CDMA & WCDMA AIR INTERFACE

ECE 2526-MOBILE COMMUNICATION SYSTEMS

Monday, March 25, 2020

SPREAD SPECTRUM OPTIONS (1)

Fast Frequency Hopping (FFSH)

Advantages:

• Has higher anti-jamming capability

SPREAD SPECTRUM OPTIONS (2)

Time Hopping Spread Spectrum (THSP)

Advantage:

• Has higher bandwidth efficiency.

SPREAD SPECTRUM OPTIONS (3)

Direct Sequence Spread Spectrum (DSSP)

Advantage:

• Can be implemented using less complex hardware and software systems.

• Widely used in cellular wireless communication systems.

CDMA FORWARD CHANNELIZATION REVIEW)

Each bit of voice data is ‘spread’ 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 UE’s transmission from other UEs‘ transmissions (uplink)

b) Separate one BS’s transmission from other BSs’ 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

…………..

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 = 𝐶ℎ𝑖𝑝 𝑅𝑎𝑡𝑒

𝑆𝑝𝑟𝑒𝑎𝑑𝑖𝑛𝑔 𝐹𝑎𝑐𝑡𝑜𝑟=

3,840,000

𝑆𝑝𝑟𝑒𝑎𝑑𝑖𝑛𝑔 𝐹𝑎𝑐𝑡𝑜𝑟

1. In the Downlink, the Scrambling Codes are used to distinguish each cell (assigned by operator – 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 “1” transmits using SC1

SC2 SC2

Cell “2” 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(𝜔𝑡)

∽

90𝑜

+

-sin(𝜔𝑡)

QPSK WCDMA

Chip rate

𝐶𝑑1 𝐺𝑑

Chip rate

𝐶𝑑3 𝐺𝑑

Chip rate

𝐶𝑑5 𝐺𝑑

+

Chip rate

𝐶𝑑2 𝐺𝑑

Chip rate

𝐶𝑑4 𝐺𝑑

Chip rate

𝐶𝑑6 𝐺𝑑

+

Chip rate

𝐶𝑐 𝐺𝑐

+

I (In-Phase)

Q (Quadrature Phase)

Scrambling Code𝐷𝑃𝐶𝐻𝑛

𝐷𝐶𝐻1

𝐷𝐶𝐻3

𝐷𝐶𝐻5

𝐷𝐶𝐻2

𝐷𝐶𝐻4

𝐷𝐶𝐻6

𝐶𝐶𝐻

Gd and Gc are 4-bit words weighted as follows:0000 – Off0001 – 1/150010 – 2/15…..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)

• Used to compensate e.g. free-space loss in the beginning of the call

• Based on distance attenuation

2. Closed loop power control (slow power control)• Used to eliminate the effect of fast fading• 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 control❖Measures 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 control❖Adjusts the target value for inner loop power

control❖Can 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 handover• WCDMA handover within the same frequency and

system. Soft, softer and hard handover supported

2. Inter-frequency handover• Handover between different frequencies but within the

same system. Only hard handover supported

3. Inter-system handover• 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

• The transition between them is seamless

• Downlink: Several Node Bstransmit the same signal to the UE which combines the transmissions

• 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

❖CHANNEL BANDWIDH : 5MHZ❖DUPLEX MODE : FDD and TDD❖ CHIP RATE : 3.84Mbps❖FRAME LENTH : 10ms❖SPREADING MODULATION BALANCED QPSK(DOWNLINK)

DUAL CHANNEL QPSK(UPLINK)❖DATA MODULATION : QPSK (DOWNLINK), BPSK(UPLINK)❖CHANNEL CODING : CONVOLUTIONAL and TURBO CODES ❖COHERENT DETECTION : USER DEDICATED TIMEMULTIPLEXED PILOT❖HANDOVER : 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

• 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

𝑐 =960 − 384

12.8= 48 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

• 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

𝑐 =960 − 480

12.8= 38 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