Multiple Access Techniques In Wireless CommunicationSOHAIL(3774) SALMAN MAYED HUSSAIN AYAZ AHMAD(3792)
MEANING OF 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
MULTIPLE VS RANDOM ACCESS Multiple Access Techniques Used to create a dedicated channel for each user Orthogonal (TD/FD with no interference) or semi-orthogonal
(CD with interference reduced by the code spreading gain) techniques may be used Random Access No dedicated channel assigned to each user Users contend for channel when they have data to send Very efficient when users rarely active; very inefficient when
users have continuous data to send Scheduling and hybrid scheduling used to combine benefits of multiple and random access
BEFORE ACCESS TECHNIQUES..In conventional telephone systems, it is possible to talk and listen simultaneously, called duplexing.
Frequency Division Duplex (FDD)Provides two distinct bands of frequencies for every user
FDD Scheme Reverse channel for uplink Forward for downlink Frequency split is to avoid self interference
TIME DIVISION DUPLEX (TDD) To use one frequency band a bidirectional flow of
information is achieved Using the simplex-type scheme Automatically alternating in time the direction of transmission on a single frequency A high enough transmission rate on the channel for a very short delay.
TDD Scheme
NOTE Both FDD and TDD uses same spectrum
difference lies in the use of two bands
For FDD both spectrums are used for uplink and
downlink communication. For TDD a single frequency channel both the downstream and upstream directions means more user.
WHY WE USE FDD and TDD These are the two basic types of systems
whereby a fixed spectrum resource is partitioned and shared among different users FDD creates channels by dividing total system bandwidth into frequency channels TDD further divide each frequency channel into time channels
Frequency Division Multiple Access Simplest scheme provide multiple access(mostly
used by analog systems) It separates users by assigning a different carrier frequency Multiple users are isolated by bandpass filters Frequency guard bands between adjacent signal are used to avoid cross talking
FDMA SCHEME
Allocation of carrier frequency
Frequency characteristic of FDMA
Number of channels in a FDMA system Bt - Bguard N= Bc N number of channels Bt total spectrum allocation Bguard guard band Bc channel bandwidth
ADVANTAGES Capacity can be increased by reducing the
information bit rate by using efficient speech coding method Capacity can be increased by reducing the information bit rate Transmit simultaneously and continuously Hardware simplicity
DISADVANTAGES Used for narrowband systems due to which not
suitable for large data rates e.g.; computer file transfer Inefficient use of spectrum I.e.; FDMA if a channel is not in use, it remains idle and cannot be used Crosstalk arising from adjacent channel interference by non linear effects
NON LINEARITY MEANS The system have to operate on saturation to
transmit more power on single antenna Causes inter modulation (signals are not integral multiples of each other) Occupies more bandwidth Definitely interference occurs RF filters needed - higher costs
TIME DIVISION MULTIPLE ACCESS Each user uses the whole channel bandwidth for
a fraction of time. Time is divided into equal time intervals, called slots. Several slots make up a frame. Guard times are used to minimize interference.
TDMA (continue) Data is transmitted via a radio-carrier in downlink
Time-sequenced and synchronized on a common
frequency for TDMA in uplink TDMA has TDD and FDD modes TDM mode for downlink and a TDMA mode for the uplink direction
TDMA SCHEME
Allocation of slots in TDMA
Time-frequency characteristic of TDMA
Number of channels in a TDMA system m*(Btot - 2*Bguard) N= Bc N number of channels m number of TDMA users per radio channel Btot total spectrum allocation
Bguard Guard Band Bc channel bandwidth
ADVANTAGES OF TDMDA TDMA permits a flexible bit rate
Frame-by-frame monitoring of signal/rates to
enable either mobiles or base stations to initiate and execute handoffs. used exclusively i.e.; not with FDMA, utilizes bandwidth more efficiently because no frequency guard band is required between channels. Transmits each signal with sufficient guard time between time slots to accommodate time inaccuracies because of clock instability and transmission delay due to propagation distance
DISADVANTAGES TDMA on the uplink demands required high peak
power in transmit mode, that shortens battery life Requires synchronization. For a TDMA system propagation time for a signal from a mobile station to a base station varies with its distance
Transmission for any user is not
continuous.
CODE DIVISION MULTIPLE ACCESS CDMA is a spread spectrum technology of
channel access used in 3G communication networks. Each user in CDMA technique uses the same spectrum at the same time A unique code is assigned to each communication by Pseudorandom Generator to isolate one communication from other.
CDMA (Continue) Spread signal means each signal is spread by
adding more bits to it. Each bit in this case is called chip and is greater than a conventional bit. Wider bandwidths are required for CDMA techniques.
CDMA scheme
ADVANTAGES Superior voice quality
High speed Broadband Connectivity Call security Increased voice and Data Transfer capacity Increased talk time and bandwidth
DISADVANTAGES Problem occurs at a CDMA Receiver if an undesired
user has a high detected power as compared to the desired user. Bandwidth wastage for less users.
Spread Spectrum Multiple Access A transmission technique in which a PN code
(Pseudorandom noise actually a 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.
SSMA TYPES Direct Sequence Spread Spectrum (DSSS)
KNOWN as CDMA. Frequency Hopped Spread Spectrum (FHSS) is the repeated switching of carrier frequency fc from one band to another during radio transmission according to some specified algorithm uses wideband, not occur naturally.
References:[1] WIRELESS COMMUNICATION AND NETWORKING by Vijay Garg [2] https://docs.google.com/viewer?a=v&q=cache:zw4CJgjNR7gJ:www.netkrom.com/support/whitepapers/TDD_vs_ FDD_in_wireless_backhaul_white_paper.pdf+tdd+vs+fdd&hl=en&pid=bl&srcid=ADGEESindWZTDz5KGE7MdE mmFpl02Ca4UWw2EElFYvbqEaESFMe2tL_CtKr4CUmNq2g52gz1ApdcqgWzCYuvUG1MGBSeHJcrZmrJ84v4ilQ-aKeaT6L7XKpfZeQd2DtwB_QSR4Zl_V&sig=AHIEtbThIzdRqKT-IjOPufzhxYUmCCp4GA [3] https:///~wl/ofdm/class/class%2520ppt/Multiple%2520Access%2520Techniques%2520for%2520Wireless%2520 Communications940706.pps+MULTIPLE+ACCESS+TECHNIQUES+PPT+SLIDES&hl=en&pid=bl&srcid=ADGEESg2dpnBauNvtpRrDU60HGCShwxYB40gIfn9HJwc3FH05YSALyStA5_Yf3JjlH5lC9zxUlOAoZDpBoWBwiI1VxJXU3RA82EyE 7pcN58cBbnxnw5P_EVvx0A_fmOBP3TXVbPVq6i&sig=AHIEtbQolvjI6JdodrBUhnw2tTof63c6Dw [4] https://docs.google.com/viewer?a=v&q=cache:v9E7EVbUif4J:www2.spsc.tugraz.at/wwwarchive/AdvancedSignalProcessing/WS03WirelessCommunication/schaeffner.ppt+MULTIPLE+ACCESS+TECHNIQUES+PPT+SLIDES&hl=en&pid=bl&src id=ADGEESjs5PbEKYEwvz46OJiSyxvC3Dsx_87HID3WyQ4DGDwzP0xxwLC4XqWJhCNuVcPkxkJnnrhDCJEE 0Z0DCty6UhPI541Dl_3dvXQ8dwr5zXL30lcUVmWD9Ajl1JWGo8qDV9HBYb7&sig=AHIEtbS7YZdH1A8hGIi9fonH6Rxk QcQFmQ [5] http://en.wikipedia.org/wiki/Intermodulation