Baseband M-ary PAM

• Consider more of pulse amplitude modulation.

• It arises at some point in almost all systems

• Its performance and design is key to many systems.

Baseband M-ary PAM

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Baseband M-ary PAM

• Illustrates 4-level PAM using NRZ pulses.

• In practice these are then passed through a shaping filter to obtain Nyquist pulses.

• Digital Subscriber Lines (DSL): Major Application of PAM• Use normal twisted pair that historically has been used for POTS.• Actually bandpass signals, but can consider as PAM since modulation

is simple frequency translation.• Used in local loop typically at distances of < 1.5 km to exchange.• Because of band limited nature of telephone channel, ISI is a major

problem and must be compensated. Operational Environment:

Baseband M-ary PAM

Are several different DSL modes.

Baseband M-ary PAMTime Compression Multiplexing:

This is actually a form of TDM, often referred to as ping-pong TDM.

Baseband M-ary PAMEcho Cancellation Mode

• Allows simultaneous transmission in both directions

• Requires a hybrid transformer to do this because channel is 2-wire.

• Almost always also requires echo cancellation

Baseband M-ary PAMBasic hybrid transformer:

Hybrids are found in all 2-4 wire transitions.

Baseband M-ary PAM• DSL achieves multiple Mbits/sec on telephone lines.

• Echo cancellation approach tends to yield better performance, although this may now be questioned with OFDM (more later).

Channel Impairments:

1. ISI

• Occurs in all bandlimited channels

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Baseband M-ary PAM• Arises due to capacitive coupling between adjacent twisted pairs in

typical telephone cable.

• Two types are1. Near-end crosstalk (NEXT)

2. Far-end crosstalk (FEXT)

NEXT is major problem since usually much stronger than FEXT.

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Baseband M-ary PAMLine Codes:

• Used to shape the transmitted spectrum. Want– Zero at d.c. since telephone lines are transformer coupled.

– Small at high frequency where there is higher attenuation and much worse cross talk.

• Possible line codes includea) Manchester codes

b) Modified duobinary (discuss later)

c) Bipolar

d) 2B1Q (already seen) which is North American standard.

Baseband M-ary PAMAsymmetric Digital Subscriber Loop (ADSL):

• Single twisted pair supports1. Downstream up to 9 Mb/s depending on length of loop

2. Upstream up ~ 1Mb/s again depending on loop length.

3. POTS

Baseband M-ary PAM

• Use FDM to accommodate

• Downstream will handle video on demand (main reason originally for asymmetry).

• Internet applications typically get– Downstream is DS1 ~ 1.544 Mb/s

– Upstream ~ 160 kb/s

• On typical loops to get data rates at DS1 and higher rates requires sophisticated modulation techniques such as OFDM or very sophisticated schemes (equalizers) to compensate the ISI.

• Now will consider equalization schemes.