CDMA & WCDMA (UMTS) AIR INTERFACE ECE 2526-WIRELESS & CELLULAR COMMUNICATION SYSTEMS Monday, June 25, 2018
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 Encoded voice data X Output Walsh coded data 1.2288 mcps Walsh code generator 1.2288 mcps
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 Walsh modulated voice data X Output Long coded data 1.2288 mcps Masked Long Code Data 1.2288 mcps
CDMA FORWARD CHANNELS 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
WCDMA AIR INTERFACE - PRINCIPLES 1. WCDMA uses a chip rate of 3.84mcps User 1 Tx 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) UE 1 UE n User n Tx CELL A CELL B
WCDMA SPREADING PROCESS
Channelization Code 1 WCDMA SPREADING & SCRAMBLING First, Individual data streams are spread to the chip rate (3.84 mcps) by applying a unique spreading code. In order to support multiple UEs each with multiple data streams, WCDMA uses a twostep approach. Stream 1 Stream 2 Channelization Code 2 Chip rate (3.84mcps) Scrambling Code (unique for every UE).. + Chip rate (3.84mcps) Chip rate (3.84mcps) Stream n Channelization Code n Chip rate (3.84mcps) 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 over a 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. Spreading Factor 4 8 16 32 64 128 256 DPDCH data rate 960 kbps Data Rate = 480 kbps 240 kbps 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. 120 kbps 60 kbps Chip Rate Spreading Factor = 3,840,000 Spreading Factor 30 kbps 15 kbps
SCRAMBLING CODES 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). Cell 1 transmits using SC1 SC1 SC1 SC3 SC4 Cell 2 transmits using SC2 SC2 SC2 SC5 SC6
UPLINK MODULATION WCDMA uses Quadrature Phase Shift Keying (QPSK) modulation in the uplink. Re(S) Pulse Shaping Complex Valued Spread and Scrambled Signal (S) SPLITTER Splits the real and imaginary parts S. Cos(ωt) 90 o + QPSK WCDMA -sin(ωt) Im(S) Pulse Shaping
DCH 1 C d1 Chip rate G d UPLINK CHANELIZATION & SCRAMBLING C d3 G d DCH 3 DCH 5 C d2 Chip rate + C d5 G d Chip rate G d + I (In-Phase) Scrambling Code DPCH n Chip rate DCH 2 C d4 G d Q (Quadrature Phase) DCH 4 DCH 6 Chip rate + C d6 G d Chip rate CCH C c G c Chip rate G d and G c are 4-bit words weighted as follows: 0000 Off 0001 1/15 0010 2/15.. 1111-15/15/ = 1
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 falling below reasonable interference levels UE1 UE2 UE1 UE2 UE3 Without Power Control, the received power levels would be unequal UE1 UE2 UE3 With Power Control, received power levels would be nearly equal UE3 UE3
Power control can be divided into two parts: TYPES OF POWER CONTROL 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
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, i.e from one MSC to another. 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 SECTOR B BS 2 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