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  1. CDMA Session 8 Nilesh Jha Notes from Stallings, modified/added to

  2. IS95 CDMA 1.25 MHz • up to 64 channels • (is the number of • orthogonal codes) • code: 1.2288 Mchips/sec. • Voice coder: 9.6 kb/s • (really 8.55 kbps plus overhead) • Cell power controlled at base stations to minimize interference (and near far problem)

  3. DIGITAL CELLULAR --- CDMA • Same frequency allocations as TDMA (but different bandwidths per signal, so grouped differently) • Started in 1996 -- 1997, standard called IS-95 • New technology had doubters, has proven successful • More spectrally efficient than TDMA because of adaptive voice coding rates (which CDMA can adapt to), power and interference control leading to reuse factor of 1, more reliable because of soft handoff • Uses 1.25 MHz bandwidth (BW) (a carrier with this BW) where multiple users share the bandwidth and are differentiated via each having a different code • Max. 26 calls/MHz/cell or 780 calls/cell (in 30 MHz) -- soft limit, could be less, sometimes stated at about half • Adopted in wideband form (about 5 MHz carriers) for 3G, with both a US version and a European versionwhich seems to be the one most of the world is going to

  4. Spread Spectrum Signal • Transmitted signal bandwidth > > information bandwidth • Some function other than the information transmitted is used to determine resultant transmitted bandwidth • Called the Spreading Function • Determines Spreading Gain or Processing Gain • PG = Bandwidth/Data Rate = BW/R • Determines How Many Users Can Share Same Frequency Band Without Affecting Each Other After Despreading

  5. General Model of SS Spreading Despreading From Stallings

  6. Advantages of CDMA Cellular • Frequency diversity – frequency-dependent transmission impairments have less effect on signal (signal is spread over 1.25 MHz, frequency selective effects average out) • Multipath resistance – chipping codes used for CDMA exhibit low cross correlation and low autocorrelation -- allows for multiple correlation receiver (called Rake receiver) to separate out multipath pieces of the signal • In TDMA multipath fading is handled through equalization, requires complex processing and not being as effective because it is narrowband • Rake does better --- uses inherent frequency diversity • Privacy – privacy is inherent since spread spectrum is obtained by use of noise-like signals -- need the codes to receive and ‘decipher’ them -- a PN code is used like the A key is in TDMA, with a unique PN code assigned to a mobile terminal • Graceful degradation – system gradually degrades as more users access the system --- soft limit, no real hard limit

  7. Drawbacks of CDMA Cellular • Some interference remains – arriving transmissions from multiple users not aligned perfectly on chip boundaries unless users are perfectly synchronized • multipath signals not synchronized, are random • Near-far problem – signals closer to the receiver are stronger than signals farther away • Requires fast and efficient closed loop power control to keep interference to weaker signals to a minimum • Soft handoff –uses signals in two cells and thus increases interference and uses more than the minimum numbers of channels • Requires more complex transmitter and receiver for spread spectrum signal generation and reception --- more expensive -- still, nowadays, chipsets available

  8. Mobile Wireless CDMA Considerations • RAKE receiver – when multiple versions of a signal arrive more than one chip interval apart, RAKE receiver attempts to recover signals from multiple paths and combine them • This method achieves better performance than simply recovering dominant signal and treating remaining signals as noise • Soft Handoff – mobile station temporarily connected to more than one base station simultaneously -- this is possible because frequency reuse is 1, and the RAKE receiver can combine signals from 2 different basestations, or pick the best in real time, or weight the strongest one more • Otherwise handoff and mobility management are done the same way as in the US TDMA system, using IS-41 for any intersystem messaging

  9. Principle of RAKE Receiver -Notice that the channel is modeled as multiple paths with different time delays and amplitude -Notice that at Rake it is necessary to estimate those channel numbers -Each Rake receiver path called a finger, includes a correlator

  10. -Forward channel maximum is 64, 64 Walsh codes but reverse channels can be more, uses PN codes -All limited by S/I

  11. From Garg

  12. Types of Channels Supported by Forward Link • Pilot (channel 0) - allows the mobile unit to acquire timing information, provides phase reference, provides means for signal strength comparison • Uses Walsh code 0 (null - pure sines and cosines), done 4-6 dB higher than others, used to acquire freq./phase reference, needed for coherent demodulation • Used by mobile for power measurements for handoff • Uses PN short code to identify BS, with time offset, 512 unique offsets • Synchronization (channel 32) - system time, system parameters • Also PN code offset for that BS, SID, network ID, long PN state • Paging (channels 1 to 7) - contain messages for one or more mobile stations • Traffic (channels 8 to 31 and 33 to 63) – the forward channel supports 55 traffic channels

  13. Forward Channels - more • SYNC -- Message can be long, in multiple frames, each 32 bits • Multiple superframes, each 3 framesmessage could be 1146 data bits, CRC • Message repeats -- has header, data, CRC --- system time from GPS • Paging -- Wsub0 to Wsub7, paging MS’s • Messages can be 1184 bits, in timeslots of 80 msec, organized so MS only looks at fraction, eg, 1 in 16 (or up to 64), sleeps rest of time • Messages have header, data, CRC -- has called MS, calling #, #messages waiting, BS ID and other parameters, alerts, unlock, registration accepted or rejected, tune to new frequency, etc • Traffic -- data rates of 9.6 or 14.4 kbps (rate sets 1 and 2) • Voice at 8.55 kbps, error detection to 9.6 kbps, dropped to 1.2 kbps during quiet periods with VOICE ACTIVITY DETECTION • 20 msec frames with 1/2 FEC (to 19.2 kbps), interleaved, scrambled, spread, modulated • Each frame has 192 bits • Multiple codes inserted --- for BS ID, scrambling and spreading • Can be blanked or dimmed and signaling inserted

  14. Forward Traffic Channel Processing Steps • Speech is encoded at a rate of 8000 bps • Additional bits added for error detection • Data transmitted in 20-ms blocks with forward error correction provided by a convolutional encoder • Data interleaved in blocks to reduce effects of errors • Data bits are scrambled, serving as a privacy mask

  15. Forward Traffic Channel Processing Steps (cont.) • Power control information inserted into traffic channel • DS-SS function spreads the 19.2 kbps to a rate of 1.2288 Mcps (cps=chips/sec) using one row of 64 x 64 Walsh matrix • Digital bit stream modulated onto the carrier using QPSK modulation scheme

  16. From Garg

  17. Forward Channel -- Comments • Notice that BS transmits all channels synchronously -- the spreading codes, W’s, are orthogonal, and stay orthogonal as they all travel the same path to each user • Also simultaneously, they all are summed, and RF modulated and amplified simultaneously with a single RF transmitter • W’s used for orthogonal spreading, short PN’s for BS ID, long PN for scrambling/privacy (each MS has its own) • Power control bit inserted 800 times/sec, puncturing the voice data

  18. Reverse Channel -- Comments • Important differences --- the MS’s do not transmit synchronously, and moreover, the paths back to the BS are different so even if orthogonal codes they would NOT stay orthogonal • W’s used for modulation on reverse, taking 6 bits and turning them into a W row, one modulation symbol made up of 64 chips • Better demodulation -- better BER for Eb/Nsub0 • Long PN code, unique to each MS, is used for spreading-- it determines the channel (on FWD it was the W’s) • Short PN code is used for phase sync • OQPSK is used, Q chip is half a chip offset, no pass tru 0

  19. Logical Channels and Messages -- Some Features • Traffic channels can carry voice/data, or signaling • Speech vocoder: QCELP, at 8.55, 4, 2, .8 kbps • Variable rate -- When no or little voice it reduces the output rate --- voice activity detection • Signaling with blanking/dimming • Also power control • Messages • Paging and Access are like FOCC and RECC in AMPS, richer • Most messages have CRC and ARQ or selective ARQ • eg, Paging channels ACK’s messages on Access channels

  20. Some Network Operations Features • RRM • Power Control -- needed for near-far problem, open and closed loop • open loop is MS measures pilot power from BS and uses a message from BS that tries to keep MS power at some level wrt BS power • closed loop BS measures power form MS and sends messages to adjust up or down by 1 dB, at 800 Hz rate • Soft Handoff • MS tells BS when to start handoff, but MSC controls it • MS receives from 2 BS (up to 6), 2 physical channels, assigning at least one correlator to each --- at BS each of 2 BS’s looks for that MS PN code • MS thus does diversity reception with Rake, MSC can combine or select • MS measures pilots in neighbors list and reports to BS • Exchange of info on traffic channels, as signaling • Can handoff to AMPS -- hard

  21. From Garg

  22. Capacity Comparison -- Ideal

  23. From IEC --- some would say the true numbers are TDMA/GSM 3-4 to 1, CDMA 6-10 to 1

  24. ITU’s Standards for Third-Generation Systems (3G) • Voice quality comparable to the public switched telephone network • 144 kbps data rate available to users in high-speed motor vehicles over large areas • 384 kbps available to pedestrians standing or moving slowly over small areas • Support for 2.048 Mbps for office use • Symmetrical / asymmetrical data transmission rates • Support for both packet switched and circuit switched data services

  25. ITU’s Standards for Third-Generation Systems (3G) (cont.) • An adaptive interface to the Internet to reflect efficiently the asymmetry between inbound and outbound traffic • More efficient use of the available spectrum in general • Support for a wide variety of mobile equipment • Flexibility to allow the introduction of new services and technologies

  26. Wideband CDMA Considerations • Bandwidth – about 5 MHz • Chip rate – depends on desired data rate, need for error control, and bandwidth limitations; 3-4 Mcps • Multirate – advantage is that the system can flexibly support multiple simultaneous applications from a given user and can efficiently use available capacity by only providing the capacity required for each service

  27. Alternative Interfaces