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 Output Encoded Walsh coded voice X data data 1.2288 mcps Walsh code 1.2288 mcps generator
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 Output modulated Long coded X voice data data 1.2288 mcps Masked Long Code 1.2288 mcps Data
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 User 1 1. WCDMA uses a chip rate of 3.84mcps Tx 2. A spreading code (pseudocode) is used to separate a UE 1 users transmission from that of others. CELL A 3. The basic design principle is to: User n a) Separate one UE’s transmission from other UEs‘ Tx transmissions (uplink) b) Separate one BS’s transmission from other BSs’ transmission (downlink) CELL B c) Separate several transmissions which a UE may UE n transmit (uplink data and control) d) Separate several transmissions which a BS may transmit (downlink data and control)
WCDMA SPREADING PROCESS
WCDMA SPREADING & SCRAMBLING In order to support multiple UEs each with First, Individual data streams multiple data streams, WCDMA uses a two- are spread to the chip rate (3.84 mcps) by applying a step approach. unique spreading code. Channelization Code 1 Stream 1 Channelization Code 2 Chip rate (3.84mcps) Scrambling Code Stream 2 (unique for every UE) + Chip rate (3.84mcps) Chip rate (3.84mcps) ………….. Second, the resulting data streams are Channelization combined and scrambled by applying a Chip rate (3.84mcps) Code n scrambling code which is unique to the Stream n 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. Spreading Factor 4 8 16 32 64 128 256 DPDCH data rate 960 480 240 120 60 30 15 kbps kbps kbps kbps kbps kbps kbps 𝐷ℎ𝑗𝑞 𝑆𝑏𝑢𝑓 3,840,000 Data Rate = 𝑇𝑞𝑠𝑓𝑏𝑒𝑗𝑜 𝐺𝑏𝑑𝑢𝑝𝑠 = 𝑇𝑞𝑠𝑓𝑏𝑒𝑗𝑜 𝐺𝑏𝑑𝑢𝑝𝑠 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.
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
EXAMPLE OF ALLOCATION OF CHANNELISATION CODES
UPLINK MODULATION WCDMA uses Quadrature Phase Shift Keying (QPSK) modulation in the uplink. Pulse Shaping Re(S) SPLITTER ∽ Splits the QPSK WCDMA Cos( 𝜕𝑢) real and + Complex Valued Spread imaginary and Scrambled Signal parts S. 90 𝑝 (S) -sin( 𝜕𝑢) Im(S) Pulse Shaping
UPLINK CHANELIZATION & 𝐻 𝑒 𝐷 𝑒1 SCRAMBLING 𝐸𝐷𝐼 1 Chip rate 𝐻 𝑒 𝐷 𝑒3 𝐸𝐷𝐼 3 Chip rate + Scrambling Code 𝐻 𝑒 𝐷 𝑒5 𝐸𝑄𝐷𝐼 𝑜 I (In-Phase) Chip rate 𝐸𝐷𝐼 5 + 𝐻 𝑒 𝐷 𝑒2 Chip rate 𝐸𝐷𝐼 2 Q (Quadrature Phase) 𝐻 𝑒 𝐷 𝑒4 Chip rate + 𝐸𝐷𝐼 4 𝐻 𝑒 𝐷 𝑒6 G d and G c are 4-bit words weighted Chip rate as follows: 0000 – Off 𝐸𝐷𝐼 6 𝐻 𝑑 𝐷 𝑑 0001 – 1/15 Chip rate 0010 – 2/15 ….. 𝐷𝐷𝐼 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 from MS within cell-coverage area falling below reasonable interference levels UE1 UE2 Without Power Control, the received UE3 power levels would be unequal UE1 UE2 UE1 UE2 UE3 With Power Control, received power levels would be nearly equal UE3 UE3
TYPES OF POWER CONTROL 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
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 UE: USER EQUIPMENT which combines the transmissions BS:BASE STATION • Uplink: Several Node Bs receive the UE transmissions. Only one of them receives the transmission correctly
SOFTER HANDOVERS SECTOR B Handover within the coverage area of one base station but between different sectors. UE1 BS 2 Procedure similar to soft handover CELLS SECTOR A
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 4 8 16 32 64 128 256 Factor DPDCH 960 480 240 120 60 30 15 data rate kbps kbps kbps kbps kbps kbps kbps
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