Wireless Communication Arjav A. Bavarva Dept. of Electronics and Communication

Multiple access techniques Arjav A. Bavarva Dept. of Electronics and Communication

Important terms  Simplex channel  Half duplex channel  Full duplex or duplex channel (Using frequency and time)  Forward channel  Reverse channel Arjav A. Bavarva Dept. of Electronics and Communication

Frequency division duplexing (FDD) two bands of frequencies for every user forward band reverse band duplexer needed frequency separation between forward band and reverse band is constant frequency seperation reverse channelforward channel f Arjav A. Bavarva Dept. of Electronics and Communication

Time division duplexing (TDD) uses time for forward and reverse link multiple users share a single radio channel forward time slot reverse time slot no duplexer is required time seperation t forward channelreverse channel Arjav A. Bavarva Dept. of Electronics and Communication

Arjav A. Bavarva Dept. of Electronics and Communication

Arjav A. Bavarva Dept. of Electronics and Communication

FDMA Frequency Division Multiple Access Arjav A. Bavarva Dept. of Electronics and Communication

Arjav A. Bavarva Dept. of Electronics and Communication

Arjav A. Bavarva Dept. of Electronics and Communication

Arjav A. Bavarva Dept. of Electronics and Communication

General Specification of FDMA Rx: MHz Tx: MHz 832 Channels spaced 30kHz apart DQPSK modulation scheme 48.6kbps bit rate Used in analog cellular phone systems (i.e. AMPS) Uses Frequency Division Duplexing (FDD) ISI (Inter symbol Interference) is low Arjav A. Bavarva Dept. of Electronics and Communication

FDMA Operation Number of FDMA Channels In the U.S. each cellular carrier is allocated 416 channels where: Arjav A. Bavarva Dept. of Electronics and Communication

Advantages of FDMA If channel is not in use, it sits idle Channel bandwidth is relatively narrow (30kHz) Simple algorithmically and lower complexity Fairly efficient when the traffic is uniformly constant Capacity increase can be obtained by reducing the information bit rate and using efficient digital code No need for network timing – fewer bits required for synchronization and framing No restriction regarding the type of baseband or type of modulation Arjav A. Bavarva Dept. of Electronics and Communication

Disadvantages to using FDMA The presence of guard bands It increases cost as duplexer is compulsory Maximum bit rate per channel is fixed Many channels use same antenna, causes nonlinear effect and signal will spread in frequency domain (Inter modulation frequency IM ). Requires right RF filtering to minimize adjacent channel interference Arjav A. Bavarva Dept. of Electronics and Communication

TDMA Time Division Multiple Access Arjav A. Bavarva Dept. of Electronics and Communication

Arjav A. Bavarva Dept. of Electronics and Communication

Arjav A. Bavarva Dept. of Electronics and Communication

Arjav A. Bavarva Dept. of Electronics and Communication

Arjav A. Bavarva Dept. of Electronics and Communication

General Specification of TDMA Rx: MHz Tx: MHz 832 Channels spaced 30kHz apart (3 users/channel) DQPSK modulation scheme 48.6kbps bit rate Interim Standard (IS) – 54 Digital AMPS (Advanced Mobile Phone System) Uses Time Division Duplexing (TDD) usually Arjav A. Bavarva Dept. of Electronics and Communication

TDMA Operation Efficiency of TDMA frame: Arjav A. Bavarva Dept. of Electronics and Communication

Time Division Multiple Access time slots one user per slot buffer and burst method Non continuous transmission digital data digital modulation Arjav A. Bavarva Dept. of Electronics and Communication

Slot 1Slot 2Slot 3 … Slot N Repeating Frame Structure Preamble Information Message Trail Bits One TDMA Frame Trail Bits Sync. Bits Information Data Guard Bits The frame is cyclically repeated over time. Arjav A. Bavarva Dept. of Electronics and Communication

Features of TDMA A single carrier frequency for several users Transmission in buffer and burst Low battery consumption Handoff process much simpler as it is discontinuous process FDD : switch instead of duplexer Very high transmission rate High synchronization overhead Guard slots necessary Arjav A. Bavarva Dept. of Electronics and Communication

Number of channels in a TDMA system N … number of channels m … number of TDMA users per radio channel B tot … total spectrum allocation B guard … Guard Band B c … channel bandwidth N= m*(B tot - 2*B guard ) BcBc Arjav A. Bavarva Dept. of Electronics and Communication

Example: Global System for Mobile (GSM) TDMA/FDD forward link at B tot = 25 MHz radio channels of B c = 200 kHz if m = 8 speech channels supported, and if no guard band is assumed : N= 8*25E6 200E3 = 1000 simultaneous users Arjav A. Bavarva Dept. of Electronics and Communication

Efficiency of TDMA percentage of transmitted data that contain information frame efficiency  f usually end user efficiency <  f, because of source and channel coding How get  f ? Arjav A. Bavarva Dept. of Electronics and Communication

Efficiency of TDMA b OH … number of overhead bits N r … number of reference bursts per frame b r … reference bits per reference burst N t … number of traffic bursts per frame b p … overhead bits per preamble in each slot b g … equivalent bits in each guard time intervall b OH = N r *b r + N t *b p + N t *b g + N r *b g Arjav A. Bavarva Dept. of Electronics and Communication

Efficiency of TDMA b T = T f * R b T … total number of bits per frame T f … frame duration R … channel bit rate Arjav A. Bavarva Dept. of Electronics and Communication

Efficiency of TDMA  f … frame efficiency b OH … number of overhead bits per frame b T … total number of bits per frame  f = (1-b OH /b T )*100% Arjav A. Bavarva Dept. of Electronics and Communication

Advantages of TDMA Flexible bit rate No frequency guard band required No need for precise narrowband filters Easy for mobile or base stations to initiate and execute hands off Extended battery life BW can be supplied on demand Arjav A. Bavarva Dept. of Electronics and Communication

Disadvantages to using TDMA Requires network-wide timing synchronization Requires signal processing for matched filtering and correlation detection Multipath distortion Arjav A. Bavarva Dept. of Electronics and Communication

CDMA Code Division Multiple Access Arjav A. Bavarva Dept. of Electronics and Communication

Arjav A. Bavarva Dept. of Electronics and Communication

Arjav A. Bavarva Dept. of Electronics and Communication

General Specification of CDMA Rx: MHz Tx: MHz 20 Channels spaced 1250kHz apart (798 users/channel) QPSK/(Offset) OQPSK modulation scheme Mbps bit rate IS-95 standard Operates at both 800 and 1900 MHz frequency bands Arjav A. Bavarva Dept. of Electronics and Communication

5: DataLink Layer5a-38 CDMA Encode/Decode Arjav A. Bavarva Dept. of Electronics and Communication

5: DataLink Layer5a-39 CDMA: two-sender interference sender 1 sender 2 uses sender 1 code to receive sender 1 data Arjav A. Bavarva Dept. of Electronics and Communication

Advantages of CDMA Many users of CDMA use the same frequency, TDD or FDD may be used Multipath fading may be substantially reduced because of large signal bandwidth No absolute limit on the number of users Easy addition of more users Impossible for hackers to decipher the code sent Better signal quality Soft hand off is performed RAKE receiver can be used to improve reception. Arjav A. Bavarva Dept. of Electronics and Communication

Disadvantages to using CDMA Power control needs to be implemented at the base station. Rapidly sampling the Radio Signal Strength Indicator (RSSI) level for each mobile and then sending power control command over a forward radio link. Near- Far- problem arises Arjav A. Bavarva Dept. of Electronics and Communication

Arjav A. Bavarva Dept. of Electronics and Communication

Arjav A. Bavarva Dept. of Electronics and Communication

Arjav A. Bavarva Dept. of Electronics and Communication

Narrowband systems Large number of narrowband channels Usually FDD Narrowband FDMA Narrowband TDMA FDMA/FDD FDMA/TDD TDMA/FDD TDMA/TDD Arjav A. Bavarva Dept. of Electronics and Communication

Logical separation FDMA/FDD f t user 1 user n forward channel reverse channel forward channel reverse channel... Arjav A. Bavarva Dept. of Electronics and Communication

Logical separation FDMA/TDD f t user 1 user n forward channelreverse channel forward channelreverse channel... Arjav A. Bavarva Dept. of Electronics and Communication

Logical separation TDMA/FDD f t user 1user n forward channel reverse channel forward channel reverse channel... Arjav A. Bavarva Dept. of Electronics and Communication

Logical separation TDMA/TDD f t user 1user n forward channel reverse channel forward channel reverse channel... Arjav A. Bavarva Dept. of Electronics and Communication

Wideband systems large number of transmitters on one channel TDMA techniques CDMA techniques FDD or TDD multiplexing techniques TDMA/FDD TDMA/TDD CDMA/FDD CDMA/TDD Arjav A. Bavarva Dept. of Electronics and Communication

Logical separation CDMA/FDD code f user 1 user n forward channelreverse channel forward channelreverse channel... Arjav A. Bavarva Dept. of Electronics and Communication

Logical separation CDMA/TDD code t user 1 user n forward channelreverse channel forward channelreverse channel... Arjav A. Bavarva Dept. of Electronics and Communication

Multiple Access Techniques in use Multiple Access Technique Advanced Mobile Phone System (AMPS) FDMA/FDD Global System for Mobile (GSM) TDMA/FDD US Digital Cellular (USDC) TDMA/FDD Digital European Cordless Telephone (DECT) FDMA/TDD US Narrowband Spread Spectrum (IS-95) CDMA/FDD Cellular System Arjav A. Bavarva Dept. of Electronics and Communication

There are three different types of CSMA protocols :- (i) 1-Persistent CSMA (ii) Non-Persistent CSMA (iii) P-Persistent CSMA CSMA 1-Persistant Non- Persistent P-Persistent CSMA Arjav A. Bavarva Dept. of Electronics and Communication

Drawback of 1-persistent The propagation delay time greatly affects this protocol. Let us suppose, just after the station 1 begins its transmission, station 2 also become ready to send its data and sense the channel. If the station 1 signal has not yet reached station 2, station 2 will sense the channel to be idle and will begin its transmission. This will result in collision. Even if propagation delay time is zero, collision will still occur. If two stations become ready in the middle of third station’s transmission both stations will wait until the transmission of first station ends and both will begin their transmission exactly simultaneously. This will also result in collision. CONTINUOUSLY SENSES SENSES & TRANSMIT TIME BUSY CHANNEL IDLE CHANNEL CHANNEL? STATION CAN TRANSMIT IDLE BUSY Arjav A. Bavarva Dept. of Electronics and Communication

channel Wait randomly idle Busy Station can transmit Sense & transmit Sense Sense Wait (Random time) wait time Busy channel Arjav A. Bavarva Dept. of Electronics and Communication

CONTINUOUSLY SENSES PROBABILITIY OUTCOME DOES NOT ALLOW TRANSMISSION Time slot TRANSMIT TIME IDLE CHANNEL? WAIT A SLOT PROBABILITY OUTCOME? CHANNEL? Acts as though collision has occurred & start again <_P BUSY STATION CAN START BUSY IDLE >P Arjav A. Bavarva Dept. of Electronics and Communication

References T. S. Rappaport, “Wireless Communication”, Prentice hall