Supporting Wireless Technologies1 3. Supporting Wireless Technologies 3.1 The Frequency Spectrum Multiple signals or noises of the same frequency will

Supporting Wireless Technologies

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3. Supporting Wireless Technologies

3.1 The Frequency Spectrum Multiple signals or noises of the same frequency will cause interference

at the receiver. To avoid interference, various wireless technologies use distinct frequency bands with well-controlled signal power which are portions of the so-called frequency spectrum.

The frequency spectrum can be divided into the following categories: very low frequency (VLF), low frequency (LF), medium frequency (MF), high frequency (HF), very high frequency (VHF), ultra-high frequency (UHF), super-high frequency (SHF), extremely high frequency (EHF), infrared, visible light, ultraviolet, x-ray, gamma-ray, and cosmic ray, each of which represents a frequency band.

Wireless communication operates at frequencies in the so-called radio spectrum, which is further divided into VLF, LF, MF, HF, VHF, UHF, SHF, and HLF. In addition, infrared data association (IrDA) is also used for short-range wireless communication.


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The frequency spectrum


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3.1.1 Public Media Broadcasting

AM radio: Amplitude modulation (AM) radio stations operate at a frequency band between 520 and 1605.5 KHz.

FM radio: Frequency modulation (FM) radio used the frequency band between 87.5 and 108 MHz.

Direct broadcast satellite (DBS)

3.1.2 Cellular Communication

Global system for mobile (GSM): The two frequency bands used by GSM are 890 to 960 MHz and 1710 to 1880 MHz.

Code-division multiple access (CDMA): The IS-95 standard defines the use of the 800- and 1900-MHz bands for CDMA cellular systems.

3G wideband CDMA (WCDMA)/universal mobile telecommunications system (UMTS): Three frequency bands are allocated for 3G UMTS services: 1900 to 1980 MHz, 2020 to 2025 MHz, and 2110 to 2190 MHz.

3G CDMA 2000: This system reused existing CDMA frequency bands.


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3.1.3 Wireless Data Communication Wireless LANs: IEEE 802.11b operates at 902 to 928 MHz and 2400 to 2483

MHz, and the industrial, scientific, and medical (ISM) radio bands operates at 2.5 GHz band in the United States.

Bluetooth: Seventy-nine 1-MHz channels are allocated from the unlicensed 2.404 ro 2.480 GHz in the United States and Europe for Bluetooth signal transmission.

WiMax: A wide rande form 2 to 11 GHz that includes both licensed and unlicensed bands will be used for 802.116a, and 11 to 66 GHz can possibly be used by 802.116c.

Ultra-wideband (UWB) Radiofrequency identification (RFID): RFID tags operate at the frequency bands o

f LF (120~140 KHz), HF (13.56 MHz), UHF (868~956 MHz), and microwave (2.4 GHz).

Infrared data association (IrDA): IrDA uses frequencies around 100 GHZ for short-range data communication.

Wireless sensors: Sensor motes support tunable frequencies in the range of 300 to 1000 MHz and the 2.4-GHz ISM band.


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3.1.4 Other Fixed or Mobile Wireless Communication Digital cordless phone Global positioning system (GPS): GPS satellites use the frequency

bands 1575.42 MHz (referred to as L1) and 1227.60 MHz (L2) to transmit signals.

Meteorological satellite services Radio frequency remote control

3.2 Wireless Communication Primer3.2.1 Signal Propagation A radio signal can be described in three domains: time

domain, frequency domain, and phase domain. Phase modulation (PM) is a form of modulation that represents

information as variations in the instantaneous phase of a carrier wave.

Phase is viewed either at a point in space over an interval of time or across an interval of space at a moment in time.

A signal emitted by an antenna travels in the air following three types of propagation modes: ground-wave propagation, sky-wave propagation, and line-of-sight (LOS) propagation.


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Attenuation In the most ideal circumstances, signal power attenuation is proportional to d2, where

d denotes the distance between the transmitter and the receiver. Noise

To cope with noises in received signals, a wireless system has to ensure that the transmitted signals are sufficiently stronger than the noises

The signal-to-noise ration (SNR) is used to represent the ration of the power in a signal to the power of the noise.

들어오는 신호의 세기 ( 단위는 마이크로볼트 ) 를 Vs 라 하고 , 잡음을 Vn 이라 하면( 이것도 단위는 역시 마이크로볼트 ), 신호 대 잡음비는 아래와 같은 공식으로 표현된다 .

S/N = 20 log10(Vs/Vn)

Multi-Path Propagation

3.2.2 Modulation Amplitude Modulation Frequency Modulation Phase Modulation

ASK, FSK, PSK


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3.2.3 Multiplexing Frequency-Division Multiplexing Time-Division Multiplexing Code-Division Multiplexing

CDMA employs spread-spectrum technology and a special coding scheme (where each transmitter is assigned a code) to allow multiple users to be multiplexed over the same physical channel.

3.3 Spread Spectrum 보내고자 하는 한 순간의 정보 요소가 한 주파수에 몰려 있지 않고 넓은 주파수대로

골고루 퍼져 있게 하는 통신 방식 . 전파 에너지가 특정 주파수대에 쏠리지 않고 넓은 주파수대로 분산되어 타 통신에

혼신을 적게 주고 , 또한 일부 주파수대의 훼손에도 원래의 정보를 복원해 낼 수 있는 장점이 있어 디지털 통신 방식에 주로 사용된다 .


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3.4 Global System for Mobile and General Packet Radio Service

3.4.1 Global System for Mobile GSM can be regarded as a TDMA-based, circuit-switching, digital

cellular system in that the channels allocated to a cell are shared by several mobile connections in a TDM fashion.

GSM Network Architecture


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3.4.2 General Packet Radio Service GSM was designed for digital voice communication. Aimed at leveraging the widely deployed existing GSM systems,

general packet radio service (GPRS) has been implemented as a 2.5 cellular system for value-added data services.

Packet Switching GPRS takes a packet-oriented approach for data transmission and a

dynamic, on-demand, bundled time-slot allocation approach for a higher data rate.

GPRS Architecture


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GPRS Services General packet radio service supports both point-to-point (PTP) packet transfer service

s and point-to-multipoint (PTM) services.

GPRS Terminal

3.5 Code-Division Multiple Access3.5.1 Code-Division Multiple Access Concept 이는 원래의 송신 데이터의 대역폭이 확산코드에 의해서 확산신호의 대역폭

만큼 넓어진 것으로 이해할 수 있다 . 이 과정을 ' 확산 (Spreading)' 이라 한다 . 이때 송신 데이터가 가지고 있는 에너지는 일정하므로 데이터에 해당하는 크기는 넓어진 대역폭만큼 반비례하여 작아진다 . 이렇게 신호를 전송할 때 대역폭이 넓어졌다고 해서 이러한 기술을 대역확산 (Spread-Spectrum) 기술이라 한다 .

대역확산 방식의 특징을 요약하면 , 첫째로 확산코드를 이용해서 대역확산을 하므로 비화 특성이 매우 우수하다는 것이고 , 둘째는 확산과 역확산 과정을 거치기 때문에 외부의 협대역 간섭에 매우 강한다는 것 , 셋째로 주파수 대역이 넓어서 마치 주파수 다이버시티 효과를 얻을 수 있어서 페이딩에 강하다는 것으로 요약할 수 있다 .


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3.5.2 IS-95

IS-95 is the underlying standard of CDMA systems.

IS-95 Key Parameters

- p. 134, Table 3.3 참조

3.5.3 Software Handoff


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3.5.4 Road to 4G


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3.6 GSM Versus CDMA Comparison of GSM and CDMA

p. 138, Table 3.4 참조

Documents

Supporting Wireless Technologies1 3. Supporting Wireless Technologies 3.1 The Frequency Spectrum Multiple signals or noises of the same frequency will