Code Division Multiple Access (CDMA)

1. Code Division Multiple Access(CDMA) Prepared by: Anil Ramroop ID-0024144 Perapong Uttarapong ID-0026852

2. Code Division Multiple Access(CDMA) • Multiple Access is a technique by which multiple users use the same physical resource. • The most prevalent multiple access techniques are TDMA,FDMA and CDMA. • CDMA is based on Spread Spectrum which evolved some 50 years back. • Each traffic channel is multiplied by a unique high speed bit stream to spread the channel in the frequency domain. • At the receiver end the spread signal is multiplied by the same high speed stream to retrieve the data. • Out of the CDMA implementations cdmaOne is the one which is most widely deployed commercially. • cdmaOne is based on the IS-95(1993) standard and is a trade mark of CDMA development group (CDG).

3. cdmaOne overview and Terminology Add check bits A/D mux Information Information Bits FEC Code Symbols Chips Chips Spreading Code Generator Spreader PSK

4. CDMA Cellular Reuse • Same Frequency is used in every cell • Interference becomes low power noise • Spectral efficiency much higher than AMPS. • 20 times theoretically. • 5~8 times in practice. • CDMA Design Parameters ( Same as AMPS ) • Forward Channel Frequency  869 – 894 Mhz • Reverse Channel Frequency  824 – 849 Mhz • Tx/Rx Frequency Spacing  45 Mhz

5. IS-95 CDMA • Existing 12.5 Mhz assigned cellular bands are used to derive 10 different CDMA bands  1.25 Mhz per band. • Frequency Reuse factor in CDMA = 1. • Channel Rate = 1.2288 Mcps (cycles per sec). • Multipath Fading exploited in CDMA. • Rake receivers are used to combine the output of several received signals. • Fading does occur on the individual signals, but each signal is affected differently and so using several of them to make a decision improves the probability of obtaining a correct decision – Multipath Diversity combining. • At Mobile • Three correlators used to receive three different signals. With a fourth one used as a roving finger which is used to detect new strong incoming signals. Process ensures that the Rake receiver always used the three strongest signals. • At Base Station • All four correlators are used to receive signals  Antenna Diversity.

6. The Rake receiver • One of the main advantages of the CDMA system is its ability to resolve different multipath components. • This is possible since CDMA is a wide band system.(??) • In order to resolve multipath signals the subscriber unit/BTS should make use of multiple receivers operating at different phases. Each of these receivers are called fingers. • The outputs of these fingers are added to form a strong output. Correlator 1 Correlator 1 Output Correlator 2 Correlator 2 Combiner Combiner Input Input Correlator 3 Correlator 3 Searcher Searcher

7. The Coding and Modulation process in CDMA • 64 bit Walsh Codes are used to provide 64 channels within each frequency band. • Walsh codes used for spreading in the forward link. • Walsh codes used to provide orthogonal modulation and not spreading to the full 1.2288 rate in the reverse link. • Besides Walsh codes, 2 other codes are used in IS-95 • Long PN code: generated from a 42 bit shift register having (242 – 1) = 4.398 x 1012 different codes. A mask is used to overlay the codes, the mask differs from channel to channel. The chip rate is 1.2288Mcps. These codes are used for: • Data scrambling/encryption in the forward path • Data spreading and encryption in the reverse path • Short PN code: generated from a pair of 15 bit shift registers having 215 –1 = 32767 codes. These codes are used for synchronization in the forward and reverse links and cell identification in the forward link • Each cell uses one of 512 possible offsets. • Adjacent cells must use different offsets. • Chip Rate is 1.2288Mcps ( i.e., not used for spreading )

8. Direct Sequence CDMA • Multiply data with a Pseudo-random noise sequence (PN)

9. Hadamard-Walsh Code The four orthogonal sequences in this Walsh code set are taken from the rows of the matrix H4 ; that is, W0 = [ 0 0 0 0 ] W1 = [ 0 1 0 1 ] W2= [ 0 0 1 1 ] W3= [ 0 1 1 0 ]

12. CDMA Channels • Forward and Reverse Channels are separated by 45 Mhz. • Forward Channel comprises of the following channels: • Pilot channel (always uses Walsh code W0 ) • Paging channel(s) ( use Walsh code W1 – W7 ) • Sync channel (always uses Walsh code W32 ) • Traffic channels ( use Walsh codes W8 – W31 and W33 – W63 ) • Reverse Channel comprises of the following channels: • Access channel • Traffic channel • Link Protocol can be summarized as follows: • Mobile acquires phase, timing and signal strength via the Pilot channel • Mobile synchronizes to Base Station via the Sync. Channel • Mobile gets system parameters via the paging channel. • Mobile and BS communicates over the traffic channels during a connection. • Mobile and BS communicate over the access and paging channels during system acquisition and paging.

14. Forward/Reverse Channel Spreading and Scrambling Process • Forward channels are spread using one of 64 orthogonal Walsh functions. Note – Perfect separation between the channels in the absence of multipath interference. • To reduce interference between mobiles that use the same Walsh function in neighboring cells, all signals in a particular cell are scrambled using the the short PN sequence for cell identification. • For the paging and traffic channels, the long PN sequence is used to scramble the signal before spreading. • Reverse channels are spread using the long PN sequence. • All 64 orthogonal Walsh functions are used to provide orthogonal modulation. • The stream is then scrambled using the short PN sequence for cell identification purposes.

15. CDMA Vocoder & Transmission Rates • IS-95 supports different transmission rates. The vocoder (QCELP) outputs 9.6 Kbps when there is a full speech signal and 1.2 Kbps when a silent period is detected. (Note 1) • Intermediate rates such as 4.8Kbps and 2.4 Kbps are progressively used to either increase or decrease rates based on the speech signal content. • Rate decisions are made every 20msec interval ( the interval over which samples are collected and processed). • In CDMA – A signal (rate set 1) is always sent for it takes too long for the receivers to ramp up again for reception. • To accommodate all the different data rates using the same air interface, bits in the lower bit rate streams are repeated to bring the rate up to 9.6Kbps. • However the bits are output at a correspondingly lower power. For example: the 1.2 Kbps bits are repeated 8 times to bring it up to 9.6 Kbps, but, the signal strength is reduced to 1/8 the power.

16. CDMA Vocoder & Transmission Rates(Cont.) • In 1995, Qualcomm introduced a higher rate coder (QCELP13) called Rate Set 2 that produces a 14.4 Kbps speech signal and 1.8 Kbps when a silent period is detected. The other intermediate rates are 7.2 Kbps and 3.6 Kbps. • So as not to change the air interface and the transmitters and receivers (in particular the interleaver), the following were done: • Reverse link  rate set 2(RC2) signal is encoded at 1/2 rate as opposed to 1/3 rate used in rate set 1(RC1). • Forward Link  puncturing of the code is used to reduce it from ½ to ¾ (i.e., 2 symbols from every 6 encoded symbols are dropped). • IS-95 also supports variable rate transmission on the reverse link as follows: • Instead of repeating the symbols and sending them at 9.6 or 14.4 Kbps, the repeated symbols are randomly deleted from the frame (after interleaving). • Thus, mobiles transmitting at the same rate do not have all their bits arrive at the same time at the BS which reduces interference. • When this mode is used, the symbols are sent at full power as oppose to reduce power when using repetition.

17. Forward Logical Channels • Pilot Channel • Transmitted at all times ( sequence of 0’s ). • Uses Walsh Code W0. • Provides phase and timing reference to the mobile terminal. • Provides signal strength to the mobile for channel acquisition. • Re-used in every cell and sector with different short PN code offset. • Sync Channel -- can be received by a mobile after it locks on to a pilot channel. Features of the Sync Channel: • Operates at 1200 bps. • Has a frame length of 26.666 msec. • Uses Walsh code W32 and uses the same PN sequence & offset as the Pilot channel. • Provides timing information to the mobile for synchronization. • Provides pilot PN offset. • Provides system time ( needed for the short PN sequence generation ). • Provides system and network Ids. • Provides paging channel rates. • Provides BS protocol revision level. • CDMA channel number

18. Forward Logical Channels (Cont.) • Paging Channel is used to page mobiles and transmit system information. • Bit rate of 9600 or 4800 bps. • Frame Length 80msec – messages can occupy several slots (1-4). • Use Walsh codes W1 – W7 ( System can use 1–7 paging channels depending on traffic load ). • Transmit the system parameter message: registration information, BS class, BS longitude/latitude, power control thresholds, etc. • Transmit the access parameter message: # of access channels, initial access power requirements, # of access attempts, authentication info., etc. • Carry the channel assignment for a traffic channel to mobile.

21. Forward Logical Channels (Cont.) • Forward Traffic Channels are used to carry user data and signaling data. Features are as follows: • Bit rates up to 9600bps (rate set 1) and up to 14.4Kbps (rate set 2). • Frame length of 20ms (192 bits for rate set 1 and 288 bits for rate set 2) • Use Walsh codes W8 – W31 and W33 – W63. • Can be used in two modes: Blank & Burst or Dim & Burst • Blank & Burst is similar to NA-TDMA, signaling data replaces speech data • Dim & Burst multiplexes signaling data or a secondary data stream with speech data (speech data sent at 4.8, 2.4 or 1.2 Kbps for RC1 and 7.2, 3.6 or 1.8Kbps for RC2.

29. Reverse Logical Channels • Access Channel: is a random access channel used by mobiles to send information (not user data) to the BS. • One or more access channels are paired with a paging channel (max. is 32 in total) • Mobiles respond to paging messages on their corresponding access channels. • Bit rate is 4800bps. • Long PN code mask consists of: • Access channel number, BS identifier, corresponding paging channel number, PN_offset (No PN offset is used for the quadrature spread). • Mobiles compete for access as follows: • Mobile chooses an access channel at random from the set associated with the paging channel. • If two mobiles choose the same access channel and PN time alignment  their transmissions will interfere with each other – Thus, the BS will not be able to distinguish between them. • No channel sensing for collision avoidance. • If a mobile does not get an ACK back before the timer expires it makes another attempt (at a higher power level) after a random wait. It repeats this process for a max. number of times, if it does not succeed, it waits a random time and then restarts the process all over again.

31. Reverse Logical Channels • Reverse Traffic Channel: used to carry user data (primary and secondary) and signaling data. A BS will support up to 61 channels. • Data transfers at 4 different levels within a rate set supported. • Signaling information is multiplexed with the user data, where possible (i.e. if variable data rates are supported). If not possible, then the signaling information takes over the channel briefly to transmit a message (blank and burst) • Instead of signaling information, a secondary traffic stream can be multiplexed (i.e., voice is primary and data is secondary). • Long PN mask is used to uniquely identify a mobile. Can be of two types: • Public consists of the mobile’s ESN. • Private derived from the encryption and authentication process. • Orthogonal modulation consists of sending one of 64 possible Walsh functions for each group of 6 coded bits. • Walsh Function number = C0 + 2C1 + 4C2 + 8C3 + 16C4 + 32C5 where the C’s represent the coded bits. Output rate is 28.8 x 64 / 6 = 307.2Kbps.

34. Power Control • Power control is of paramount importance for a CDMA system. In order to have max. efficiency, the power received at the BS from all Mobiles must be nearly equal. • Mobile’s power too low, then many bit errors will occur. • Mobile’s power too high, then the interference level increases. • Power Control at Mobile • Closed Loop: power control information is sent to the mobile from the BS. Puncturing is used, 2 data symbols are replaced by one power control symbol (double the power). This bit either indicates a transition up or down in power in 1db increments. The power bit is sent 16 times per 20ms (every 1.25ms) (Pclosed.) • Open Loop: The mobile senses the strength of the pilot signal and can adjust its power based upon that. If the signal is very strong, the assumption can be made that the mobile is very close to BS and the power should be dropped. The mobile uses Ptarget sent in the access param. Msg. – (Popen). • The transmitted power at the mobile (in units of dBm) is Ptran= Popen + Pclosed • Power Control at BS • The BS decreases its power level gradually and wait to hear from the mobile what the frame error rate (FER) is (power measurement report). If high, the BS then increases its power level.

37. Handoffs • CDMA supports three types of handoffs • Hard handoff ( Similar to the NA-TDMA (IS-136) ) • Soft handoff • Handoff between two different cells (between two different sites) operating on the same frequency. • Softer handoff • Handoff between two different sectors of the same cellular site. • Mobile assists in the handoff process, therefore it is referred to as Mobile Assisted Hand Off (MAHO). • Mobile report signal measurements to the BS. The roving finger of the Rake receiver is used to measure the pilot signals of neighboring BSs (neighbor list messages sent to mobiles periodically). • During call setup, a mobile is given a list of handoff thresholds and a list of likely new cells. The mobile keeps track of those cell that fall above the threshold and sends this information to the MSC whenever requested. • Mobile and MSC classify the neighboring BSs to keep track of the handoff process. • Based upon data received from the mobile the MSC constantly re-classifies the BSs with regard to the mobile: • Active list: contains BSs currently used for communication at least one BS. • Candidate list: contains list of BSs that could be used for communication based upon current signal strength measurements.

38. Handoffs (Con’t) • Neighbor list: contains a list of BSs that could soon be promoted to candidate list. • Remaining list: all other BSs that do not qualify. • When the MSC moves a BS from the candidate list into the active list, it directs BS to serve the mobile. • MSC informs both the new BS and the mobile and assigns a forward channel number (Walsh code) for communication. • Soft handoffs consist of the mobile being served by two BSs. This means the following: • Mobile receives the signal from two BSs. • Two BSs also receives the signal from the mobile. • Soft handoffs also eliminate the ping pong effect (i.e., when traveling along the boundary of two cells) as the mobile is being served by two BSs and does not have to switch BSs until absolutely necessary.

39. Handoffs (Con’t) • Mobile initiates the handoff • The mobile analyze the measurements and inform the MSC when a handoff might be necessary. (If one BS’s signal strength becomes much higher that the other). • Handoff process is controlled by the MSC. • When a handoff occurs all three correlators are switched over to the new cell and used as a Rake receiver again. • The connection to the current BS is cutoff and the new BS becomes the current BS. • Summary of handoff process is as follows: • Mobile communicates with original/current BS. • Mobile communicates with current cell BS and new cell BS. • Mobile communicates with the new cell BS (which becomes the current cell).

42. Mobile Management • Mobiles must register with a system if they want to receive or make calls. • There are 5 different types of autonomous registration messages in IS-95. System msgs on the Sync channel indicates will ones are in effect. • Power up • Power down (de-registration) • Timer exceeds a threshold • Distance between new and old BS exceeds a certain limit. • BS’s sends out GPS info. in system’s messages which includes distance threshold. • New zone (cells under one MSC are clustered in zones). • There are 4 other types of registration that are not mobile initiated, i.e., BS asks for it – mobile changes some parameter and informs the BS implicitly in the page response.

43. Mobile Management (Cont.) • When a mobile registers it also will indicate which slots it will listen to when the paging channel is in slotted mode. • It also provides other parameters such as protocol version and class type that it is using so that the MSC knows how to communicates with it and what services to provide. • Roaming: CDMA system consists of system Ids (SID) and network Ids (NID). • System has many networks within it so a mobile has to keep track of the SID/NID pair of the area it is in (broadcast by the BSs). • Each mobile has a list of home SIDs and NIDs. If it enters an area that has an NID that is not on the list, but the SID is  classified as NID roaming. • If the SID is not on the list  it is SID roaming. • Once the mobile knows it’s a roamer it will figure out what kind of services it will be able to access in this foreign (non home) environment.