IN INTERFACE ECE 2526-MOBILE COMMUNICATION SYSTEMS Monday, March - - PowerPoint PPT Presentation

CDMA & WCDMA AIR IR IN 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 generator 1.2288 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

• ccupies 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 Data 1.2288 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 carry

a) multirate rate voice/data parameters b) user data c) 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 n Tx User 1 Tx

UE1 UEn

CELL A CELL B

WCDMA SPREADING PROCESS

WCDMA SPREADING & SCRAMBLING

Stream 1 Stream 2

…………..

Stream n Channelization Code 2 Channelization Code n

+

Scrambling Code (unique for every UE) Channelization Code 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 network b) Send a single burst of data c) 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 =

𝐷ℎ𝑗𝑞 𝑆𝑏𝑢𝑓 𝑇𝑞𝑠𝑓𝑏𝑒𝑗𝑜𝑕 𝐺𝑏𝑑𝑢𝑝𝑠 = 3,840,000 𝑇𝑞𝑠𝑓𝑏𝑒𝑗𝑜𝑕 𝐺𝑏𝑑𝑢𝑝𝑠

• 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.

SPLITTER Splits the real and imaginary parts S. Pulse Shaping Pulse Shaping Complex Valued Spread and Scrambled Signal (S) Re(S) Im(S) Cos(𝜕𝑢) ∽ 90𝑝 +

• sin(𝜕𝑢)

QPSK WCDMA

Chip rate 𝐷𝑒1 𝐻𝑒 Chip rate 𝐷𝑒3 𝐻𝑒 Chip rate 𝐷𝑒5 𝐻𝑒

+

Chip rate 𝐷𝑒2 𝐻𝑒 Chip rate 𝐷𝑒4 𝐻𝑒 Chip rate 𝐷𝑒6 𝐻𝑒

+

Chip rate 𝐷𝑑 𝐻𝑑

+ I (In-Phase) Q (Quadrature Phase)

Scrambling Code 𝐸𝑄𝐷𝐼𝑜

𝐸𝐷𝐼1 𝐸𝐷𝐼3 𝐸𝐷𝐼5 𝐸𝐷𝐼2 𝐸𝐷𝐼4 𝐸𝐷𝐼6 𝐷𝐷𝐼 Gd and Gc are 4-bit words weighted as follows: 0000 – Off 0001 – 1/15 0010 – 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

UE1 UE2 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 Bs

transmit 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

IM IMT (W (WCDMA) FREQUENCY ALLOCATION IN IN KENYA

COMMUNICATION AUTHORITY (C (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 DPDCH data 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 DPDCH data 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 𝑑 = 960 − 384 12.8 = 48 𝑑ℎ𝑏𝑜𝑜𝑓𝑚𝑡

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

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 𝑑 = 960 − 480 12.8 = 38 𝑑ℎ𝑏𝑜𝑜𝑓𝑚𝑡

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