CDMA & WCDMA AIR INTERFACE
ECE 2526-MOBILE COMMUNICATION SYSTEMS
Monday, March 25, 2020
SPREAD SPECTRUM OPTIONS (1)
Fast Frequency Hopping (FFSH)
Advantages:
β’ Has higher anti-jamming capability
SPREAD SPECTRUM OPTIONS (2)
Time Hopping Spread Spectrum (THSP)
Advantage:
β’ Has higher bandwidth efficiency.
SPREAD SPECTRUM OPTIONS (3)
Direct Sequence Spread Spectrum (DSSP)
Advantage:
β’ Can be implemented using less complex hardware and software systems.
β’ Widely used in cellular wireless communication systems.
CDMA FORWARD CHANNELIZATION REVIEW)
Each bit of voice data is βspreadβ by a factor of 64
Each Walsh code has 64 bits
X
Walsh code
generator1.2288 mcps
Output
Walsh coded
data
1.2288 mcps
Encoded
voice
data
CDMA FORWARD CHANNELS
The IS-95 channels in the forward link are arranged in the following fashion:1. Pilot channel - transmitted as a reference by
the base station to provide timing and phase reference for the mobiles
2. Paging channels (up to seven) - used to carry information to enable mobiles to be paged, SMS and other broadcast messages. It occupies Walsh codes 1 - 7 dependent upon the system requirements.
3. Sync channel - used to provide the timing reference to access the cell . Uses Walsh code 32.
4. Forward Traffic Channel - used to carry voice, user data, and also signalling information.
CDMA REVERSE CHANNELIZATION - REVIEW
1. Long code is used to provide channelization
2. Walsh codes not used; they would provide only 64 channels compared
to 4.3 billion
X
Masked Long Code
Data1.2288 mcps
Output
Long coded
data
1.2288 mcps
Walsh
modulated
voice data
There are only two basic CDMA reverse channels:
1. Access channel - used for a) gaining access to the network b) call origination requests c) sending responses to paging.
2. Reverse traffic channel - used to carrya) multirate rate voice/data parameters b) user datac) signalling
CDMA FORWARD CHANNELS
WCDMA AIR INTERFACE - PRINCIPLES
1. WCDMA uses a chip rate of 3.84mcps
2. A spreading code (pseudocode) is used to separate a users transmission from that of others.
3. The basic design principle is to:
a) Separate one UEβs transmission from other UEsβ transmissions (uplink)
b) Separate one BSβs transmission from other BSsβ transmission (downlink)
c) Separate several transmissions which a UE may transmit (uplink data and control)
d) Separate several transmissions which a BS may transmit (downlink data and control)
User nTx
User 1Tx
UE1
UEn
CELL A
CELL B
WCDMA SPREADING PROCESS
WCDMA SPREADING & SCRAMBLING
Stream 1
Stream 2
β¦β¦β¦β¦..
Stream n
ChannelizationCode 2
ChannelizationCode n
+
Scrambling Code (unique for every UE)
ChannelizationCode 1
Chip rate (3.84mcps)
Chip rate (3.84mcps)
Chip rate (3.84mcps) Chip rate (3.84mcps)
In order to support multiple UEs each with multiple data streams, WCDMA uses a two-step approach.
First, Individual data streams are spread to the chip rate (3.84 mcps) by applying a unique spreading code.
Second, the resulting data streams are combined and scrambled by applying a scrambling code which is unique to the UE.
UPLINK SPREADING, SCRAMBLING & MODULATION
1. User information (data and control) is carried over the air interface ( physical channel).
2. Different physical channels used in the uplink direction depending on what the user wants to do. Examples include:a) Request for access to the networkb) Send a single burst of datac) Send a stream of data
d) When a UE is transmitting a stream of data two physical channels are employed. These are:e) Dedicated Physical Data Channel (DPDCH)f) Dedicated Physical Control Channel (DPCCH)
DEDICATED PHYSICAL DATA CHANNEL (DPDCH)
1. A spreading factor for a DPDCH can be 4, 8, 16, 32, 64,128 or 256 which corresponds to the data rates shown below.
1. A significant amount of data is used for Forward Error Correction and the true data rate is approximately half the DPDCH rate.
2. Therefore a DPDCH with a spreading factor of 4 will carry approximately 480 Kbps of usable data. The rest is used for error correction.
3. If the user desires higher data rates, then multiple DPDCHs (up to 6) can be used.
Spreading Factor 4 8 16 32 64 128 256
DPDCH data rate 960 kbps
480 kbps
240 kbps
120 kbps
60 kbps
30 kbps
15 kbps
Data Rate = πΆβππ π ππ‘π
πππππππππ πΉπππ‘ππ=
3,840,000
πππππππππ πΉπππ‘ππ
1. In the Downlink, the Scrambling Codes are used to distinguish each cell (assigned by operator β SC planning).
2. In the Uplink, the Scrambling Codes are used to distinguish each UE (assigned by network).
SC3 SC4
SC5 SC6
SC1 SC1
Cell β1β transmits using SC1
SC2 SC2
Cell β2β transmits using SC2
SCRAMBLING CODES
EXAMPLE OF ALLOCATION OF CHANNELISATION CODES
UPLINK MODULATION
WCDMA uses Quadrature Phase Shift Keying (QPSK) modulation in the uplink.
SPLITTERSplits the real and imaginary parts S.
PulseShaping
PulseShaping
Complex Valued Spreadand Scrambled Signal
(S)
Re(S)
Im(S)
Cos(ππ‘)
β½
90π
+
-sin(ππ‘)
QPSK WCDMA
Chip rate
πΆπ1 πΊπ
Chip rate
πΆπ3 πΊπ
Chip rate
πΆπ5 πΊπ
+
Chip rate
πΆπ2 πΊπ
Chip rate
πΆπ4 πΊπ
Chip rate
πΆπ6 πΊπ
+
Chip rate
πΆπ πΊπ
+
I (In-Phase)
Q (Quadrature Phase)
Scrambling Codeπ·ππΆπ»π
π·πΆπ»1
π·πΆπ»3
π·πΆπ»5
π·πΆπ»2
π·πΆπ»4
π·πΆπ»6
πΆπΆπ»
Gd and Gc are 4-bit words weighted as follows:0000 β Off0001 β 1/150010 β 2/15β¦..1111 - 15/15/ = 1
UPLINK CHANELIZATION & SCRAMBLING
POWER CONTROL IN WCDMA
The purpose of power control is to ensure that each user receives and transmits just enough energy to prevent:
1. Blocking of distant users (near-far-effect)
2. Signal from MS within cell-coverage area falling below reasonable interference levels
UE1UE2
UE1
UE2
UE3
UE1 UE2 UE3
Without Power Control, the received power levels would be unequal
With Power Control, received power levels would be nearly equal
UE3
UE3
Power control can be divided into two parts:
1. Open loop power control (fast power control)
β’ Used to compensate e.g. free-space loss in the beginning of the call
β’ Based on distance attenuation
2. Closed loop power control (slow power control)β’ Used to eliminate the effect of fast fadingβ’ Applied 1,500 times per second
TYPES OF POWER CONTROL
CLOSED LOOP POWER CONTROL
Closed loop power control can also be divided into two parts:
1. Inner loop power controlβMeasures the signal levels and compares this
to the target value and if the value is higher than target then power is lowered otherwise power is increased
2. Outer loop power controlβAdjusts the target value for inner loop power
controlβCan be used to control performance e.g. the
Quality of Service (QoS)
WCDMA HAND-OVERS
WCDMA handovers can be categorized into three different types which support different handover modes
1. Intra-frequency handoverβ’ WCDMA handover within the same frequency and
system. Soft, softer and hard handover supported
2. Inter-frequency handoverβ’ Handover between different frequencies but within the
same system. Only hard handover supported
3. Inter-system handoverβ’ Handover to the another system, e.g. from WCDMA to
GSM or WCDMA to LTE. Only hard handover supported
SOFT HANDOVER
1. Handover between different base stations
2. MS is connected simultaneously to multiple base stations
β’ The transition between them is seamless
β’ Downlink: Several Node Bstransmit the same signal to the UE which combines the transmissions
β’ Uplink: Several Node Bs receive the UE transmissions. Only one of them receives the transmission correctly
UE: USER EQUIPMENT
BS:BASE STATION
SOFTER HANDOVERS
Handover within the coverage area of one base station but between different sectors.
Procedure similar to soft handover
UE1
BS 2
SECTOR B
SECTOR A CELLS
WCDMA SPECIFICATIONS
βCHANNEL BANDWIDH : 5MHZβDUPLEX MODE : FDD and TDDβ CHIP RATE : 3.84MbpsβFRAME LENTH : 10msβSPREADING MODULATION BALANCED QPSK(DOWNLINK)
DUAL CHANNEL QPSK(UPLINK)βDATA MODULATION : QPSK (DOWNLINK), BPSK(UPLINK)βCHANNEL CODING : CONVOLUTIONAL and TURBO CODES βCOHERENT DETECTION : USER DEDICATED TIMEMULTIPLEXED PILOTβHANDOVER : SOFT HANDOVER and FREQUENCY HANDOVER
IMT (WCDMA) FREQUENCY ALLOCATION IN KENYA
COMMUNICATION AUTHORITY (CA) NOTES
WORKED EXAMPLES
What is the spreading factor for wideband CDMA when the bit rate used for voice communication is 12.8 Kbps.
MODEL ANSWER
(i) The chiprate for WCDMA is 3.84
Therefore the spreading factor = 3,840,000/12,800 = 300
Spreading Factor
4 8 16 32 64 128 256
DPDCHdata rate
960 kbps
480 kbps
240 kbps
120 kbps
60 kbps
30 kbps
15 kbps
WORKED EXAMPLE 2
How many simultaneous voice connections can be supported in the WCDMA cell considering that when the spreading factor 4?
MODEL ANSWER
Chiprate of WCDMA is 3.84 Mcps
If the spreading factor is 4, then,
data rate =3,840,000/4 = 960 Kbps
But a standard voice channels runs at 12.8Kbps
Therefore 960Kbps will support 960/12.8 = 75 voice channels.
Spreading Factor
4 8 16 32 64 128 256
DPDCHdata rate
960 kbps
480 kbps
240 kbps
120 kbps
60 kbps
30 kbps
15 kbps
WORKED EXAMPLE 3
β’ How many voice connections can be supported in the cell if there already exists a data session at the bit rate of 384 Kbps in the cell?
MODEL ANSWER (1)
Data capacity with the lowest spreading factor is 960 Kbps
Number of available voice channels, c is therefore
π =960 β 384
12.8= 48 πβππππππ
Spreading Factor
4 8 16 32 64 128 256
DPDCHdata rate
960 kbps
480 kbps
240 kbps
120 kbps
60 kbps
30 kbps
15 kbps
WORKED EXAMPLE 3
β’ How many voice connections can be supported in the cell if there already exists a data session at the bit rate of 384 Kbps in the cell?
MODEL ANSWER (1)
Data capacity with the nearest spreading factor is 480 Kbps
Number of available voice channels, c is therefore
π =960 β 480
12.8= 38 πβππππππ
Spreading Factor
4 8 16 32 64 128 256
DPDCHdata rate
960 kbps
480 kbps
240 kbps
120 kbps
60 kbps
30 kbps
15 kbps