit data in a buffer-and-burst method, thus the transmission for any user is noncontinuous. ◼ Unlike in FDMA systems which accommodate analog FM, digital data and digital modulation must be used with TDMA.
- 13. DIT TDMA Frame Structure ◼ Each frame is made up of a preamble, an information message, and tail bits. ◼ In TDMA/TDD, half of the slot in the frame used for the forward link channels and half used for reverse link channels. ◼ In TDMA/FDD, frame structure used for either forward or reverse transmission, but carrier frequency is different for the forward and reverse links.
- 14. DIT The Features of TDMA ◼ TDMA shares a single carrier frequency with several users, where each user makes use of non-overlapping time slots. ◼ Data transmission for users of a TDMA system is not continuous, but occurs in bursts. ◼ Because of DTX in TDMA, the handoff process is much simpler for MS, since it is able to listen for other BS during idle time slots. ◼ TDMA uses different time slots for transmission and reception, thus duplexing are not required. ◼ Adaptive equalization is necessary in TDMA systems, since the transmission rates are very high as compared to FDMA channels. ◼ TDMA has an advantage to allocate different numbers of time slots per frame to different users.
- 15. DIT Efficiency of TDMA ◼ The efficiency of a TDMA system is a measure of the percentage of transmitted data that contains information as opposed to providing overhead for the access scheme. ◼ The frame efficiency, ηf is the percentage of bits per frame which contain transmitted data. ◼ The number of overhead bits per frame is where, Nr is the number of reference bursts per frame, Nt is the number of traffic bursts per frame, br is the number of overhead bits per reference burst, bp is the number of overhead bits per preamble in each slot, and Bg is the Number of equivalent bits in each guard time interval. ◼ The total number of bits per frame, bT is where Tf is the frame duration, and R is the channel bit rate. The frame efficiency ηf is thus given as b 1 100% b OH f T = −
- 16. DIT Number of Channels in TDMA The number of TDMA channel slot that can be provided in a TDMA system is given by where m is the max. number of TDMA users supported on each radio channel. Example: Solution: tot guard c m(B 2B ) B N − =
- 17. DIT Spread Spectrum Multiple Access (SSMA) ◼ SSMA uses signals which have a transmission bandwidth that is several orders of magnitude greater than the minimum required RF bandwidth. ◼ A pseudo-noise (PN) sequence converts a narrowband signal to wideband noise-like signal before transmission. ◼ SSMA also provides immunity to multipath interference and robust multiple access capability. ◼ SSMA is not very bandwidth when used by single user, so spread spectrum systems become bandwidth efficient in a multiple user environment. ◼ There are two types of SSMA techniques: Frequency Hoped Multiple Access (FH) and Direct Sequence Multiple Access (DS), also is called Code Division Multiple Access (CDMA).
- 18. DIT FHMA ◼ FHMA is a digital multiple access system in which the carrier frequencies of the individual users are varied in a pseudorandom fashion within a wideband channel. ◼ The digital data is broken into uniform sized bursts which are transmitted on different carrier frequencies. ◼ The difference between FHMA and FDMA system is that the frequency hoped signal changes channels at rapid intervals. ◼ If the rate of change of the carrier frequency is greater than the symbol rate then the system is referred to as a fast frequency hopping system. ◼ If the channel changes at a rate less than or equal to the symbol rate, it is called slow frequency hopping.
- 19. DIT Comparison of AMPS with Digital TDMA Based Cellular System In practice, TDMA systems improve capacity by a factor of 3 to 6 times as compared to analog cellular radio systems
- 20. DIT CDMA ◼ In CDMA system, the narrowband message signal is multiplied by a very large bandwidth signal called spreading signal. ◼ The spreading signal is a PN code sequence that has a chip rate which is orders of magnitudes greater than the data rate of the message. ◼ All users in a CDMA system, use the same carrier frequency and may transmit simultaneously.
- 21. DIT CDMA (2) ◼ Each user has its own pseudorandom codeword which is approximately orthogonal to all other codeword. ◼ For detection of the message signal the receiver needs to know the codeword used by the transmitter. ◼ Each user operates independently with no knowledge of the other users. ◼ In CDMA, if the power control of each user within a cell is not controlled at the BS receiver, then the near-far problem occurs.
- 22. DIT The 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. ◼ Multipath fading may be reduced because the signal is spread over a large bandwidth. ◼ Channel data rates are very high in CDMA systems. ◼ The near-far problem occurs at a CDMA receiver if an undesired user has high detected power as compared to the desired user.
- 23. DIT Capacity of Cellular CDMA ◼ The capacity of CDMA systems is interference limited, while it is bandwidth limited in FDMA and TDMA. ◼ Therefore, any reduction in the interference will cause a linear increase in the capacity of CDMA. ◼ Interference is reduced by using multi-sectorized antennas, which results in spatial isolation of users. ◼ Another way of increasing CDMA capacity is to operate in a DTX. ◼ The number of users can access the system is given by ◼ For large number of users and for system interference limited rather than noise limited, number of users is
- 24. DIT Example
- 25. DIT Hybrid FDMA/CDMA (FCDMA) ◼ This technique can be used as an alternative to the DS-CDMA techniques. ◼ The available wideband spectrum is divided into a number of subspectras with smaller bandwidths. ◼ Hybrid system has an advantage that different users can be allocated different subspectrum bandwidths depending on their requirements. ◼ The capacity of this FDMA/CDMA technique is calculated as the sum of the capacity of a system operating in the subspectra.
- 26. DIT Hybrid DS/FHMA ◼ This technique consists of a direct sequence modulated signal whose center frequency is made to hop periodically in a pseudorandom fashion. ◼ Direct sequence, frequency hopped systems have an advantages in that they avoid the near-far effect. ◼ However, frequency hopped systems are not adaptable to the soft handoff process since it is difficult to synchronize the frequency hoped BS receiver to the multiple hopped signals.
- 27. DIT Time Division CDMA (TCDMA) ◼ In a TCDMA (also called TDMA/CDMA) system, different spreading codes are assigned to different cells. ◼ Within each cell, only one user per cell is allotted a particular time slot. ◼ Thus at any time, only one CDMA user is transmitting in each cell. ◼ When a handoff takes place, the spreading code of the user is changed to that of the new cell. ◼ Using TCDMA has an advantage in that it avoids the near-far effect since only one user transmits at a time within a cell.
- 28. DIT Time Division Frequency Hopping (TDFH) ◼ This multiple access technique has an advantage in severe multipath or when severe co-channel interference occurs. ◼ The subscriber can hop to a new frequency at the start of a new TDMA frame, thus avoiding a severe fade on a particular channel. ◼ This technique has been adopted for the GSM standard, where the hopping sequence is predefined and the subscriber is allowed to hop only on certain frequencies which are assigned to a cell. ◼ It avoids co-channel interference problems between neighboring cells if two interfering BS transmitters are made to transmit on different frequencies at different times. ◼ The use of TDFH can increase capacity in GSM system.
- 29. DIT SDMA ◼ SDMA controls the radiated energy for each user in space. ◼ SDMA serves different users by using spot beam antennas. ◼ Sectorized antenna may be thought of as a primitive application of SDMA. ◼ In the future, adaptive antennas will be used to steer energy in the direction of many users at once.
- 30. DIT Capacity of Cellular System ◼ Channel capacity for a radio system is the maximum number of channels or users that can be provided in a fixed frequency band. ◼ Radio capacity is a parameter which measures spectrum efficiency of a wireless system. ◼ This parameter is determined by the required carrier-to- interference ratio (C/I) and the channel bandwidth Bc. ◼ In a cellular system the interference at a BS receiver will come from the subscriber units in the surrounding cells, which is called reverse channel interference. ◼ For a particular subscriber unit, the desired BS will provide the desired forward channel while the surrounding co-channel BS will provide the forward channel interference. ◼ Co-channel reuse ratio is given by, Q=D/R
- 31. DIT Thanks! Technology changes but communication lasts.