David WaittKate Disney

2008 April

Digitizing An Analog World

Is “Going Digital” Enough?Is “Going Digital” Enough?

Simply digitizing information (Music, Simply digitizing information (Music, video, voice, pictures) for transmission is video, voice, pictures) for transmission is not enough to use the RF spectrum not enough to use the RF spectrum efficientlyefficiently

The information must be transmitted as The information must be transmitted as efficiently as possibleefficiently as possible

Common Wireless Devices TodayCommon Wireless Devices Today

CellphoneCellphone• GSM GSM

• (850, 900, 1800, 1900 MHz)(850, 900, 1800, 1900 MHz)

• CDMA CDMA • (900, 1900 MHz)(900, 1900 MHz)

Wireless (WiFi, 802.11) Wireless (WiFi, 802.11) (2400, 5100, 5800 MHz)(2400, 5100, 5800 MHz)

Bluetooth Bluetooth (2400 MHz)(2400 MHz)

How do these devices use the RF How do these devices use the RF spectrum efficiently?spectrum efficiently?

GSM – GSM – GGlobal lobal SSystem for ystem for MMobile obile communications communications

Time Division Time Division MultiplexingMultiplexing

Cell site Cell site communicates with up communicates with up to 8 phones on a to 8 phones on a single frequency for a single frequency for a short amount of time, short amount of time, “Time Slot”“Time Slot”

Each slot is .004615 Each slot is .004615 SecondsSeconds

AT&T (Cingular), T-AT&T (Cingular), T-MobileMobile

slot 1 slot 8slot 7slot 6slot 5slot 4slot 3slot 2

TIME

CDMA – CDMA – CCode ode DDivision ivision MMultiple ultiple AAccessccess

Phones are assigned Phones are assigned codes that are codes that are embedded into the embedded into the signalsignal

The cell site can The cell site can distinguish between the distinguish between the different codesdifferent codes

The number of phones The number of phones communicating on a communicating on a given frequency is given frequency is determined by the “Bit determined by the “Bit Error Rate”Error Rate”

Verizon, SprintVerizon, Sprint

Code 1 Code NCode 6Code 4Code 3Code 2

Communication with many phonesoccurs on ONE frequency all the time

What Else Can Be DoneWhat Else Can Be Done

What else can be done What else can be done to encourage to encourage “Frequency Re-Use?“Frequency Re-Use?

This drawing shows very poor use of the radio spectrum. if this was a GSM cellular system with 10 different frequencies, this system could accommodate 80 phones for the state of California

Reducing Coverage AreaReducing Coverage Area

Reducing the coverage area of each cell site increases capacity, but costs more to deploy the network

BluetoothBluetooth

Most common Most common use is Bluetooth use is Bluetooth headsets for use headsets for use with cellular with cellular phonesphones

Frequency Frequency Hopping Spread Hopping Spread SpectrumSpectrum

Frequency HoppingFrequency Hopping 79 hopping channels79 hopping channels Used in a random orderUsed in a random order 0.000625 seconds per frequency0.000625 seconds per frequency

TIME, (Time Slots)

Frequency 1

Frequency 8

Frequency 7

Frequency 6

Frequency 5

Frequency 4

Frequency 3

Frequency 2

Why Frequency Hopping?Why Frequency Hopping?

If a few packets of data is lost due to If a few packets of data is lost due to interference, only a very small portion of interference, only a very small portion of the information is lost!the information is lost!

TIME, (Time Slots)

Frequency 1

Frequency 8

Frequency 7

Frequency 6

Frequency 5

Frequency 4

Frequency 3

Frequency 2