CDMA Technologies for Cellular Phone System, Dec. 22, CDMA Technologies for Cellular Phone System

CDMA Technologies for Cellular Phone System, Dec. 22, Contents 1.Introduction 2.Spread Spectrum Technology 3.DS-CDMA 4.Spreading Codes 5.Features of CDMA –RAKE Receiver –Power Control –Frequency Allocation –Soft Handoff 6.Conclusion

CDMA Technologies for Cellular Phone System, Dec. 22, Introduction: Overview of Cellular systems

CDMA Technologies for Cellular Phone System, Dec. 22, Evolution of Cellular Systems 1st.Generation (1980s) Analog NMT CT0 TACS CT1 AMPS 3rd. Generation (2000s) 2nd. Generation (1990s) Digital GSM DECT DCS1800 CT2 PDC PHS IS-54 IS-95 IS-136 UP-PCS IMT-2000 CDMA2000 W-CDMA

CDMA Technologies for Cellular Phone System, Dec. 22, Japan’s Cellular Subscriber Growth Record No. of Subscribers (Million) end December of Year end Nov ． ,679K cdmaOne/ CDMA2000 1x/ EV-DO 21 ， 222K PDC (TDMA) 47,787K W-CDMA 20,670K PDC AnalogcdmaOne/CDMA2000 1x/EV-DO W-CDMA /Nov.

CDMA Technologies for Cellular Phone System, Dec. 22, Requirements for 3G mobile systems High Capacity Tolerance for interference Privacy Tolerance for fading Ability to various data rate transmission Flexible QoS

CDMA Technologies for Cellular Phone System, Dec. 22, Duplex & Multiple Access Methods

CDMA Technologies for Cellular Phone System, Dec. 22, Duplex Methods of Radio Links Mobile Station Base Station Forward link Reverse link

CDMA Technologies for Cellular Phone System, Dec. 22, Frequency Division Duplex (FDD) Forward link frequency and reverse link frequency is different In each link, signals are continuously transmitted in parallel. Mobile Station Base Station Forward link (F1) Reverse link (F2)

CDMA Technologies for Cellular Phone System, Dec. 22, Time Division Duplex (TDD) Forward link frequency and reverse link frequency is the same. In each link, signals are in continuously transmitted by turns just like a ping-pong. Mobile Station Base Station Forward link (F1) Reverse link (F1)

CDMA Technologies for Cellular Phone System, Dec. 22, Example of FDD systems Transmitter Receiver BPF: Band Pass Filter BPF Transmitter Receiver BPF F1 F2 F1 F2 Mobile Station Base Station

CDMA Technologies for Cellular Phone System, Dec. 22, Example of TDD Systems Transmitter Receiver BPF: Band Pass Filter BPF Transmitter Receiver BPF F1 Mobile Station Base Station Synchronous Switches

CDMA Technologies for Cellular Phone System, Dec. 22, Multiple Access Methods Mobile Station Base Station Mobile Station Forward link Reverse link

CDMA Technologies for Cellular Phone System, Dec. 22, FDMA Overview A A B B C C Frequency Time f 2 f 1 f 0

CDMA Technologies for Cellular Phone System, Dec. 22, TDMA Overview CBACBACBACBA C A B Time f 0 Frequency

CDMA Technologies for Cellular Phone System, Dec. 22, What is CDMA ? SenderReceiver Code A A Code B B A B A B C B C A Code A A B C Time Frequency B C B A Base-band SpectrumRadio Spectrum spread spectrum

CDMA Technologies for Cellular Phone System, Dec. 22, Summary of Multiple Access FDMA TDMA CDMA time power frequency

CDMA Technologies for Cellular Phone System, Dec. 22, Spread Spectrum Technology

CDMA Technologies for Cellular Phone System, Dec. 22, How to spread spectrum... Direct Sequence (DS) Modulation (primary modulation) Modulation (primary modulation) user data Spreading (secondary modulation) Spreading (secondary modulation) Tx Base-band Frequency Power Density Radio Frequency Power Density TIME data rate spreading sequence (spreading code)

CDMA Technologies for Cellular Phone System, Dec. 22, Demodulating DS Signals (1/2) If you know the correct spreading sequence (code), Radio Frequency Power Density Accumulate for one bit duration Accumulate for one bit duration Demodulated data Base-band Frequency gathering energy ! spreading sequence (spreading code) received signal TIME =0 1+0=1 0+1=1 1+1=0

CDMA Technologies for Cellular Phone System, Dec. 22, Demodulating DS Signals (2/2) If you don’t know the correct spreading sequence (code) Base-band Frequency received signal spreading sequence (spreading code) Accumulate for one bit duration Accumulate for one bit duration Demodulated data Radio Frequency Power Density TIME No data can be detected

CDMA Technologies for Cellular Phone System, Dec. 22, Feature of SS Privacy, Security Radio Frequency Power Density Power density of SS-signals would be lower than the noise density. transmitted SS-signal Noise Power Density Radio Frequency Noise received signal de- modulator Base-band Frequency Power Density With incorrect code (or carrier frequency), SS-signal itself cannot be detected. Other system cannot recognize the existence of communication, because of signal behind the noise. With correct code (and carrier frequency), data can be detected. Base-band Frequency Power Density

CDMA Technologies for Cellular Phone System, Dec. 22, DS-CDMA

CDMA Technologies for Cellular Phone System, Dec. 22, Freq. BPF Despreader Code B Freq. BPF Despreader Code A DS-CDMA System Overview (Forward link) CDMA is a multiple spread spectrum. Difference between each communication path is only the spreading code Data B Code B BPF Freq. Data A Code A BPF Freq. MS-A MS-B BS Data A Data B

CDMA Technologies for Cellular Phone System, Dec. 22, Freq. BPF Despreader Code B Freq. BPF Despreader Code A DS-CDMA System Overview (Reverse Link) CDMA is a multiple spread spectrum. Difference between each communication path is only the spreading code Data B Code B BPF Freq. Data A Code A BPF Freq. MS-B MS-A BS Data A Data B

CDMA Technologies for Cellular Phone System, Dec. 22, Spreading Code

CDMA Technologies for Cellular Phone System, Dec. 22, Cross-Correlation between Code A and Code B = 6/16 Self-Correlation for each code is 1. one data bit duration Spreading Code A one data bit duration Spreading Code A Spreading Code B =0 1+0=1 0+1=1 1+1=0

CDMA Technologies for Cellular Phone System, Dec. 22, Preferable Codes In order to minimize mutual interference in DS-CDMA, the spreading codes with less cross-correlation should be chosen. Synchronous DS-CDMA : Orthogonal Codes are appropriate. (Walsh code etc.) Asynchronous DS-CDMA : Pseudo-random Noise (PN) codes / Maximum sequence Gold codes

CDMA Technologies for Cellular Phone System, Dec. 22, Multiplexing using Walsh Code

CDMA Technologies for Cellular Phone System, Dec. 22, Synchronous DS-CDMA Forward Link (Down Link) Synchronous Chip Timing A A Signal for B Station (after de-spreading) Less Interference for A station Synchronous CDMA Systems realized in Point to Multi-point System. e.g., Forward Link (Base Station to Mobile Station) in Mobile Phone.

CDMA Technologies for Cellular Phone System, Dec. 22, Asynchronous DS-CDMA In asynchronous CDMA system, orthogonal codes produce bad cross-correlation. Reverse Link (Up Link) B A Signal for B Station (after re-spreading) Big Interference from A station Asynchronous Chip Timing Signals from A and B are interfering each other. ABAB

CDMA Technologies for Cellular Phone System, Dec. 22, Features of CDMA

CDMA Technologies for Cellular Phone System, Dec. 22, Mobile Propagation Environment ･･･ Multi-path Fading The peaks and bottoms of received power appear, in proportion to Doppler frequency. Base Station (BS) Mobile Station (MS) multi-path propagation Path Delay Power path-2 path-3 path-1 Time Power

CDMA Technologies for Cellular Phone System, Dec. 22, Fading in non-CDMA System Path Delay Power path-1 path-2 path-3 With low time-resolution, different signal paths cannot be discriminated. These signals sometimes strengthen, and sometimes cancel out each other, depending on their phase relation. This is “fading”. In this case, signal quality is damaged when signals cancel out each other. In other words, signal quality is dominated by the probability for detected power to be weaker than minimum required level. Time Power Detected Power In non-CDMA system, “fading” damages signal quality. Required signal level

CDMA Technologies for Cellular Phone System, Dec. 22, Fading in CDMA System... As the CDMA system has high time-resolution, different path delay of CDMA signals can be discriminated. Energy from all paths can be summed by adjusting their phases and path delays. This is a principle of RAKE receiver. Path Delay Power path-1 path-2 path-3 CDMA Receiver CDMA Receiver Synchronization Adder CODE A with timing of path-1 Path Delay Power path-1 Power path-1 path-2 path-3 CODE A with timing of path-2 Path Delay Power path-2 interference produced by path-2 and path-3

CDMA Technologies for Cellular Phone System, Dec. 22, Fading in CDMA System (continued) In CDMA system, multi-path propagation improves the signal quality by adopting RAKE receiver. Time Power Detected Power RAKE receiver Power path-1 path-2 path-3 Detected power of CDMA signal will be less fluctuated by combining all energy

CDMA Technologies for Cellular Phone System, Dec. 22, Near-Far Problem CODE B CDMA Transmitter DATA B CODE A CDMA Receiver CODE A CDMA Transmitter DATA A P Desired Signal Power = P/Lp-a Interfered Signal Power = P/Lp-b/G G: processing gain Demodulated DATA P Lp-a Lp-b When user B is close to the receiver and user A is far from the receiver, Lp-a could be much bigger than Lp-b. In this case, desired signal power is smaller than the interfered power.

CDMA Technologies for Cellular Phone System, Dec. 22, Power Control... Ａ Ｂ Time Detected Power from A from B As the propagation losses between BS and MSs are different according to individual communication distances, the received levels at the base station are different from each other when all mobile stations transmit their signals at the same power. Moreover, the received level fluctuates quickly due to fading. In order to maintain the strength of received signal level at BS, power control technique must be employed in CDMA systems.

CDMA Technologies for Cellular Phone System, Dec. 22, Power Control (continued) （（（（（（ ② ① Open Loop Power ControlClosed Loop Power Control estimating path loss calculating transmission power transmit measuring received power transmit receive decide transmission power transmit measuring received power power control command about 1000 times per second ① ②

CDMA Technologies for Cellular Phone System, Dec. 22, Effect of Power Control Ａ Ｂ Time Detected Power from MS B from MS A closed loop power control for MS B. for MS A. Effect of Power Control Power control is capable of compensating the fading fluctuation. Received power from all MS are controlled to be equal. → Near-Far problem is mitigated by the power control.

CDMA Technologies for Cellular Phone System, Dec. 22, Frequency Allocation (1/2) In FDMA or TDMA, radio resource is allocated not to be interfered among neighbor cells. f1 f2 f3 f4 f5 f6 f7 cell : a “cell” means covered area by one base station. Neighbor cells cannot use the same (identical) frequency band (or time slot). The left figure shows the simple cell allocation with seven frequency sub-bands. In actual situation, it is difficult to allocate the frequency (or time slot) appropriately because of complicated radio propagation and irregular cell allocation.

CDMA Technologies for Cellular Phone System, Dec. 22, Frequency Allocation (2/2) In the CDMA system, identical radio resource can be allocated among all cells as explained in Introduction. Frequency allocation in CDMA is not necessary. In this sense, CDMA cellular system is easy to be designed.

CDMA Technologies for Cellular Phone System, Dec. 22, Soft Handover (1/2) Handover : Cellular system tracks mobile stations in order to maintain their communication links. When mobile station goes to neighbor cell, communication link switches from current cell to the neighbor cell. Hard Handover : In FDMA or TDMA cellular system, a new communication link is established after breaking the current communication link by hard handover. Communication between MS and BS instantaneously breaks by switching a frequency or a time slot. Hard handover: make connection (new cell B) after break (old cell A) switching Cell B Cell A

CDMA Technologies for Cellular Phone System, Dec. 22, Soft Handover (2/2) Σ Cell B Cell A Soft handover : break (old cell A) after make connection (new cell B) BS A and BS B transmit the same signal to the MS simultaneously In CDMA cellular system, communication link keeps a connection even in the handover procedure. Because the system does not require the frequency or time slot switching.

CDMA Technologies for Cellular Phone System, Dec. 22, Conclusion CDMA is based on the spread spectrum technology which has been used in military field. CDMA cellular system has many advantages compared with the FDMA and TDMA cellular systems. CDMA system was adopted as the international standard for the 3 rd generation mobile communications. The number of CDMA users will dramatically increase in the next five years all over the world.