Mobile Communications Mobile Communications Multiple Access SDMA - - PDF document

Mobile Communications Mobile Communications Multiple Access

 SDMA  TDMA  FDMA  CDMA

Mobile Communication Multiple Access 1

Multiple Access

• All users of a cell in cellular networks use the same frequency

spectrum spectrum multiplexing schemes:

• SDMA (Space Division Multiple Access)
• FDMA (Frequency Division Multiple Access)

TDMA (Time Division Multiple Access)

• TDMA (Time Division Multiple Access)
• CDMA (Code Division Multiple Access)

Mobile Communication Multiple Access 2

SDMA (Space Division Multiple Access)

• Is used in all cellular networks
• Re-use of frequency bands in non-neighbouring cells
• Re use of frequency bands in non neighbouring cells
• A cell is assigned frequency bands in such a way that there is no

interference with neighbouring cells

• Example: 4-cell repetition scheme
• Example: 4-cell repetition scheme

Mobile Communication Multiple Access 3

SDMA (Space Division Multiple Access)

• 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

Mobile Communication Multiple Access 4

SDMA (Space Division Multiple Access)

• Additionally the cells can be subdivided into sectors by

use of directed antennas use of directed antennas

Mobile Communication Multiple Access 5

SDMA (Space Division Multiple Access)

Cell planning

larger cells for rural areas smaller cells for densly smaller cells for densly populated areas

Mobile Communication Multiple Access 6

FDMA (Frequency Division Multiple Access)

• Multiple access through frequency division
• Example: GSM 900 provides 125 x 200Khz channels within 25 Mhz
• Example: GSM 900 provides 125 x 200Khz channels within 25 Mhz
• 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 ...

Mobile Communication Multiple Access 7

TDMA (Time Division Multiple Access)

• traditional technology in fixed networks
• is often used together with FDMA
• is often used together with FDMA
• prerequisites:
• voice coding
• data compression
• data compression
• access on a frequency channel is only allowed during predefined

time slots example GSM TDMA frame:

• example GSM TDMA frame:

mobile station A 6 7 0 1 2 3 4 5 6 7 1

 

TDMA frame 4,615 ms

Mobile Communication Multiple Access 8

CDMA (Code Division Multiple Access)

• spread spectrum (because the frequency spectrum is spread)
• all mobile stations use the same channel, they are distinguished
• all mobile stations use the same channel, they are distinguished

from each other by codes

• advantages:
• no coordination and synchronization necessary
• no coordination and synchronization necessary
• uniform usage of the whole spectrum by each of the users
• different CDMA methods:

Direct Sequence (DS)

• Direct Sequence (DS)
• Frequency Hopping (FH)
• Time Hopping (TH)

Mobile Communication Multiple Access 9

DS-CDMA

1 bit stream (19,2 Kbit/s) chip stream (1 23 Mc/s) 1 1 code generator (1 23 Mc/s) chip stream (1,23 Mc/s) 1 1 1 code generator (1,23 Mc/s) 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

Mobile Communication Multiple Access 10

DS-CDMA

Power Levels from MS Received Power Levels at BTS

CA CC CD CD

• exact adjustment of transmit power levels are constantly

necessary

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DS-CDMA Example

A Data 1 1 A Key 1 0 0 1 0 0 0 1 0 1 0 1 0 1 0 0 1 A Data XOR A Key 0 1 1 1 0 0 1 0 1 0 1 1 0 0 0 1 0 1 A Signal A Signal B Data 1 B Key B Data XOR B Key 0 1 1 0 1 0 0 1 1 0 0 0 1 1 0 1 0 1 0 1 1 1 1 1 0 0 1 0 1 B Data XOR B Key 0 1 1 B Signal 0 1 1 1 1 1 0 0 1 0 1 Composite A+B Signal

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DS-CDMA Example

A Key 1 0 0 1 0 0 0 1 0 1 0 1 0 1 0 0 1 Composite A+B ey 1 0 0 1 0 0 0 1 0 1 0 1 0 1 0 0 1 Composite A+B Signal (A+B)*A Key A Data 1 1 Integrator A Data 1 1 * - Operator: (A+B)*1 = (A+B) , (A+B)*0 = (A+B)

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DS-CDMA Example

B Key 0 1 1 0 0 1 0 0 0 1 1 0 1 0 1 B Key 0 1 1 0 0 1 0 0 0 1 1 0 1 0 1 Composite A+B Composite A+B Signal (A+B)*B Key Integrator B Data 1

Mobile Communication Multiple Access 14

DS-CDMA Example

Composite A+B p Signal False Key 1 0 0 1 0 0 0 1 1 1 0 0 1 1 1 (A+B)* False Key ( ) y

Integrator

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FH-CDMA

• frequency Hopping (FH-CDMA), e.g. Bluetooth
• carrier frequency of the transmitted signal is not constant biut
• carrier frequency of the transmitted signal is not constant biut

varies over time

• in one time slot the carrier remains constant. In the following one

it “hops” to a different frequency it hops to a different frequency.

Frequency

Frequency Hopping CDMA

. . . time

2402-2403 2403-2404

T

Mobile Communication Multiple Access 16

FH-CDMA

• there are several different hopping algorithms
• cyclic hopping
• cyclic hopping
• pseudo random hopping
• there is sometimes a distinction between slow frequency hopping

and fast frequency hopping and fast frequency hopping

• with S-FH (slow FH) for each data packet the frequency

remains constant with F FH (fast FH) for each data packet the frequency

• with F-FH (fast FH) for each data packet the frequency

changes irrespectively of bits and packets, one bit can be distributed among several hops h l h b f f b d h l b bl

• the larger the number of frequency bands, the less probable

collisions become

• with F-FH a collision within one frequency band may not

significant, it may be corrected by the other bands

• with S-FH there is normally an error correction on a higher level

Mobile Communication Multiple Access 17

TH-CDMA

• with time hopping CDMA the data are sent in short bursts with

pseudo random intervals between them pseudo random intervals between them

• time line is separated into equal frames which are themselves

divided into time slots. A sender occupies one slot randomly

Mobile Communication Multiple Access 18

Multiple Access summary

Mobile Communication Multiple Access 19