Coding Methods in CDMA

Coding Methods in CDMA Barker Code Pseudo-noise code Walsh code Scrambling code Channelization code Carrier Modulation code

Barker code Barker code of length n= 11 (+1,-1,+1,+1,-1,+1,+1,+1,-1,-1,-1) Used in IEEE 802.11 Strong autocorrelation The absolute value of the inner product of a vector multiplied with itself should be large. Barker code c11 has 6 1s and 5 0s

Pseudo-noise code (PN code) Autocorrelation characteristics A code appearing random like noise but is actually not random Used to generate one or multiple sequences PN codes useful for soft handover

Soft handover in CDMA A second BTS added for the users on the edge of a cell and new PN code used in new cell for soft handover. GSM - separate operate frequencies in adjacent cells to avoid inter-cell interference. CDMA- adjacent cells use same set of carrier and chipping frequencies but different codes. Each cell has distinct pseudo-noise code offsets.

M-sequences (maximum length sequences) code Code generated by using m small length shift registers The feedback generates a large number of sets, each set having m sequences Example: a set of 15 registers (m = 15) can be used to generate a set of (215 – 1) sequences Application as scrambling code

Linear feedback shift register (LFSR)

Generator Polynomial GQ = z15 + z12 + z 11 + z10 + z6 + z5 + z4 + z3 + 1. A generator polynomial must have at least first term zn present if the degree of the polynomial is n and last term z0 = 1 GQ = {1001 1100 0111 1001} Can be written as (15,12,11,10,6,5,4,3,0) Initial State Vector Starting sequence should not be 0. {000 1110 0011 1100}

Gold Codes WCDMA uses Gold codes Created from two M-sequence codes M1 and M2. M1 and M2 are added Modulo 2 Different M1 and M2 are created by just using different starting registers.

Orthogonal Codes When there is no effect of interference between the two sets of signals on the received output Require synchronization between the transmitter and receiver Do not show a strong autocorrelation property

Orthogonal Code Zero cross-correlation Cross-correlation refers to the product of the ith symbol in two codes and the sum of products for all values of i

Synchronization Instant of the received first bit of coded symbols and first bit of generated code for extracting symbols are in the same phase

Best codes Optimized codes which enable significant correlation Do not cause significant interference among the different channels

Three coding schemes in CDMA Orthogonal codes almost zero cross correlation and are used in identifying the user, user channel, and carrier. Long M-sequence PN codes have strong autocorrelation and are used in synchronizing and detecting the user channel signals. Short PN codes also have strong autocorrelation and are used in synchronizing and detecting the user carriers.

Walsh Codes Used in IS-95 cdmaOne 64 × 64 matrix All pairs of rows orthogonal Generated from a matrix called the Hadamard matrix

Walsh Code Pilot channel─ W0, zero-th row Walsh code = {0, 0, …,0,0} Synchronization channel─ W32, 32nd row Walsh code = {0, 0, …,0,0} all 0s for first half columns elements and {1,1,..., 1,1} all ones for next half elements Paging channel - W1, 1st row Walsh code = {0, 0, …,0,0} all 0s for first half columns elements and {1,1, .., 1,1} all ones for next half elements Traffic channels - W2–31, 33–63

W0 and W2 orthoganilty 0th row in the 8 ×8 matrix Walsh code W0 = {0, 0, 0, 0, 0, 0, 0, 0} 2nd row Walsh code W2 = {0, 0, 1, 1, 0, 0, 1, 1} The codes can be rewritten as {–1, –1, – 1, –1, –1, –1, –1, –1} and {–1, –1, +1, +1, –1, –1, +1, +1} Σ pi . qi = 0

Scrambling Codes A scrambling code can be a PN Msequence code Must exhibit strong autocorrelation property To support large number of users and user channels.

Channelization Codes Channelization code has a short length sequence and must exhibit the orthogonality property It is used for differentiating among different channels during CDMA transmission. Walsh codes used for channelization due to their orthogonality property

Carrier Modulation Codes Short PN codes Orthogonal phase modulation is in time-space To synchronize the carriers of different base stations Orthogonal code or PN code modulation (spreading) in code-space To identify the multiple user channels

Development of mobile telecommunication systems Universität Karlsruhe Institut für Telematik Mobilkommunikation SS 1998 Development of mobile telecommunication systems CT0/1 FDMA AMPS CT2 NMT IMT-FT DECT IS-136 TDMA D-AMPS EDGE IMT-SC IS-136HS UWC-136 TDMA GSM GPRS PDC IMT-DS UTRA FDD / W-CDMA HSPA IMT-TC UTRA TDD / TD-CDMA CDMA IMT-TC TD-SCDMA IS-95 cdmaOne IMT-MC cdma2000 1X EV-DO cdma2000 1X 1X EV-DV (3X) 1G 2G 2.5G 3G Prof. Dr. Dr. h.c. G. Krüger E. Dorner / Dr. J. Schiller

3G Technologies

3G Technologies 3G wireless communications standard First 3G system was deployed in Japan in 2001. Defined by the recommendations of the International Telecommunication Union (ITU) Below 3G─ Data rates lower than 153.6 kbps are considered 2G systems only provide speeds ranging from 9.6 kbps to 28.8 kbps 3G technologies ─ High quality of service and high data rate support for multimedia (audio, pictures, text, and video) transfer

UMTS (Universal Mobile Telecommunication System Data rates – 100 kbps to 2 Mbps It combines CDMA for bandwidth efficiency and GSM for compatibility.

WCDMA Supports data rates of 2 Mbps or higher for short distances 384 kbps for long distances

WCDMA-FDD WCDMA access is either FDD or TDD (time division duplex) WCDMA-FDD Also referred to as UTRAFDD (universal (or sometimes UMTS) terrestrial radio access-frequency division duplex) FDD separates reverse link (called uplink in GSM) and forward link (called downlink in GSM) frequencies

WCDMA-FDD 1.920–1.980 GHz for uplink 2.110–2.170 GHz for downlink Each 5 MHz bandwidth Wider than the 1.25 MHz of the IS-95 (2G CDMA) system

Protocol Layers between the WCDMA MS and BTS

Physical Layer Physical layer Uu radio interface Supports asynchronous transmission Can also support synchronous transmission

Data-link layer MAC Controls the flow of packets to and from the network layer Ciphering function Sends and receives data from control and user plane service access points at the radio link control (RLC) layer and sends it to the physical layer

Network layer above the RLCs Provides access to multiple services such as BMC (broadcast and multicast control protocol) for the user applications PDCP (packet data convergence protocol) for the user applications

Application Layer GMM (GPRS mobility management) CC [call (connection) control] • MM (mobility management) • SM (session management) • SS (supplementary service) • SMS (short message service) • User Applications

WCDMA Chipping rate used in WCDMA is 3.84 Mchip/S Multirate transmission Certain types of data need to be transmitted at fast rates and some (power-control data and SMS text) require slow transmission rates. A single code is used when transmitting small data rate signals. Multiple codes are used when transmitting large data rate signals. Gold codes, S(2) and Walsh codes