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CELLULAR COMMUNICATIONS 7. Multiple Access. Multiple Access  Radio spectrum is shared among number of transmissions  Uplink and downlink voice and data.

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Presentation on theme: "CELLULAR COMMUNICATIONS 7. Multiple Access. Multiple Access  Radio spectrum is shared among number of transmissions  Uplink and downlink voice and data."— Presentation transcript:

1 CELLULAR COMMUNICATIONS 7. Multiple Access

2 Multiple Access  Radio spectrum is shared among number of transmissions  Uplink and downlink voice and data transmission from the single handset Duplexing methods  Unrelated communications sessions Many voice conversations by different parties  Random Access Requests New handset /session requests

3 Traditional MAC Protocol Classification  Contention Protocols  Transmit when you feel like transmitting  Retry if collision, try to minimize collisions, additional reservation modes  Problem: Receiver must be awake as well  Scheduling Protocols  Use a “pre-computed” schedule to transmit messages  Distributed, adaptive solutions are difficult

4 Quality of Service  Data networks are usually “best-effort” networks  No guarantee on data delivery time  Usually use packet switching(routing)  Decision when and how to send data for each packet  No resource allocation for session  Guaranteed quality  Make promises that certain amount of data will be deliveries within specified time  Usually use circuit switching  Route (circuit) is established at the beginning of the session with all required resources (e.g. spectrum/bandwidth) allocated

5 Call Admission Control (CAC)  Decide if to accept request for new session call  Admin call only if QoS constrains could be met without affecting existing sessions  Network is busy  Depends on type of required service

6 UMTS Quality of Service (QoS) Classes 3GPP (3 rd Generation Partnership Project) defines four classes for UMTS  Conversation Class: Delay Constrained / Connection Oriented/ Constant Bit Rate  Streaming Class: Delay Constrained / Connection Oriented / Variable Bit rate  Interactive Class: Longer Delay Constraints / Connectionless  Background Class: Best Effort Connectionless Services

7 Duplexing  Time Division Duplexing  Frequency Division Duplexing

8 Multiple Access  FDMA  Different frequency for different users  Multicarrier(MC) FDMA: set of different frequencies  TDMA  Different time slots for different users  FHMA  Different frequency for different times for different users governed by code  CDMA  Same carriers for different users but modulated differently

9 9 Some medium access control mechanisms for wireless TDMA CDMA FDMA SDMA Fixed Aloha Reservations DAMA Multiple Access with Collision Avoidance Polling Pure CSMA Used in GSM Slotted Non-persistent p-persistentCSMA/CA Copes with hidden and exposed terminal RTS/CTS Used in (optional) MACAW MACA-BIFAMA CARMA Used in (mandatory) Used in (optional) FHSS: Frequency-Hopping Spread Spectrum DSSS: Direct Sequence Spread Spectrum CSMA: Carrier Sense Multiple Access CA: Collision Avoidance DAMA: Demand-Assigned Multiple Access MACA-BI: MACA by invitation FAMA: Floor Acquisition Multiple Access CARMA: Collision Avoidance and Resolution Multiple Access FHSS: Frequency-Hopping Spread Spectrum DSSS: Direct Sequence Spread Spectrum CSMA: Carrier Sense Multiple Access CA: Collision Avoidance DAMA: Demand-Assigned Multiple Access MACA-BI: MACA by invitation FAMA: Floor Acquisition Multiple Access CARMA: Collision Avoidance and Resolution Multiple Access FHSS DSSS Used in GSM Fixed Used in Bluetooth Used in UMTS

10 Frequency Division Multiple Access (FDMA)  The frequency spectrum is divided into unique frequency bands or channels  These channels are assigned to users on demand  Multiple users cannot share a channel  Users are assigned a channel as a pair of frequencies (forward and reverse channels)  FDMA requires tight RF filtering to reduce adjacent channel interference

11 Channel-1Channel-6Channel-5Channel-7Channel-8 Channel-9 FDMA TIME FREQUENCY Channel-2Channel-3Channel-6Channel-4Channel-5Channel-7Channel-8 Channel-9

12 FDMA Guide Bands

13 Time Division Multiple Access (TDMA)  TDMA systems divides the radio spectrum into time slots, and in each time slot only one use is allowed to either transmit or receive  Transmission for any user is non-continuous  In each TDMA frame, the preamble contains the synchronization information  TDMA shares a single carrier frequency with several users  TDMA could allocate varied number of time slots per frame to different users

14 TDMA TIME FREQUENCY Channel-7 Channel-10 Channel-8Channel-9Channel-6Channel-5Channel-4Channel-3 Channel-2 Channel-1

15 TDMA Guard Time

16 CDMA

17 CDMA Family

18 Hidden Terminal Problem  A sends to B, C cannot receive A  C wants to send to B, C senses a “free” medium (CS fails)  collision at B, A cannot receive the collision (CD fails)  A is “hidden” for C BAC

19 Exposed Terminal Problem  B sends to A, C wants to send to D  C has to wait, CS signals a medium in use  since A is outside the radio range of C waiting is not necessary  C is “exposed” to B BAC D

20 Motivation - Near and Far Terminals  Terminals A and B send, C receives  the signal of terminal B hides A’s signal  C cannot receive A  This is also a severe problem for CDMA networks  precise power control required ABC

21 Jean-Paul Linnartz21 ALOHA Protocol  Any terminal is allowed to transmit without considering whether channel is idle or busy  If packet is received correctly, the base station transmits an acknowledgement.  If no acknowledgement is received by the mobile,  1) it assumes the packet to be lost  2) it retransmits the packet after waiting a random time, usually with probability Pr in every slot. itb

22 Jean-Paul Linnartz 22 ALOHA Protocol  Unslotted ALOHA: transmission may start anytime  Slotted ALOHA: packets are transmitted in time slots  Critical performance issue: "How to choose the retransmission parameter?"  Too long: leads to excessive delay  Too short: stirs instability Instability: Number of terminals in backlog grows without bounds

23 Summary

24 OFDM-FDMA and OFDM-TDMA  OFDM-FDMA  Each user occupies a subset of subcarriers for a given time. The frequency bands assigned to a specific user is not changed over the time.  OFDM-TDMA  Each user occupies more than one OFDM symbols, and transmits on different time slots.

25 OFDMA  Each user occupies a subset of subcarriers for a given time. Users should not be overlapped in frequency domain at any given time. But, the frequency bands assigned to a specific user may change over the time.

26 26 Diversity  Fading : Rapid fluctuations of signal strength due to constructive and destructive interference between multi-paths.  Diversity : Technique to compensate for fading channel impairments. It can be obtained over: Time - Interleaving of coded bits Frequency – Spread spectrum & frequency hopping Space – Multiple antennas

27 OFDMA  Multiuser diversity  achievable data rate of a given resource varies from one user to another.  assign each resource to the user who can exploit it best → multiuser diversity.  For example, consider an antenna with two users:

28 OFDMA – multiuser diversity  Transmitter receives feedback from users

29 OFDMA – multiuser diversity  For OFDM-TDMA, the SINR on each subcarrier is the average of two users  For OFDMA with resource allocation, each subcarrier are allocated to the specific user that has the best channel frequency response. Thus the SINR for OFDMA is the maximum of two users.

30 SDMA  Space Division Multiple Access

31 31 Antenna Diversity Multiple antennas at the base station to transmit the same signal. Fundamental difference : “Multi-user diversity takes advantage of rather than Compensate fading”

32 32 Opportunistic Beam forming The information bearing signal at each of the transmit antenna is multiplied by a random complex gain. Formation of random beam.

33 History of Adopted Access Technique Adopted Not Adopted 2G systems G systems G systems G systems 2020 CDM TDM OFDM CDMA MIMO-SCM OFDM MIMO-SCM ? ? [ Multi-carrier techniques for 4G systems]  Robust against frequency selective fading  A lot of know-how obtained through research and development of wireless LANs and digital broadcasting  Synergistic effects when combined with CDMA


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