Multiplex

STC Systel Training Center 1

Multiple access techniques

• 1-FDMA

• 2-TDMA

• 3-CDMA

• 4-OFDMA

• 5-DATA FRAMES


FDMA

Strength

f1 f2 f3


TDMA

Frequency

Strength

f1 f2 f3

Time


CDMA

Frequency

Strength



OFDM and CDMA

Communications Systems


Outline• Multiple Access Techniques• Code Division Multiple Access (CDMA)

- Spread Spectrum Technique- Direct Sequence Spreading/De-spreading- PN Sequences and orthogonal codes- Immunity to interference- Applications

• Orthogonal Frequency Division Multiplexing (OFDM)- FDM vs OFDM- Basic System Architecture- Applications



- Spread Spectrum Technique- Direct Sequence Spreading/De-spreading- PN Sequences and orthogonal codes- Immunity to interference- Applications - Orthogonal Frequency Division Multiplexing (OFDM)- FDM vs OFDM- Basic System Architecture- Applications


Multiple Access Techniques

• Lack of resources.• Trying to optimize the usage of the available ones.


Time Division Multiple Access (TDMA)

• The whole system spectrum is allocated for single user for pre-defined burst of time.



• Advantages- More efficient use of spectrum, compared to FDMA - No need for precise narrowband filters- Flexible bit rate- Easy for mobile or base stations to initiate and execute hands off- Extended battery life



• Disadvantages- Requires network-wide timing synchronization- Multipath distortion


Frequency Division Multiple Access (FDMA)

• Pre-defined band of the system spectrum is allocated for single user all the time.



• Advantages- Sub-bands can be treated as independent systems - Capacity increase can be obtained by reducing the information bit rate and using efficient digital code- No need for network timing



• Disadvantages- The presence of guard bands- Requires right RF filtering to minimize adjacent channel interference- Channel bandwidth is relatively narrow (30kHz)- Maximum bit rate per channel is fixed- Idle channels add to spectrum inefficiency





Code Division Multiple Access (CDMA)

• The whole system spectrum is allocated for single user all the time.





Spread Spectrum Technique• Spectrum: Power spectrum associated with

the base-band signal.• Spread Spectrum: Spreading the base-band

spectrum over a given bandwidth.• Total energy under the power spectrum curve

remains the same after spreading and amplitude may reach the noise level.





Direct Sequence Spreading


Direct Sequence De-Spreading





PN Sequences• Orthogonal Codes• Walsh Codes





Immunity to Interference


Code Division Multiple Access (CDMA)• Advantages

- Greatest spectrum efficiency - No absolute limit on the number of users- Easy addition of more users- Impossible for hackers to decipher the code sent- No sense of handoff when changing cells “Soft Hand-off”- Simplified frequency planning- Random Walsh codes enhance user privacy- Precise power control increases talk time and battery life for mobile phones


Code Division Multiple Access (CDMA)

• Disadvantages- As the number of users increases, the overall quality of service decreases- Near- Far- problem arises- Low ability of complying with existing systems


Multiple Access Techniques Comparison

TDMA FDMA CDMA

Spectrum Efficiency Medium Low High

Security Low Low High

Backward Compatibility Medium High Low

Ease of Implementation High High Low

Frequency Reuse Same frequency Depends on

interference level Same frequency

Transmission Mode Burst Continuous Continuous





Applications

• IS-95: The North American DS-CDMA Standard (cdmaOne)

• WCDMA (UMTS)• GPS





OFDM Modulation


OFDM Modulation






Basic System Architecture



Serial to parallel

converterIFFT

First Data Symbol

Last Data Symbol

TPS and Pilots

First Pilot sub-carrier

Last Pilot sub-carrier

First TPS sub-carrier

r172r173 =y0

r206

r1875 =y1511r1876

r2047

r1877

r171

r0

zeros

zeros





IFFT

Parallel to Serial

Converter

x0

x1535x1536

x2047

x1536

x2047




Pilots


Pilots






Questions


Applications

• DAB• DVB-T/H• ADSL• VDSL2• WiMAX


Data Frames

• E1 Frame

• T1 Fram


E1 Frame



T1 Frame

-- Each T1 line is split into 24 channels referred to as DS0's. Each channel is 8 bits wide and the frame is sent on a 8KHz boundary.

- Every frame has synchronization bit at the beginning (8000 Hz).

The data capacity is therefore (24x8x8000)+8000 = 1.544Mbits/sec.

Multiplex

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