W-CDMA for UMTS – Principles u Introduction u CDMA Background/ History u Code Division Multiple Access (CDMA) u Why CDMA ? u CDMA Principles / Spreading Codes u Multi-path Radio Channel and Rake Receiver u Problems to Solve u Macro Diversity and Soft Handover u Near-Far Problem and Power Control u UMTS General Requirements u FDD vs. TDD u Key Parameters u Spectrum Allocation
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W-CDMA for UMTS – Principles
u Introductionu CDMA Background/ History
u Code Division Multiple Access (CDMA)u Why CDMA ?u CDMA Principles / Spreading Codesu Multi-path Radio Channel and Rake Receiver
u Problems to Solveu Macro Diversity and Soft Handoveru Near-Far Problem and Power Control
u UMTS General Requirementsu FDD vs. TDDu Key Parametersu Spectrum Allocation
Cellular Communication Networks 2Andreas Mitschele-Thiel, Jens Mueckenheim Nov. 2014
CDMA History
Pioneer Era (Spread Spectrum)40s and 50s: Spread Spectrum technique for military anti-jam applications
1949: Claude Shannon and Robert Pierce develop basic ideas of CDMA
1970s: Several developments for military systems (e.g. GPS)
Cellular Communication Networks 3Andreas Mitschele-Thiel, Jens Mueckenheim Nov. 2014
Spread Spectrum Technology
Problem of radio transmission: frequency dependent fading can wipe out narrowband signals for duration of the interferenceSolution: spread the narrow band signal into a broad band signal using a specialcode
Þ protection against narrow band interference
Side effects:u coexistence of several signals without dynamic coordinationu tap-proof
Alternatives:u Direct Sequence (UMTS)u Frequency Hopping (slow FH: GSM, fast FH: Bluetooth)
detection atreceiver
interferencespreadsignal
signal (despreaded)
spreadinterference
f f
power power
Cellular Communication Networks 4Andreas Mitschele-Thiel, Jens Mueckenheim Nov. 2014
Spreading and Frequency Selective Fading
FDMA: Relatively small bandwidth oneach channel
u Guard bands to avoid interferencebetween the users
u Channels maybe (temporary)unavailable due to channelselective fading
CDMA: relatively large bandwidth ofthe spread signal
u Frequency selective fading causesonly some reduction in the level ofthe received signal
u Users are separated by thespreading sequence
22
22
2
frequency
channelquality
1
spreadsignals
frequency
channelquality
1 23
4
5 6
small bandwidth guard band
Cellular Communication Networks 5Andreas Mitschele-Thiel, Jens Mueckenheim Nov. 2014
CDMA Multiple Access
CDMA (Code Division Multiple Access)u all terminals send on the same frequency probably at the same time and
can use the whole bandwidth of the transmission channelu each sender has a unique random number (spreading sequence), the
sender modulates the signal with this random numberu the receiver can “tune” into this signal if it knows the pseudo random
number, tuning is done via a correlation function
Advantages:u all terminals can use the same frequency, less planning neededu huge code space (e.g. 232) compared to frequency spaceu interference (e.g. white noise) is not codedu forward error correction and encryption can be easily integrated
Disadvantages:u higher complexity of a receiver (receiver cannot just listen into the medium
and start receiving if there is a signal)u all signals should have the same strength at a receiver (power control)
Cellular Communication Networks 6Andreas Mitschele-Thiel, Jens Mueckenheim Nov. 2014
CDMA Multiple Access (contd.)
Principle of CDMA Communication
Cellular Communication Networks 7Andreas Mitschele-Thiel, Jens Mueckenheim Nov. 2014
DSSS (Direct Sequence Spread Spectrum) I
Modulation of the signal with pseudo-random number (code sequence)
u Many chips per bit (e.g., 128)result in higher bandwidth of thesignal
Spreading factor SF: ratio betweenchip rate RC and data rate Rb
u RC = Rb · SFu Tb = TC · SF
Processing Gainu GS = 10 · log10(SF)
user data(data rate)
code sequence(chip rate)
resulting signal(chip rate)
1
0
=
Tc
Ts
´
Cellular Communication Networks 8Andreas Mitschele-Thiel, Jens Mueckenheim Nov. 2014
DSSS (Direct Sequence Spread Spectrum) II
Xuser data
codesequence
modulator
radiocarrier
spreadspectrumsignal
transmitsignal
transmitter
demodulator
receivedsignal
radiocarrier
X
codesequence
basebandsignal
receiver
integrator
products
decisiondata
sums
correlator
Cellular Communication Networks 9Andreas Mitschele-Thiel, Jens Mueckenheim Nov. 2014
CDMA Principle (Downlink)
Code 0
Code 1
Code 2
S
data 0
data 1
data 2
Code 0
Code 1
Code 2
data 0
data 1
data 2
sender (base station) receiver (terminal)
Transmission overair interface
Cellular Communication Networks 10Andreas Mitschele-Thiel, Jens Mueckenheim Nov. 2014
CDMA Principle (Uplink)
Code 0
Code 1
Code 2
S
data 0
data 1
data 2
Code 0
Code 1
Code 2
data 0
data 1
data 2
sender (terminal) receiver (base station)
transmission overair interface
Cellular Communication Networks 11Andreas Mitschele-Thiel, Jens Mueckenheim Nov. 2014
UMTS Spreading
u Constant chip-rate of 3.84 Mchip/s (FDD)u Variable data rates are realized by different spreading factors of the
orthogonal channelization codesu Higher data rates: less chips per bit (and vice-versa)
u Senders are separated by unique, quasi-orthogonal scrambling codesu Simple code management: each station can reuse the same orthogonal
channelization codesu No need for precise synchronization as the scrambling codes remain
quasi-orthogonal
data1 data2 data3
scramblingcode1
chan.code3
chan.code2
chan.code1
data4 data5
chan.code4
chan.code1
sender1 sender2
scramblingcode2
Cellular Communication Networks 12Andreas Mitschele-Thiel, Jens Mueckenheim Nov. 2014
Functionality of Channelization and Scrambling Codes
Channelization Code Scrambling CodeUsage UL: Separation of physical data
(DPDCH) and control channels(DPCCH) from same terminalDL: Separation of DL connectionsto different users within one cell
Detection mechanism takesinto account interferencefrom other users as all signalsare known in the receiver(known interference can becanceled)
Multi-userDetector
(JointDetection/
InterferenceCancellation)
Despreading(Correlator)
c1(t–Td1)
RAKE 1
c2(t–Td2)
RAKE 2
cn(t–Tdn)
RAKE n
Cellular Communication Networks 29Andreas Mitschele-Thiel, Jens Mueckenheim Nov. 2014
Interference Cancellation – Realization
Subtractive interference cancellation
Cellular Communication Networks 30Andreas Mitschele-Thiel, Jens Mueckenheim Nov. 2014
FDD vs. TDD Mode
UMTS supports FDD and TDD
FDD mode:u Multiple access scheme: DS-CDMA (Direct Sequence-CDMA)u Symmetric capacity of up- and down-linku Better suited for low bit rate transmission in larger cells
(no timing advance, no synchronization from MS required)
TDD mode:u Multiple access scheme: TD-CDMA (JD-CDMA)u Asymmetric capacity allocation for up- and down-linku Strict synchronization required for MS (timing advance)u Relaxed power control and near-far resistance by the use of intra-cell