Radio Access
Johan Montelius
jm@sics.se
Shannon
C = W x log2(1 + S/N)
The capacity [C] in bits/s is
directly proportional to the
available bandwidth [W] and
log2 proportional to the
signal to noise ratio [S/N].
bandwidth & power
S/N = 1
Capacity increase
8
6
increase W
increase S
4
2
0
1
2
3
4
5
S or W increase
6
7
8
Attenuation in open space
Sr = S0/4r2
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 costs
signal strength
Sr = S/r3,5
0,035
0,030
0,025
0,020
0,015
0,010
0,005
0,000
S=1
S=2
S=4
1
2
3
4
5
6
7
distance from sender
8
Too bad for broadcast but
good for cellular systems
Not a problem
good quality
detectable
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
MSS
ITU
EU
IMT 2000
GSM 1800
UMTS
DECT
MDS
US
PCS
Jp/Ko
China
IMT 2000
GSM 1800
1800
IMT 2000
1850
1900
1950
2000
2050
2100
2150
2200
2250
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
A
B
C
3 cells per site
typically used in urban environment
Frequency planning
4 sites, 3 cells per site
12 carriers needed
minimum distance
A
B
D
E
C
J
F
G
H
K
L
I
A
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 sender must adjust its transmission to meet the
time slot at the receiver. The farther away the earlier
you send . The base station will tell you if your late or early.
a
b
B
A
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
0
1
2
3
4
5
6
7
down
up
The up link is delayed 3 slots in order to
give the terminal time to adjust to the new
frequency. Time slots 5 and 6 can be used to
listen for better frequencies.
GPRS
• Dynamically allocate time slots
– normally 1:4 one up, four down
• Data and voice can be combined
• Coding schemes (user data rates)
– CS
– CS
– CS
– CS
1:
2:
3:
4:
9,05
13,4
15,6
21,4
kb/s
kb/s
kb/s
kb/s
total 72,4 kb/s
total 107,2 kb/s
total 124,8 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
11 Mb/s 8 b/symbol
8 chips/symbol 1,375 Msymb/s QPSK
5,5 Mb/s 4 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
1
message di
d1
d2
-1
1
code cik
-1
1
out zik
-1
Zik= dik * cik
Code division: decoding
1
out zik
-1
1
code cik
-1
S
m
di =
1
m
zikcik
k=1
d1 =
1
8
(-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
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+1-1-1-1
Db = +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
Za = -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
Zab= +0-2+0+2+0+0+2+2+2+0+2+0+2-2+0+0
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
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