C5 - Signals Transmission

Cables

• Unshielded twisted pair (UTP)
  • Not suited for long distance due to attenuation (capacitor between wires)
• Co-axial
  • High bandwidth broadcast (one-to-many)
  • Costly
  • Bulky

UTP and co-axial cable are designed to reduce noise

• Optical fiber
  • Uses near-infrared light with wavelength $1500\mathrm{nm}$
  • Single-mode fibers
    • $125\mu \mathrm{m}$ diameter including the cladding
    • $8-10\mu \mathrm{m}$ core diameter
  • Dense wavelength division multiplexing (DWDM)

For comparison, human hair are $60\mu \mathrm{m}$ in diameter

Developed in 1970’s by Prof. Charles Kao

Radio

Uses EM wave with frequency $10\mathrm{kHz}-300\mathrm{GHz}$
Practical systems use frequency $300\mathrm{kHz}-30\mathrm{GHz}$

Low frequency waves can travel around obstacles by diffraction, therefore transmit over a longer distance but carrying less information.

Usual frequency allocation

In descending order

1. Satellite
2. Cellular
3. TV
4. AM/FM
5. Ship Telephone (Marine Communication)

1G to 5G

Generation	Signal	Access Technique	Speed
1G	Analog	FDMA	$2.4\mathrm{kbps}$
2G	Digital	TDMA, FDMA	$14.4\mathrm{kbps}$
3G	Digital	CDMA	$384\mathrm{kbps}-2\mathrm{Mbps}$
4G	Digital	OFDMA	$1\mathrm{Gbps}$
5G	Digital	OFDMA, MIMO	$10\mathrm{Gbps}$

Cellular Network

Cell phone: two-way radio communication device.
800 MHz - 5.9 GHz

• Base stations (antenna, transceiver, and control equipment)
  • Omni-directional
  • Communicate using multiple access techniques
• Mobile telephone switching office (MTSO)
  • Connect base stations and PSTN
• Public switched telephone network (PSTN)
• Central office (CO)

Hexagon rather than circle for best coverage

Frequency Reuse Plan

For 1G and 2G, adjacent cells cannot use same frequency.

Larger cluster size $\to$ smaller interference $\to$ less number of frequencies available for each cell $\to$ low frequency reuse

Smaller cluster size $\to$ higher frequency reuse

Higher power, higher coverage, lower number of cells, reduce network capacity

• Macro, Micro and Pico cells

To increase network capacity, more cells should be used instead of higher power per cell

Hand-off

3G limit: 200 km/hr
Lower practical limit to cell size (no hand-off all the time)

Roaming

Handset needs to be compatible to the cellular system and frequency bands used in the host operator

Multiple Access Technologies

Frequency-Division Multiple Access (FDMA)

• 1G
  • Frequency channels were assigned in pairs (30kHz each for uplink and downlink)
• Analog system
• Divide the total frequency spectrum bandwidth into narrow frequency bands.

e.g. FM radio stations broadcast in different frequency bands with 200kHz bandwidth

Time Division Multiple Access (TDMA)

• 2G (GSM)
  • Divided into channels of 200kHz, 8 time slots
• Digital system
• Multiple frequency channels, different time slots

For GSM, time is partitioned into frames with duration of 4.6ms.
Each frame is divided into 8 time slots of ~0.57ms

2G is about 16% more efficient than 1G (25kHz vs 30kHz)

Synchronization

In the upstream direction, mobiles at different distances might not be perfectly synchronize.

A guard time of 0.03ms (9km) is added for each time slot to prevent collison.

Code Division Multiple Access (CDMA)

Spread Spectrum technology invent by Hedy Lamarr during WW2

Multiplying a slow data rate information rate stream by a much higher data rate code (PN code)

Adjacent cells can use the same frequency

Match-filtering

Multiply the received signal with the PN code and integrate the result.

If a correct PN code is used, the output would be an amplified magnitude of m or -m.

If a wrong PN code is used, the output will be zero or noise-like.

Steps of PN Coding

1. Encode $A\times {\text{PN}}_A$
2. Decode $A\times {\text{PN}}_A\times {\text{PN}}_A=mA$

m is the number of chips

Multiple PN Coding

1. Encode $A\times {\text{PN}}_A$ and $B\times {\text{PN}}_B$
2. Addition $A\times {\text{PN}}_A+B\times {\text{PN}}_B$
3. Decode into $A\cdot {\text{PN}}_A\cdot {\text{PN}}_A+B\cdot {\text{PN}}_B\cdot {\text{PN}}_A=mA$ and $A\cdot {\text{PN}}_A\cdot {\text{PN}}_B+B\cdot {\text{PN}}_B\cdot {\text{PN}}_B=mB$

PN code should be orthogonal, i.e. ${\text{PN}}_A\cdot {\text{PN}}_B=0$

To transfer multiple signals, encode is done before addition. Then, information can be retrieved using the respective PN code.

OFDMA

Massive MIMO