cdma tdma fdma

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Manoj Kr. Shukla Assistant Professor Dept. of Electronics Engineering

Harcourt Butler Technological InstituteKanpur 208002 Email:,

Question The EM spectrum is a limited resource How can we share it? Time Space Frequency Polarization Spread Spectrum - use a wider bandwidth?

Multiple Access techniques Goalallow many users to simultaneously share a communications resource

Time Division Multiple Access (TDMA) Space Division Multiple Access (SDMA) Frequency Division Multiple Access (FDMA) Polarization Division Multiple Access (PDMA) Code Division Multiple Access (CDMA) Interleave Division Multiple Access (IDMA) Orthogonal Frequency Division Multiple Access

Key Issue separate the signals at the receiver toextract your informationTwo methods Do not mix the signals in the first place can use space or time (SDMA or TDMA)

Use distinctive properties of each signal as ameans to identify Frequency spectrum (FDMA) Polarization of waves (PDMA) code sequence attached to each message (CDMA)

International Cocktail Party FDMA Large room divided up into small rooms withlimited microphones. Each pair of people takes turns speaking.

TDMA Large room divided up into small rooms with

limited microphones. Certain pairs of people per room, however, each pair gets limited seconds to speak.

CDMA No small rooms. Everyone is speaking in

different languages with own microphones. If voice volume is minimized, the number of people is maximized.

Definitions TDMA Time Division Multiple Access FDMA Frequency Division Multiple Access CDMA Code Division Multiple Access IDMA- Interleave Division Multiple Access

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

TDMA Details The incoming data from each source arebriefly buffered and scanned to to form a composite digital data stream mc ( t ) .Buffer Buffer Frame mc ( t )preamble preamble

U1 U2

m1 ( t ) m2 ( t )


1 2



1 2Time slot



mN ( t )

BufferScan operation


Each slot may be empty or occupied. + has preamble & guard bits

TDMA System Each user receives half ofthe frame and the full bandwidth.Interval of Interest

Users can resolve both multipath

s0 h1

s0 h2 s1 h1 s1 h2 s2 h1 s2 h2

Time allocation is

independent of power allocation. Nonlinear ISI cancellation. Cancel edge effects as well.

TDMA Block DiagramUser 1 Data Estimate Channel


ISI Cancellation


Detect and Decode

User 2 Data Output

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 TDMA installations offer savings in base station equipment, space and maintenance The most cost-effective technology for upgrading a current analog system to digital

Disadvantages to using TDMA Requires network-wide timingsynchronization Requires signal processing fro matched filtering and correlation detection Demands high peak power on uplink in transient mode Multipath distortion

General Specification of FDMA Rx: 869-894MHz Tx: 824-849MHz 832 Channels spaced 30kHz apart

(3 users/channel) DQPSK modulation scheme 48.6kbps bit rate Used in analog cellular phone systems (i.e. AMPS) Uses Frequency Division Duplexing (FDD) ISI (Intersymbol Interference) is low

Advantages of FDMA Channel bandwidth is relatively narrow (30kHz) Simple algorithmically, and from a hardware

standpoint Fairly efficient when the number of stations is small and 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 No restriction regarding the type of baseband or type of modulation

Disadvantages to using FDMA The presence of guard bands Requires right RF filtering to minimize

adjacent channel interference Maximum bit rate per channel is fixed Small inhibiting flexibility in bit rate capability Does not differ significantly from analog system If channel is not in use, it sits idle


Space Division Multiple Access

Use highly directional The receiver selects the beam that provides the greatest signal enhancement and interference reduction Smart antenna systems can adjust their antenna pattern to enhance the desired signal, null or reduce interference.

Desired Signal Direction

SDMA Pros and ConsAdvantages BW increases with km2 Disadvantages Restricted Geometry terminals in same direction cannot share

Simple system

May have unused BW if no terminals in given zone, bw not used


Polarization Division Multiple Access

Two methods Two antennas with orthogonal polarizations an antenna with dual-polarization (SATCOM)

Each polarization provides one separatechannel

PDMA Pros and ConsAdvantages doubles BW Disadvantages Large specialized Ae

Spread SpectrumCDMA - FHMA - DSMA - SSMA

Definition - Spread Spectrum The transmission bandwidth must be muchlarger than the information bandwidth

The resulting RF bandwidth is determinedby a function other than the information being sent

Spread Spectrum - illustratedPower Density Conventional Transmission


same total powerSpread Spectrum Transmission




How Two main methods Frequency Hopped Multiple Access (FHMA) Direct Sequence Multiple Access (DSMA) THMA does exist, but not common

Both depend on pseudo randomorthogonal codes often called pseudo noise


Frequency Hopping Multiple Access

message is "cut" into small "chunks"

Each chunk is modulated by a different fc(determined by pseudo-random code)

A band pass filter accepts the signals thatfollow the hopping sequence and rejects all other requires synchronization predictable patterns

note - some early systems used short

FHSS - illustratedFrequency

Frequency Hop Tune Time Dwell Time



Direct Sequence Multiple Access

Each bit is chipped Example - time domainData0.1 ms

1 bit0.1 s

Chips1000 chipsRequires much wider bandwidth

Cross Correlation Mathematical process used to determine thesimilarity between two signals

15-bit Code Received Signal Modulo-2 sum

111101011001000 011110101100100100011110101100

Correlation = -1/15 (very poor)

Used for despreading

to determine start of code to lock onto correct code

Pseudo Random Orthogonal... Different sequences are said to beorthogonal if they do not interfere with one another (ie have low cross correlation)

A sequence is pseudo random if it is

orthogonal with a time shifted version of itself number of codes available No application developer pool Some unsolved technical problems: Cell interference large cell area functions high speed mobility poor stability of existing ICs Power consumption of handsets

3GPP TDD technology, fully compatible with GSM and GPRS Easy to upgrade from existing infrastructure Efficient use of spectrum Effective data transmission. Asynchronous uplink downlink, suitable for Internet traffic Use of Smart Antenna technology Good mobility: > 120 km/h Large cells, with diameter up to 40 km

5. Third Generation Mobile 5.3 TD-SCDMA

TD-SCDMA forum Industry consortium devoted to developand support TD-SCDMA technology Established in Dec/2000 by China Mobile, China Telecom, China Unicom, Datang, Huawei, Motorola, Nortel and Siemens More than 420 members 16 Board Members 18 Senior Members 390 ordinary members

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 No sense of handoff when changing cells

Disadvantages to using CDMA As the number of users increases, theoverall quality of service decreases Self-jamming Near- Far- problem arise higher complexity of a receiver all signals should have the same strength at a receiver

near and far terminals

Terminals A sends and B receives signal strength decreases proportional to the square of the distance the signal of terminal B therefore drowns out As signal



Comparison SDMA/TDMA/FDMA/CDMAApproach Idea SDMAsegment space into cells/sectors

TDMAsegment sending time into disjoint time-slots, demand driven or fixed patterns all terminals are active for short periods of time on the same frequency synchronization in the time domain

FDMAsegment the frequency band into disjoint sub-bands

CDMAspread the spectrum using orthogonal codes


only one terminal can be active in one cell/one sector cell structure, directed antennas

every terminal has its own frequency, uninterrupted filtering in the frequency domain simple, established, robust inflexible, frequencies are a scarce resource

Signal separation

all terminals can be active at the same place at