Radio Access Johan Montelius [email protected]. Shannon C = W x log 2 (1 + S/N) The capacity [C] in bits/s is directly proportional to the available bandwidth.

Radio Access

Johan [email protected]

Shannon

C = W x log2(1 + S/N)

The capacity [C] in bits/s is directly proportional to the available bandwidth [W] andlog2 proportional to the signal to noise ratio [S/N].

bandwidth & power

S/ N = 1

0

2

4

6

8

1 2 3 4 5 6 7 8S or W increase

Capa

city

incr

ease

increase Wincrease S

Attenuation in open space

Sr = S0/4r2

The signal strength at a distance [Sr] is directly proportional to the sending strength [S0]and indirectly proportional to square of the

distance [r]

Real life

In urban environment the signal strength is proportional to 1/rk where k = 1,6 … 3,8

Distance costsSr = S/ r3,5

0,0000,0050,0100,0150,0200,0250,0300,035

1 2 3 4 5 6 7 8

distance from sender

signal st

rength

S = 1 S = 2S = 4

Too bad for broadcast but good for cellular systems

good quality

detectable

Not a problem

What is interference

Rules of thumb

• Bandwidth– most important factor to increase

capacity

• Power– will buy you distance but at a high cost

• Noise– your own signal can be the worst

problem

Divide the resources

• Space– systems ”far” apart don’t interfere with each other

• Frequency– modulate the signal to use a specified frequency

band

• Time– synchronize and allocate time slots

• Code– Information coding

National/International regulations

1800 1850 1900 1950 2000 2050 2100 2150 2200 2250

ITU

EU

US

Jp/Ko

China

GSM 1800

GSM 1800

PCS

UMTS

IMT 2000

IMT 2000

IMT 2000

DECT

MSS

MDS

Frequency division

• By modulating a carrier frequency, the radiated power can be limited to a specified frequency range.

• The width of the range is the bandwidth of the carrier.

• A guard band is needed to protect adjacent carriers.

Frequency planning

3 cells per sitetypically used in urban environment

A

B

C

Frequency planning

E

D

C

A

B

A

minimum distance4 sites, 3 cells per site

12 carriers needed

F

GH

LK

J

I

Time division

• Enabled by faster processors.• A carrier is divided into time slots.• Each channel is allocated a time slot.• A guard period is needed between

adjacent time slots

Timing advance

A

B

a b

A sender must adjust its transmission to meet thetime slot at the receiver. The farther away the earlier you send . The base station will tell you if your late or early.

Locating a mobile terminal

500 m

What is left ?

• when bandwidth is fixed• and power is limited• do the best modulation possible

Modulation

• frequency modulation• amplitude modulation• phase modulation• combination of above• … no modulation ?

Wireless systems

• Often use a phase modulation• Could change modulation depending

on quality of signal• Spectral efficiency

– up to 2 bits raw data per Hz under good conditions

– aprx 0,5 to 1 bit user data per Hz– limited by signal to noise ratio

How do we compare?

• What is the maximum user capacity?• What is the maximum capacity of a

system?• How many carriers do we have?• What is the total capacity of a carrier?• How many carriers can be used at any

given point?

GSM• Each duplex carrier is 2x200 KHz wide• 900

– up 890-915 MHz down 935-960 MHz– 124 duplex carriers– 2x25MHz in total

• 1800– up 1710-1785 MHz down 1805-1880 MHz– 374 duplex carriers– 2x75MHz in total !!!!!

• 1900 (in the US)– up 1850-1910 MHz down 1930-1990– 2x60MHz in total

GSM

• Time division– 8 time slots per carrier– one carrier up one carrier down

• Gaussian Minimum Shift Keying (GMSK)

– user bitrate 9,6 kb/s or 14,4 kb/s per timeslot

– raw bitrate 272 kb/s per carrier • HSCSD

– Two or more time slots

Up and down

down

up

The up link is delayed 3 slots in order togive the terminal time to adjust to the new frequency. Time slots 5 and 6 can be used to listen for better frequencies.

0 1 2 3 4 5 6 7

GPRS

• Dynamically allocate time slots– normally 1:4 one up, four down

• Data and voice can be combined • Coding schemes (user data rates)

– CS 1: 9,05 kb/s total 72,4 kb/s– CS 2: 13,4 kb/s total 107,2 kb/s– CS 3: 15,6 kb/s total 124,8 kb/s– CS 4: 21,4 kb/s total 171,2 kb/s

EDGE

• Enhanced data rate for GSM Evolution

• Change the modulation to 8-PSK i.e. 3 bits per symbol

• User data rate – 22,8 kb/s to 69,2 kb/s– Total of 553 kb/s– don’t move

UMTS/WCDMA

• Each paired carrier is 2x5MHz• 1900-1980, 2010-2025, 2110-2170

MHz • 155 MHz in total• Unpaired carriers can be used using

time-division duplex mode (TDD)• A typical operator

– Two or three paired, one unpaired– Up to six operators share the spectrum

ISM 2.4 GHz

• Industrial, Scientific and Medical– US 2400 – 24835 83,5MHz in total– Japan 2400 – 2497 89,7MHz in total

• Open for anyone, no license• Limitation on power < 0.1W (<1W US)

• Using a spread spectrum technique

Spread spectrum

• Why spread the signal over a wider spectrum?– more robust, will survive if part of the

spectrum is noisy– will allow other systems to operate in the

same environment

• Two techniques– frequency hopping– direct sequence

Frequency Hopping

• divide the spectrum into separate carriers– In ISM, FCC regulated at least 70

carriers• transmit and hop

– In ISM, FCC regulates < 400 ms• a code determines where to hop

– how do we synchronize?

• low cost, low power, very robust

Direct Sequence

• Increase the bandwidth by sending a pattern, chipping sequence, at a higher bitrate

• sequence can be static or dynamic– dynamic patterns are used in CDMA

• high bitrate, robust

Bluetooth 1.1

• Frequency hopping, GFSK modulation– Gaussian Frequency Shift Key

• 79 carriers of 1 MHz, 1600 hops per s• Power

– Class 1: 20dBm (100mW) range aprx 100m– Class 2: 4dBm (2,5 mW) range aprx 10m– Class 3: 0dbM (1 mW) range aprx 10 cm

• Master & Slave– Master determines hopping sequence

• Capacity 712 Kb/s per channel

802.11b

• DSSS, BPSK (1Mbps) QPSK (11Mbps)• ISM 2.4

– US 11 carriers– Europe (except France and Spain) 13

carriers– Japan 14 carriers

• Carrier– 22 MHz wide – can use 3 carriers without overlap!

802.11b

• 1 Mb/s using BPSK – Barker spread sequence of 11 bits

• 2 Mb/s using QPSK– Barker sequence of 11 bits (22 Mb/s raw data)

• 5,5 and 11 Mb/s– QPSK, same as for 2Mb/s– complementary code keying– 1,375M symbols/s – each symbol is 8 bits long (11 Mb/s raw data)

– each symbol represents 4 or 8 bits

802.11b

5,5 Mb/s 4 b/symbol 8 chips/symbol 1,375 Msymb/s QPSK

11 Mb/s 8 b/symbol 8 chips/symbol 1,375 Msymb/s QPSK

2 Mb/s 1 b/symbol 11 chips/symbol 2 Msymb/s QPSK

1 Mb/s 1 b/symbol 11 chips/symbol 1 Msymb/s BPSK

Code division

• Same frequency can be used• No cell planning• How do we decode the message?

Code division: coding

message di-1

1

code cik

out zikZik= dik * cik

-1

1

d1 d2

-1

1

Code division: decoding

di = zikcik

k = 1

m

m 1

code cik

out zik

-1

1

-1

1

d1 = 8

1 (-1 –1 – 1 –1 – 1 – 1 –1 – 1) = -1

Code division: multiple senders

Da = -1-1-1-1-1-1-1-1+1+1+1+1+1+1+1+1

Db = +1+1+1+1+1+1+1+1+1+1+1+1+1+1+1+1

Ca = +1+1+1-1+1-1-1-1+1+1+1-1+1-1-1-1Za = -1-1-1+1-1+1+1+1+1+1+1-1+1-1-1-1

Cb = +1-1+1+1+1-1+1+1+1-1+1+1+1-1+1+1

Zb = +1-1+1+1+1-1+1+1+1-1+1+1+1-1+1+1

Code division

Zab= +0-2+0+2+0+0+2+2+2+0+2+0+2-2+0+0

Ca = +1+1+1-1+1-1-1-1+1+1+1-1+1-1-1-1

Za = -1-1-1+1-1+1+1+1+1+1+1-1+1-1-1-1Zb = +1-1+1+1+1-1+1+1+1-1+1+1+1-1+1+1

Zab= +0-2+0+2+0+0+2+2+2+0+2+0+2-2+0+0

ZCa= +0-2+0-2+0+0-2-2+2+0+2+0+1+2+0+0

Sa= -8/8 = -1 +8/8 = +1

UWB

• Ultra wide band – More than 1.5 GHz or 20% of central

frequency

• Use low power, low enough to disappear in noise level of other systems

• Compensate by using large bandwidth, up to several GHz

• Distance is, due to low power, limited < 10 m

Shannon revisited

• Shannon’s theorem sets a limit for one receiver listening to one message.

• What happens if we have several channels open, multiple receivers.

• Is there a limitation on capacity in space?

WCDMA

• 5 MHz carrier• QPSK modulation• 3,84 Mcps chipping rate