radio access johan montelius [email protected]. shannon c = w x log 2 (1 + s/n) the capacity [c] in bits/s...
TRANSCRIPT
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