Transcript 1. Multiple access techniques
Wireless Communication 171004 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 reverse channel forward channel frequency seperation
Arjav A. Bavarva Dept. of Electronics and Communication
f
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 reverse channel forward channel time seperation
Arjav A. Bavarva Dept. of Electronics and Communication
t
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: 869-894MHz Tx: 824-849MHz 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
N
f
c
guard
f
- total spectrum
guard
- guard band
c
- channel bandwidth In the U.S. each cellular carrier is allocated 416 channels where:
f
12.5
MHz
guard
c
10
kHz
30
kHz N
12.5
MHz
30
kHz kHz
416
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: 869-894MHz Tx: 824-849MHz 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:
b OH
- overhead bits per frame
N r
- number of reference bursts per frame
N t
- number of traffic bursts per frame
b r
- number of overhead bits per reference burst
b p
- number of overhead bits per preamble p er slot
b g
- number of equivalent bits in each guard time interval
T f
- frame duration
R rf
- bit rate of the radio-frequency channel
b OH
N b r r
N b t p
N t
r
g b total
T f
R rf
f
1
b OH b total
100%
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
Repeating Frame Structure
One TDMA Frame Preamble Information Message Trail Bits Slot 1 Slot 2 Slot 3 … Slot N 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= m*(B tot - 2*B guard ) B c • • • • • 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
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 : = 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 = N r *b r + N t *b p + N t *b g + N r *b g • • • • • • 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
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 = (1-b OH /b T )*100% • • • f … frame efficiency b OH … number of overhead bits per frame b T … total number of bits per frame
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: 869-894MHz Tx: 824-849MHz 20 Channels spaced 1250kHz apart (798 users/channel) QPSK/(Offset) OQPSK modulation scheme 1.2288Mbps bit rate IS-95 standard Operates at both 800 and 1900 MHz frequency bands
Arjav A. Bavarva Dept. of Electronics and Communication
CDMA Encode/Decode
Dept. of Electronics and Communication
5a-38
sender 1 sender 2
CDMA: two-sender interference
uses sender 1 code 5a-39
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
user 1 user n forward channel reverse channel ...
forward channel reverse channel t
Arjav A. Bavarva Dept. of Electronics and Communication
f
Logical separation FDMA/TDD
forward channel user 1 reverse channel ...
forward channel user n reverse channel t
Arjav A. Bavarva Dept. of Electronics and Communication
f
Logical separation TDMA/FDD
user 1 forward channel reverse channel forward channel ...
user n reverse channel t
Arjav A. Bavarva Dept. of Electronics and Communication
f
Logical separation TDMA/TDD
user 1 user n forward channel reverse channel ...
forward channel reverse channel f t
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
forward channel user 1 reverse channel ...
forward channel user n reverse channel f
Arjav A. Bavarva Dept. of Electronics and Communication
code
Logical separation CDMA/TDD
forward channel user 1 reverse channel ...
forward channel user n reverse channel t
Arjav A. Bavarva Dept. of Electronics and Communication
code
Multiple Access Techniques in use
Multiple Access Cellular System Technique Advanced Mobile Phone System (AMPS) Global System for Mobile (GSM) US Digital Cellular (USDC) FDMA/FDD TDMA/FDD TDMA/FDD Digital European Cordless Telephone (DECT) FDMA/TDD US Narrowband Spread Spectrum (IS-95) CDMA/FDD
Arjav A. Bavarva Dept. of Electronics and Communication
CSMA
•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
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.
SENSES & TRANSMIT CONTINUOUSLY SENSES CHANNEL?
BUSY BUSY CHANNEL IDLE CHANNEL TIME IDLE
Arjav A. Bavarva
STATION CAN TRANSMIT
Dept. of Electronics and Communication
channel Busy idle Sense & transmit Station can transmit Sense Sense Wait (Random time) wait Wait randomly time Busy channel
Arjav A. Bavarva Dept. of Electronics and Communication
CONTINUOUSLY SENSES PROBABILITIY OUTCOME DOES NOT ALLOW TRANSMISSION TRANSMIT Time slot Time slot Time slot TIME CHANNEL?
BUSY IDLE >P CHANNEL?
WAIT A SLOT BUSY Acts as though collision has occurred & start again
Arjav A. Bavarva
PROBABILITY OUTCOME?
<_P IDLE
References
T. S. Rappaport, “Wireless Communication”, Prentice hall