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Multiple Access Techniques for Wireless Communications

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					Multiple Access Techniques for Wireless
            Communications


            2005/07/06
           Weng Chien-Erh
Wireless Access Tech. Lab.

                             Table of Contents(1)

         Introduction
         FDMA
         TDMA
         CDMA
         SS (Spread Spectrum)
            FHSS
            DSSS
            Hybrid




                                                              CCU
                                                    Wireless Access Tech. Lab.
Wireless Access Tech. Lab.

                             Table of Contents(2)

         Packet Radio
            Pure ALOHA
            Slotted ALOHA
            CSMA
            Reservation Protocol
                Reservation-ALOHA
                PRMA
                NC-PRMA




                                                              CCU
                                                    Wireless Access Tech. Lab.
Wireless Access Tech. Lab.

                             Introduction (1)

         Multiple Access:
            Enable many mobile users to share simultaneously radio
            spectrum.

            Provide for the sharing of channel capacity between a
            number of transmitters at different locations.

            Aim to share a channel between two or more signals in
            such way that each signal can be received without
            interference from another.




                                                                     CCU
                                                           Wireless Access Tech. Lab.
Wireless Access Tech. Lab.

                             Introduction (2)




                                                          CCU
                                                Wireless Access Tech. Lab.
Wireless Access Tech. Lab.

                                 Introduction (3)

         In conventional telephone systems, it is possible to
         talk and listen simultaneously, called duplexing.
         Duplexing
            Allow the possibility of talking and listening simultaneously.
            Frequency Division Duplex (FDD)
                Provides two distinct bands of frequencies for every user


            Time Division Duplex (TDD)
                Multiple users share a signal channel by taking turns in time domain
                Each duplexing channel has both a forward time slot and a reverse time
                slot to facilitate bidirectional communication.




                                                                                      CCU
                                                                            Wireless Access Tech. Lab.
Wireless Access Tech. Lab.

                             Introduction (4)




                                                          CCU
                                                Wireless Access Tech. Lab.
Wireless Access Tech. Lab.
          Frequency Division Multiple Access (FDMA)
                             (1)

         Each transmitter is allocated a channel with a
         particular bandwidth.

         All transmitters are able to transmit simultaneously.




                                                              CCU
                                                    Wireless Access Tech. Lab.
Wireless Access Tech. Lab.
          Frequency Division Multiple Access (FDMA)
                             (2)

         Allocation of separate channels to FDMA signals




                                                            CCU
                                                  Wireless Access Tech. Lab.
Wireless Access Tech. Lab.
          Frequency Division Multiple Access (FDMA)
                             (3)

         Time-frequency characteristic of FDMA




                                                           CCU
                                                 Wireless Access Tech. Lab.
Wireless Access Tech. Lab.
          Frequency Division Multiple Access (FDMA)
                             (4)

         Features of FDMA
            If an FDMA channel is not in sue, then it sits idle and can’t
            be used by other users.

            Transmit simultaneously and continuously.

            FDMA is usually implemented in narrowband systems.
                Its symbol time is large as compared to the average delay spread.




                                                                                   CCU
                                                                         Wireless Access Tech. Lab.
Wireless Access Tech. Lab.
          Frequency Division Multiple Access (FDMA)
                             (5)

         Features of FDMA (Cont.)
            For continuous transmission, fewer bits are needed for
            overhead purposes (such as synchronization and framing
            bits) as compared to TDMA.

            FDMA uses duplexers since both TX and RX operate at
            the same time.




                                                                    CCU
                                                          Wireless Access Tech. Lab.
Wireless Access Tech. Lab.

           Time Division Multiple Access (TDMA) (1)

         Transmitter share a common channel.

         Only one transmitter is allowed to transmit at a
         time.
            Synchronous TDMA: access to the channel is restricted to
            regular.
            Asynchronous TDMA: a station may transmit at any time
            that the channel is free.




                                                                   CCU
                                                         Wireless Access Tech. Lab.
Wireless Access Tech. Lab.

           Time Division Multiple Access (TDMA) (2)

         Allocation of time slot in TDMA




                                                     CCU
                                           Wireless Access Tech. Lab.
Wireless Access Tech. Lab.

           Time Division Multiple Access (TDMA) (3)

         Time-frequency characteristic of synchronous
         TDMA




                                                           CCU
                                                 Wireless Access Tech. Lab.
Wireless Access Tech. Lab.

           Time Division Multiple Access (TDMA) (4)

         Features of TDMA (Cont.)

            TDMA systems divide the radio spectrum into time slots.
            Each user occupies a cyclically repeating time slot.

            Transmit data in a buffer-and-burst method, thus the
            transmission for any user is not continuous.

            TDMA has TDD and FDD modes.




                                                                    CCU
                                                          Wireless Access Tech. Lab.
Wireless Access Tech. Lab.

           Time Division Multiple Access (TDMA) (5)

         TDMA Frame Structure




                                                     CCU
                                           Wireless Access Tech. Lab.
Wireless Access Tech. Lab.

           Time Division Multiple Access (TDMA) (6)

         TDMA Frame Structure (Cont.)
            In TDMA, the preamble contains the address and
            synchronization information that both the base station
            and the mobiles use to identify each other.

            Different TDMA standards have different TDMA frame
            structures.




                                                                     CCU
                                                           Wireless Access Tech. Lab.
Wireless Access Tech. Lab.

           Time Division Multiple Access (TDMA) (7)

         Features of TDMA (Cont.)

            Share a single carrier frequency with several users.

            Data transmission is not continuous, but occurs in bursts.

            No duplexers is required since users employ different
            time slots for transmission and reception.

            TDMA can allocate different numbers of time slots per
            frame to different users, allowing bandwidth be supplied
            on demand to different users.


                                                                      CCU
                                                            Wireless Access Tech. Lab.
Wireless Access Tech. Lab.

           Time Division Multiple Access (TDMA) (8)

         Combined used of synchronous TDMA and FDMA




                                                        CCU
                                              Wireless Access Tech. Lab.
Wireless Access Tech. Lab.

           Time Division Multiple Access (TDMA) (8)

         Asynchronous TDMA: Carrier-Sense Multiple
         Access (CSMA)
            Allows a transmitter to access the channel at any time
            that is not being used by another transmitter.




                                                                     CCU
                                                           Wireless Access Tech. Lab.
Wireless Access Tech. Lab.

           Code Division Multiple Access (CDMA) (1)

         Transmitter may transmit at the same time, in the
         same channel.

         Each signal is modified by spreading it over a large
         bandwidth.
            This spreading occurs by combining the transmitter signal
            with a spreading sequence.




                                                                     CCU
                                                           Wireless Access Tech. Lab.
Wireless Access Tech. Lab.

           Code Division Multiple Access (CDMA) (2)




                                                     CCU
                                           Wireless Access Tech. Lab.
Wireless Access Tech. Lab.

           Code Division Multiple Access (CDMA) (3)

         example




                                                     CCU
                                           Wireless Access Tech. Lab.
Wireless Access Tech. Lab.

           Code Division Multiple Access (CDMA) (4)

         Features of CDMA
            Many users of a CDMA system share the same frequency.

            The symbol (chip) duration is very short and usually much
            less than the channel delay spread.

            The near-far problem occurs at a CDMA RX if an
            undesired user has a high detected power as compared to
            the desired user.




                                                                      CCU
                                                            Wireless Access Tech. Lab.
Wireless Access Tech. Lab.

                             Frequency Hopping (1)

         Frequency hopping is a form of FDMA

         Each transmitter is allocated a group of channels,
         known as hop set .

         The transmitter transmits data in short bursts,
         choosing one of these channels on which to
         transmit each burst.




                                                               CCU
                                                     Wireless Access Tech. Lab.
Wireless Access Tech. Lab.

                             Frequency Hopping (2)

         Time-frequency characteristic of a single
         transmitter.




                                                               CCU
                                                     Wireless Access Tech. Lab.
Wireless Access Tech. Lab.

                             Frequency Hopping (3)

         Signal received form a pair of frequency-hopping
         transmitters.




                                                               CCU
                                                     Wireless Access Tech. Lab.
Wireless Access Tech. Lab.

                             Time Hopping (1)

         Each bit is transmitted as a single pulse, with the
         value of j-th bit determined by whether it arrives
         before or after the reference time tj.




                                                              CCU
                                                    Wireless Access Tech. Lab.
Wireless Access Tech. Lab.

                                    Time Hopping (2)

         TH-PPM

                                          (                              )
                             Ns −1
               Str (t ) = ∑ ∑ wtr t − iTs − jT f − ciN s + jTc − d i δ
                         i   j =0



                                     Tc
                                                                                            t


                              Tf
                                               Ts
         User1 : C(1)=[1 0 0 2]               d1=0
         User2 : C(2)=[0 1 2 0]               d2=1
         User3 : C(3)=[2 2 1 1]               d3=0
                                                                                   CCU
                                                                         Wireless Access Tech. Lab.
Wireless Access Tech. Lab.

              Spread Spectrum Multiple Access (1)

         A transmission technique in which a PN code,
         independent of information data, is employed as a
         modulation waveform to “spread” the signal energy
         over a bandwidth much greater than the signal
         information bandwidth.

         At the receiver the signal is “despread” using a
         synchronized replica of the PN code.

         Direct Sequence Spread Spectrum (DSSS)

         Frequency Hopping Spread Spectrum (FHSS)
                                                              CCU
                                                    Wireless Access Tech. Lab.
Wireless Access Tech. Lab.

              Spread Spectrum Multiple Access (2)

         Direct Sequence Spread Spectrum (DSSS)
            A carrier is modulated by a digital code in which the code
            bit rate is much larger than the information signal bit rate.
            These systems are also called pseudo-noise systems.

            Also called code division multiple access (CDMA)

            A short code system uses a PN code length equal to a
            data symbol.

            A long system uses a PN code length that is much longer
            than a data symbol.


                                                                       CCU
                                                             Wireless Access Tech. Lab.
Wireless Access Tech. Lab.

              Spread Spectrum Multiple Access (3)

         Direct Sequence Spread Spectrum (DSSS)




                                                            CCU
                                                  Wireless Access Tech. Lab.
Wireless Access Tech. Lab.

              Spread Spectrum Multiple Access (4)

         Basic principle of DSSS
            For BPSK modulation




                                                      CCU
                                            Wireless Access Tech. Lab.
Wireless Access Tech. Lab.

              Spread Spectrum Multiple Access (5)

         Frequency Hopping Spread Spectrum (FHSS)
            It divides available bandwidth into N channels and hops
            between these channels according to the PN sequence.

            Fast hopping

            Slow hopping




                                                                     CCU
                                                           Wireless Access Tech. Lab.
Wireless Access Tech. Lab.

              Spread Spectrum Multiple Access (6)

         Frequency Hopping Spread Spectrum (FHSS)




                                                         CCU
                                               Wireless Access Tech. Lab.
Wireless Access Tech. Lab.

              Spread Spectrum Multiple Access (7)

         Modulation




                                                      CCU
                                            Wireless Access Tech. Lab.
Wireless Access Tech. Lab.

              Spread Spectrum Multiple Access (8)




                                                      CCU
                                            Wireless Access Tech. Lab.
Wireless Access Tech. Lab.

              Spread Spectrum Multiple Access (9)




                                                      CCU
                                            Wireless Access Tech. Lab.
Wireless Access Tech. Lab.

             Spread Spectrum Multiple Access (10)

         Performance in the presence of interference
            Narrowband interference
            Wideband interference
            Gaussian noise




                                                            CCU
                                                  Wireless Access Tech. Lab.
Wireless Access Tech. Lab.

             Spread Spectrum Multiple Access (11)




                                                     CCU
                                           Wireless Access Tech. Lab.
Wireless Access Tech. Lab.

             Spread Spectrum Multiple Access (12)
        Narrowband interference




                                                     CCU
                                           Wireless Access Tech. Lab.
Wireless Access Tech. Lab.

             Spread Spectrum Multiple Access (13)
         Wideband interference




                                                     CCU
                                           Wireless Access Tech. Lab.
Wireless Access Tech. Lab.

             Spread Spectrum Multiple Access (14)
         Gaussian noise




                                                     CCU
                                           Wireless Access Tech. Lab.
Wireless Access Tech. Lab.

             Spread Spectrum Multiple Access (15)

       Hybrid FDMA/CDMA (FCDMA):
          The available wideband spectrum is divided into a number of
          subspectras with smaller bandwidths.
          Each of these smaller suchannels becomes a narrowband
          CDMA system having processing gain lower than the original
          CDMA system.




                                                                     CCU
                                                           Wireless Access Tech. Lab.
Wireless Access Tech. Lab.

             Spread Spectrum Multiple Access (16)


       Hybrid Direct Sequence/Frequency Hopped Multiple
       Access (DS/FHMA)
          This technique consists of a direct sequence modulated
          signal whose center frequency is made to hop periodically in
          a pseudorandom fashion.
          Having an advantage in that they avoid the near-far effect.




                                                                      CCU
                                                            Wireless Access Tech. Lab.
Wireless Access Tech. Lab.

             Spread Spectrum Multiple Access (17)


       Time Division Frequency Hopping (TDFH)
          The subscriber can hop to a new frequency at the start of a
          new TDMA frame.
          Has been adopted in GSM.




                                                                     CCU
                                                           Wireless Access Tech. Lab.
Wireless Access Tech. Lab.

                             Packet Radio (1)

       In packet radio (PR) access techniques, many subscribers
       attempt to access a single channel in an uncoordinated (or
       minimally coordinated manner.

       Collision from the simultaneous transmissions of multiple
       transmitters are detected at the BS, in which case an ACK or
       NACK signal is broadcast by the BS to alert the desired user of
       received transmission.

       PR multiple access is very easy to implement but has low
       spectral efficiency and may include delays.

       The subscribers use a contention technique to transmit on a
       common channel.

                                                                      CCU
                                                            Wireless Access Tech. Lab.
Wireless Access Tech. Lab.

                             Packet Radio (2)

       ALOHA protocols, developed for early satellite systems, allow
       each subscriber to transmit whenever they have data to sent.

       The transmitting subscribers listen to the acknowledgement
       feedback to determine if transmission has been successful or
       not.

       If a collision occurs, the subscriber waits a random amount of
       time, and then transmits the packet.

       The performance of contention techniques can be evaluated by
       throughput (T), which is defined as the average number of
       message successfully transmitted per unit time, and the average
       delay (D) experienced by a typical message burst.


                                                                      CCU
                                                            Wireless Access Tech. Lab.
Wireless Access Tech. Lab.

                               Packet Radio (3)

      Packet Radio Protocols

         V p , vulnerable period is defined as
         the time interval during which the
         packets are susceptible to
         collisions with transmission form
         other user.

         Packet A suffer a collision if
         other terminals transmit packets
         during the period t1 to t1 + 2τ




                                                            CCU
                                                  Wireless Access Tech. Lab.
Wireless Access Tech. Lab.

                         Packet Radio (4)

       Assume that packet transmissions occur with Poisson
       distribution having mean arrival rate of λ packets per second
          τ
       and is the packet duration in seconds. The traffic occupancy
       or throughput R is given by R = λτ .

       R is the normalized channel traffic (measured in Erlangs) and if
       R > 1, then the packets generated by the users exceed the
       maximum transmission rate of the channel. For reasonable
       throughput, 0 < R < 1.

       Under normal loading, the throughput T is the same as the total
       offered load L.

       The load L is the sum of the newly generated packets and the
       retransmitted packets that suffered collisions.
                                                                      CCU
                                                            Wireless Access Tech. Lab.
Wireless Access Tech. Lab.

                             Packet Radio (5)

       The normal throughput is given as the total offered load times
       the probability of successful transmission, i.e.
               T = R Pr[nocollision ] = λτ Pr[nocollision ]
       The probability that n packets are generated by the user
       population during a given packet duration interval is assumed to
       Poisson distributed and is given as
                                           e− R
                                 Pr( n ) =
                                            n!
       The probability that zero packets are generated (i.e., no
       collision) during this interval is given by


                                 Pr(0) = e − R
                                                                        CCU
                                                              Wireless Access Tech. Lab.
Wireless Access Tech. Lab.

                             Packet Radio (6)

       Type of Access
          Contention protocols are categorized as:
              Random Access: there is no coordination among that users and
              the messages are transmitted from the users as they arrive at
              the transmitter.

              Scheduled Access: based on a coordinated access of users on
              the channel and the users transmit messages within allotted
              slots or time intervals.

              Hybird Access: a combination of random access and scheduled.




                                                                          CCU
                                                                Wireless Access Tech. Lab.
Wireless Access Tech. Lab.

                             Packet Radio (7)

     Pure ALOHA
        The pure ALOHA protocol is random access protocol used for
        data transfer and a user accesses a channel as soon as a
        message is ready to be transmitted.
        After a transmission, the user waits for an acknowledgment on
        either the same channel or a separate feedback channel.
        In case of collisions, the terminal waits for a random period of
        time and retransmits the message.
        For pure ALOHA, the vulnerable period is double the packet
        duration:                    ,




                                                                      CCU
                                                            Wireless Access Tech. Lab.
Wireless Access Tech. Lab.

                             Packet Radio (8)

       Slotted ALOHA
          In slotted ALOHA, time is divided into equal time slots of
          length greater than the packet durationτ .
          The subscribers each have synchronized clocks and transmit
          a message only at the beginning of a new time slot.
          The vulnerable period of slotted ALOHA is only one packet
          duration, since partial collisions are prevented through
          synchronization.
          The probability that no other packets will be generated
                                             −R
          during the vulnerable period is e .
          The throughput for the case of slotted ALOHA is thus given
          by T = Re − R .


                                                                   CCU
                                                         Wireless Access Tech. Lab.
Wireless Access Tech. Lab.

                             Packet Radio (9)

        Carrier Sense Multiple Access (CSMA)
           CSMA protocols are based on the fact that each terminal on
           the network is able to monitor the status of the channel
           before transmitting information.

           In CSMA, detection delay and propagation delay are two
           important parameters.
               Detection delay is a function of he receiver hardware and is the
               time required for a terminal to sense whether or not the
               channel is idle.
               Propagation delay is a relative measure of how fast it takes for
               a packet to travel from a BS to a MS.




                                                                            CCU
                                                                  Wireless Access Tech. Lab.
Wireless Access Tech. Lab.

                             Packet Radio (10)

      Several variations of the CSMA strategy
         1-persistent CSMA

          Non-persistent CSMA

          p-persistent CSMA

          CSMA/CD

          Data sense multiple access(DSMA)




                                                           CCU
                                                 Wireless Access Tech. Lab.
Wireless Access Tech. Lab.

                                Packet Radio (11)
    Reservation Protocols
       Reservation ALOHA (R-ALOHA)
           R-ALOHA is a packet scheme based on time division multiplexing.
           Two phase: contention phase and transmission phase
           Mobiles contend the channel in reservation phase (slotted-ALOHA)
           Mobiles that succeed in making reservation can transmit without
           interference

                                         Frame N


                          ACK      B-M             B-M                   ACK



                          M-B      M-B

            Reservation             Transmission
               phase                   phase


                                                                                  CCU
                                                                        Wireless Access Tech. Lab.
Wireless Access Tech. Lab.

                             Packet Radio (12)

       PRMA (Packet Reservation Multiple Access)
           A combination of TDMA and reservation ALOHA
           Ask channel resource in the talkspurt
           Release channel resource in the silent gap
           Permission probability                        Effect of voice activity detector



                                              Frame N




                   B->M      ACK    B->M     ACK                  B->M        ACK




                   M->B             M->B                          M->B


                                                                                  CCU
                                                                        Wireless Access Tech. Lab.
Wireless Access Tech. Lab.

                               Packet Radio (13)

     NC-PRMA (Non-Collision Packet
     Reservation Multiple Access)
         The existing users inform the
         BS about their demands in a
         non-collision manner (time-
         frequency signaling scheme)




         Ii     : Information slot
         CM i   : Control minislot
         C      : Control slot
         UID    : Uplink user identifier
         DID    : Downlink user identifier



                                                             CCU
                                                   Wireless Access Tech. Lab.

				
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