Mobile Communications Mobile Communications
Multiple Access
SDMA TDMA
FDMA CDMA
Multiple Access
o All users of a cell in cellular networks use the same frequency spectrum
spectrum
multiplexing schemes:
o SDMA (Space Division Multiple Access)
o FDMA (Frequency Division Multiple Access) TDMA (Time Division Multiple Access)
o TDMA (Time Division Multiple Access)
o CDMA (Code Division Multiple Access)
SDMA (Space Division Multiple Access)
o Is used in all cellular networks
o Re-use of frequency bands in non-neighbouring cells o Re use of frequency bands in non neighbouring cells
o A cell is assigned frequency bands in such a way that there is no interference with neighbouring cells
o Example: 4-cell repetition scheme
o Example: 4-cell repetition scheme
SDMA (Space Division Multiple Access)
o 7-cell repetition scheme
repetition schemes exist for K = 3, 4, 7 and multiples p The bigger K, the:
• smaller the number of channels per cell smaller the number of channels per cell
• lesser the interference
SDMA (Space Division Multiple Access)
o Additionally the cells can be subdivided into sectors by use of directed antennas
use of directed antennas
SDMA (Space Division Multiple Access)
Cell planning
larger cells for rural areas
smaller cells for densly
smaller cells for densly
populated areas
FDMA (Frequency Division Multiple Access)
o Multiple access through frequency division
o Example: GSM 900 provides 125 x 200Khz channels within 25 Mhz o Example: GSM 900 provides 125 x 200Khz channels within 25 Mhz o Assignment of individual channels by a control channel
891 0 Mhz channel 45 891,2 Mhz ...
890,6 Mhz channel 43 890,8 Mhz channel 44 891,0 Mhz
...
TDMA (Time Division Multiple Access)
o traditional technology in fixed networks o is often used together with FDMA
o is often used together with FDMA o prerequisites:
o voice coding
o data compression o data compression
o access on a frequency channel is only allowed during predefined time slots
example GSM TDMA frame:
o example GSM TDMA frame:
mobile station A
6 7 0 1 2 3 4 5 6 7 0 1
TDMA frame
CDMA (Code Division Multiple Access)
o spread spectrum (because the frequency spectrum is spread) o all mobile stations use the same channel, they are distinguished o all mobile stations use the same channel, they are distinguished
from each other by codes o advantages:
o no coordination and synchronization necessary o no coordination and synchronization necessary
o uniform usage of the whole spectrum by each of the users o different CDMA methods:
Direct Sequence (DS)
o Direct Sequence (DS)
o Frequency Hopping (FH)
o Time Hopping (TH)
DS-CDMA
1
bit stream (19,2 Kbit/s)
chip stream (1 23 Mc/s) 0
0 0 1 1
code generator (1 23 Mc/s)
chip stream (1,23 Mc/s) 1
1 1 0 0
code generator (1,23 Mc/s)
1 1 1 1 0 0
0 1 1 1 0 0
1 1 1 1 0 0 0
0
0 1 1 1 0 0 1 1 0 1 1 0 0
DS-CDMA
Power Levels from MS
Received Power Levels at BTS
C
AC
CC
DC
DDS-CDMA Example
A Data
1 1 0
A Key
1 0 0 1 1 0 0 1 0 1 0 0 1 0 0 0 1 0 A Data XOR A Key 0 1 1 0 0 1 1 0 1 0 1 1 1 0 0 0 1 0 A Signal
A Signal
B Data 0 1 0
B Key
B Data XOR B Key
0 1 0 0 1
0 1 0
0 0 1
1 0 0 0 1 1 0 1 0 1 0
0 1 0 1 0 1 1 1 1 0 1 0 0 0
B Data XOR B Key 1 0 0 1
B Signal
0 1 0 1 0 1 1 1 1 0 1 0 0 0
Composite A+B
DS-CDMA Example
A Key
1 0 0 1 1 0 0 1 0 1 0 0 1 0 0 0 1 0
Composite A+B
ey 1 0 0 1 1 0 0 1 0 1 0 0 1 0 0 0 1 0
Composite A+B Signal
(A+B)*A Key
A Data 1 1 0
Integrator
A Data 1 1 0
DS-CDMA Example
B Key 0 1 0 1 0 1 0 0 0 1 1 0 1 0 1 0 0 0
B Key 0 1 0 1 0 1 0 0 0 1 1 0 1 0 1 0 0 0
Composite A+B Composite A+B Signal
(A+B)*B Key
Integrator
B Data 0 1 0
DS-CDMA Example
Composite A+B p Signal
False Key 1 0 0 0
1 0 0 0
1 0 1
1 0 0
1
1 1 0
(A+B)* False Key
( ) y
Integrator
FH-CDMA
o frequency Hopping (FH-CDMA), e.g. Bluetooth
o carrier frequency of the transmitted signal is not constant biut o carrier frequency of the transmitted signal is not constant biut
varies over time
o in one time slot the carrier remains constant. In the following one it “hops” to a different frequency
it hops to a different frequency.
