Back to Study material
WC

Unit - 3

Multiple access techniques in wireless communication

Q1) Explain FDMA?

A1)

  • It sub-divide frequency into a several frequency band with non-overlapping.
  • Fig. 1 FDMA

  • FDMA allows user to transmit signals simultaneously to satellite transponder with the help of assigning a specific frequency to every user among a channel.
  • Each transaction of signal has own unique radio channel. Channel are probably 30 KHz or less used to transmit or receive channels.
  • Frequency allocation made by national policies. Uplink (i.e. from mobile station to base station). Downlink (i.e. from base station to mobile station) uses frequencies bands like. Uplink and downlink with frequencies mentioned below :
  • Uplink

    890.2 MHz, 915 MHz

    Downlink

    935.2 MHz to 960 MHz

    Uplink (UL) and downlink (DL) can be described with the help of one relation among them as follows:

    fd = fu + 45 MHz

  • For Example :
  • If UL frequency is :

    fu = 890 MHz + n∙ 0. 2 MHz

    Then

    fd = 935 MHz + n ∙ 0. 2 MHz

  • Fixed assigned frequency makes scheme very inflexible limits to no. of sender turn into disadvantage of FDMA.
  •  

    Q2) Explain TDMA?

    A2)

  • It is based on time slots. TDMA more flexible than FDMA scheme.
  • Dynamic allocation of channel or by some fixed pattern channels get allotted for time slots to takes places synchronized communication.
  • Fig. 2 TDMA

  • Fixed channel are allocated for communication as best practice solution for wireless phone system in Medium Access Control (MAC) reserved time slot access is for crucial.
  • Q3) Explain SDMA?

    A3)

    Fig. 3 SDMA

    A cable consists of several fiber which are used as bi-directional pairs.\

    Fig. 4 Guard Spaces

    Interference’s risk is reduced by guard space between frequency space.

  • It is used to assign distinct space for subscribers. It is bound to provide a base station to mobile phone user. According to availability of frequency (FDM), time slot (TDM) or code (CDM) MAC algorithm available best base station to mobile station.
  • SDM doesn’t need any multiplexing equipment. It gives well utilize individual physical channel by some multiplexing technique.
  •  

    Q4) Explain CDMA?

    A4)

  • CDMA stands for Code Division Multiple Access. CDMA is a technique for spread spectrum multiple access.
  • Data signals are XOReD with pseudorandom code and then transmitted over a channel.
  • Different codes are used to modulate their signals, selection of code to modulate is important aspect as it relates with performance of system.
  • In CDMA, single channel uses entire bandwidth and it does not share time as all stations transmit data simultaneously. Due to this CDMA differ from FDMA and TDMA.
  • Let us assume we have 4 different stations A, B, C and D. All 4 stations connected to same channel. Data from station A is d1, B is d2, C is d3 and D is d4 similarly code assigned to A is C1, B is C2, C is C3 and D is C4.
  • To transmit data, all 4 stations have multiplication strategy i.e. either code multiply by itself. It results in 4 stations or code multiply results in 4 stations or code multiply results in 4 station of code multiply by another, it results in zero station.
  •  

    Fig. 5: Communication of station with code

  • If station C and station D are talking to each other, D wants to listen whatever C is saying. It multiplies data on channel by C3 of station C.
  • As C3 C3 is 4 and rest of combination i.e. C1 C3, C2 C3  and C4 C3 are all zero.
  • Every station has some code, which is sequence of numbers known as chips.
  • Sequence should be selected in proper manner to get appropriate codes. Orthogonal sequence has some properties :
  • Multiplication of sequence by a scalar.
  • Inner product of two equal sequences.
  • Inner product of two different sequences.
  • Sequence generation uses Walsh table to generate chip sequence.
  •  

    Q5) Compare all the multiple access techniques?

    A5)

    Approach

    SDMA

    TDMA

    FDMA

    CDMA

    Idea

    Segment space into cells/sectors

    Segment sending time into disjoint time-slots, demand driven or fixed patters

    Segment the frequency band into disjoint sub-bands

    Spread the spectrum using orthogonal codes

     

    Terminals

    Only one terminal can be active in one cell/one sector

    All terminals are active for short periods of time on the same frequency

    Every terminal has its own frequency. uninterrupted

    All terminals can be active at the same place at the same moment uninterrupted.

    Signal separation

    Cell structure, directed antennas

    Synchronization in the time domain

    Filtering in the frequency domain

    Code plus special receivers

    Advantages

    Very simple, increases capacity per km2

    Established, fully digital, flexible

    Simple, established robust

    Flexible, less frequency planning needed, soft handover 

    Disadvantages

     Inflexible, antennas typically fixed

    Guard space needed (multipath propagation), synchronization difficult

    Inflexible, frequencies are a scarce resource

    Complex receivers, needs more complicated power control for senders

    Comment

    Only in combination with TDMA, FDMA or CDMA useful

    Standard in fixed networks, together with FDMA/SDMA used in many mobile networks

    Typically combined with TDMA (frequency hopping patterns) and SDMA (frequency reuse)

    Still faces some problems, higher complexity, lowered expectations: will be integrated with TDMA/FDMA

     

    Q6) Derive Spectral efficiency of different wireless access technologies?

    A6) As we know radio frequency resource is very vital in wireless communication system. Hence RF frequencies should be maximally utilized. Following are the methods used to improve the spectrum utilization.
     Cell planning

     Frequency reuse

     channel assignment

     multiple access techniques

    In general, spectral efficiency depends upon following parameters.

     Channel spacing in KHz

     frequency reuse factor • cell area in Km2 

     Modulation techniques

     Multiple Access techniques of types

     

    Spectral efficiency =

     

    Q7) Derive Spectral efficiency in FDMA system?

    A7) η = Ndata/cluster / (BWt * N * Acell)

    = N * Ndata/cell / (BWt * N * Acell) channels/MHz/Km2

    Where, Acell is the area of each cell

    Ndata/cell = [ {(BWt - 2*Bg)/Bch} - Ncntl]/N

    BWt is total bandwidth,

     Bch is channel bandwidth,

    Bg is guard band bandwidth,

    N is number of cells per cluster

    Q8) Derive Spectral efficiency in TDMA system

    A8) Spectral efficiency of Wide band TDMA and narrow band TDMA is expressed as follows:

    ηWTDMA = {(Tf - Tp - Tt)/Tf} * (Ni/Ns)

    Where,

    Tp = preamble period,

    Tt= trailer time period,

    Tf= frame duration

    Ns= number of symbols in a time slot, Ni = number of information bits

    ηNTDMA = {(Tf - Tp - Tt)/Tf} * (Ni/Ns) * {(Nsub * Bch)/BWt}

    Nsub = Number of sub bands

    Nslot = Number of slots

     

    Q9) Spectral efficiency for DS-CDMA system.

    A9) Spectral efficiency of DS-CDMA system is expressed as follows:

    η = Us * Rb/W ... bits/S/Hz

    Where Rb is information bit rate

    W = system bandwidth for one direction

    Us =system utilization