Teknik Jaringan Akses CDMA Mobile Communication & IS-95ct
Spread Spectrum Priniciples • Does not attempt to allocate disjoint frequency or time slot resources • Instead, this approach allocates all resources to simultaneous users, controlling the power transmitted by each user to the minimum required to maintain a given SNR • Each user employs a noise-like wideband signal occupying the entire frequency allocation • Each user contributes to the background noise affecting all users, but to the least extent possible.
Spread Spectrum Priniciples • This restriction on interference limits capacity, but because time and bandwidth resource allocations are unrestricted, the resulting capacity is significantly higher than the conventional system
Spread Spectrum Priniciples • Suppose each user use a wideband Gaussian noise carrier • Suppose each user’s transmission is controlled so that all signals received at the BS are of equal power • Let Ps be the power of each user, and the background noise be negligible. • Then the total interference power, I, presented to each user’s demodulator is I = (K-1) Ps (1) where K is the number of users
Spread Spectrum Priniciples • Let’s say demodulator of each user operates at bit-energy-to-noise-density level of Eb/N0. • So the noise density received by each user’s demodulator is N0 = I/W (2), where W Hz is the bandwidth of the wideband noise carriers • The received energy per bit is the received signal power divided by the data rate R (bits/s), i.e., Eb = Ps/R (3)
Spread Spectrum Priniciples • Combining (1), (2) and (3) we get K – 1 = I/Ps = (W/R) / (Eb/N0) (4) • If W >> R then the capacity of the system can be large • i.e., transmission bandwidth should be much larger than the message bandwidth • If Eb/N0 is small, then also the capacity can be large. (since Eb/N0 α SNR, this means SNR should be as small as possible)
Code Division Multiple Access - CDMA • Multiple users occupying the same band by having different codes is known as CDMA - Code Division Multiple Access system Let W - spread bandwidth in Hz R = 1/Tb = Date Rate S - received power of the desired signal in W J - received power for undesired signals like multiple access users, multipath, jammers etc in W Eb - received energy per bit for the desired signal in W N0 - equivalent noise spectral density in W/Hz
CDMA (contd…) What is the tolerable interference over desired signal power?
CDMA (contd…) • In conventional systems W/R 1 which means, for satisfactory operation J/S < 1 • Example Let R = 9600; W = 1.2288 MHz (Eb/N0)min = 6 dB (values taken from IS-95) Jamming margin (JM) = 10log10(1.2288*106/9.6*103) - 6 = 15.1 dB 32 • This antijam margin or JM arises from Processing Gain (PG) = W/R = 128 • If (Eb/N0)min is further decreased or PG is increased, JM can be further increased
CDMA (contd…) • JM can be used to accommodate multiple users in the same band • If (Eb/N0)min and PG is fixed, number of users is maximized if perfect power control is employed. • Capacity of a CDMA system is proportional to PG.
Spreading Codes • A noise-like and random signal has to be generated at the transmitter. • The same signal must be generated at the receiver in synchronization. • We limit the complexity by specifying only one bit per sample, i.e., a binary sequence.
Desirable Randomness Properties • Relative frequencies of “0” and “1” should be ½ (Balance property) • Run lengths of zeros and ones should be (Run property): • Half of all run lengths should be unity • One - quarter should be of length two • One - eighth should be of length three • A fraction 1/2n of all run lengths should be of length n for all finite n
Desirable Randomness Properties (contd…) • If the random sequence is shifted by any nonzero number of elements, the resulting sequence should have an equal number of agreements and disagreements with the original sequence (Autocorrelation property)
PN Sequences • A deterministically generated sequence that nearly satisfies these properties is referred to as a Pseudorandom Sequence (PN) • Periodic binary sequences can be conveniently generated using linear feedback shift registers (LFSR) • If the number of stages in the LFSR is r, P 2r - 1 where P is the period of the sequence
PN Sequences (contd…) • However, if the feedback connections satisfy a specific property, P = 2r - 1. Then the sequence is called a Maximal Length Shift Register (MLSR) or a PN sequence. • Thus if r=15, P=32767. • MLSR satisfies the randomness properties stated before
Randomness Properties of PN Sequences • Balance property - Of the 2r - 1 terms, 2r-1 are one and 2r-1–1 are zero. Thus the unbalance is 1/P. For r=50; 1/P10-15 • Run length property - Relative frequency of run length n (zero or ones) is 1/ 2n for n r-1 and 1/(2r - 1) for n = r • One run length each of r-1 zeros and r ones occurs. There are no run lengths for n > r • Autocorrelation property - The number of disagreements exceeds the number of agreements by unity. Thus again the discrepancy is 1/p
PN Sequences Specified in IS-95 • A “long” PN sequence (r =42) is used to scramble the user data with a different code shift for each user • The 42-degree characteristic polynomial is given by: • x42+x41+x40+x39+x37+x36+x35+x32+x26+x25+x24+x23+x21+x20+x17+x16+x15+x11+x9+x7+1 • The period of the long code is 242 - 1 4.4*102 chips and lasts over 41 days
PN Sequences Specified in IS-95 (contd…) • A short PN sequence (r = 15) is specific to a base station and its period is (215−1)Tc = 27ms. • Two “short” PN sequences (r=15) are used to spread the quadrature components of the forward and reverse link waveforms
Power Control in CDMA • CDMA goal is to maximize the number of simultaneous users • Capacity is maximized by maintaining the signal to interference ratio at the minimum acceptable • Power transmitted by mobile station must be therefore controlled • Transmit power enough to achieve target BER: no less no more
Two factors important for power control • Propagation loss • due to propagation loss, power variations up to 80 dB • a high dynamic range of power control required • Channel Fading • average rate of fade is one fade per second per mile hour of mobile speed • power attenuated by more than 30 dB • power control must track the fade
Power Control in IS-95A • At 900 MHz and 120 km/hr mobile speed Doppler shift =100Hz • In IS 95-A closed loop power control is operated at 800 Hz update rate • Power control bits are inserted (‘punctured’) into the interleaved and encoded traffic data stream • Power control step size is +/- 1 dB • Power control bit errors do not affect performance much
Rake Receiver • Mobile station receives multiple attenuated and delayed replicas of the original signal (multipath diversity channels). • Two multipath signals are resolvable only if their relative delay exceeds the chip period Tc • Amplitudes and phases of multipath components are found by correlating the received waveform with multiple delayed versions of the signal (delay = nTc). • Searcher performs the above task for up to 3 different multipath signals. • 3 parallel demodulators (RAKE fingers) isolate the multipath components and the RAKE receiver combines them.
Handoff in CDMA System • In GSM hard handoff occurs at the cell boundary • Soft Handoff • Mobile commences Communication with a new BS without interrupting communication with old BS • same frequency assignment between old and new BS • provides different site selection diversity • Softer Handoff • Handoff between sectors in a cell • CDMA to CDMA hard handoff • Mobile transmits between two base stations with different frequency assignment
Soft Handoff- A unique feature of CDMA Mobile Advantages • Contact with new base station is made before the call is switched • Diversity combining is used between multiple cell sites • Diversity combining is the process of combining information from multiple transmitted packets to increase the effective SNR of received packets • additional resistance to fading • If the new cell is loaded to capacity, handoff can still be performed for a small increase in BER • Neither the mobile nor the base station is required to change frequency
References • Lee JS and Miller LM, CDMA System Engineering Handbook, Arttech Publishing House, 1998. • Viterbi A, CDMA-Spread Spectrum Communication, Addison Wesley 1995.
GSM & CDMA A comparison
GSM • Time Division Multiple Access Based Technology • 200kHz bandwidth per carrier • Deployed in reuse pattern 3/9, 4/12, 7/21 • Available operating frequency 900, 1800, 1900 MHz • Using SIM Card
CDMA • Code Division Multiple Access Based Technology • 1.25 MHz bandwidth per carrier • Reuse factor 1 • Available operating frequency 450, 800, 1900 MHz • Using RUIM Card
CDMA • Inherently superior receive sensitivity (approx. -121 dB) • Tradeoff between Capacity, Coverage and Quality • Soft/Softer hand-off (make before break): • Precise power control algoriths minimize interference • Multiple diversities: • Receive Spatial Diversity trough two receive antennas • Path diversity trough rake receivers • Frequency diversity trough spread spectrum • Time diversity trough interleaving
GSM • Fixed coverage • Receive sensitivity improvement (approx. -108dB), relies on external solutions (masthead pre-amplifier, high power amplifier) • Hard hand-off (break before make)
Summary • CDMA, compared with GSM (TDMA) technology, provide : • better spectrum efficiency (more capacity) • better coverage (less sites required) • better voice quality • better data capability • better forward compatibility (same spectrum can be reused)
Preface... • The authors would like to acknowledgement material contributions from: • Murtaza Amiji, NMS Communications • Samuel S. May, Senior Research Analyst, US Bancorp Piper Jaffray • Others as noted on specific slides • We intend ongoing improvements to this tutorial and solicit your comments at: • firstname.lastname@example.org • and/or email@example.com • For the latest version go to: • http://www.nmscommunications.com/3Gtutorial
Outline • History and evolution of mobile radio • Brief history of cellular wireless telephony • Radio technology today: TDMA, CDMA • Demographics and market trends today • 3G vision, 3G migration paths • Evolving network architectures • Based on GSM-MAP or on IS-41 today • 3GPP versus 3GPP2 evolution paths • 3G utilization of softswitches, VoIP and SIP • Potential for convergence
Outline (continued) • Evolving services • SMS, EMS, MMS messaging • Location • Video and IP multimedia • Applications & application frameworks • Is there a Killer App? • Business models • What’s really happening? When?
3G Tutorial • History and Evolution of Mobile Radio • Evolving Network Architectures • Evolving Services • Applications • Business Models
First Mobile Radio Telephone1924 Courtesy of Rich Howard
Landline Subs (millions) Mobile Subs World Telecom Statistics Crossover has happened May 2002 !
2 7 5 2 3 3 3 1 1 1 6 6 6 4 4 4 2 2 2 7 7 7 5 5 5 3 1 Cellular Mobile Telephony • Frequency modulation • Antenna diversity • Cellular concept • Bell Labs (1957 & 1960) • Frequency reuse • Typically every 7 cells • Handoff as caller moves • Modified CO switch • HLR, paging, handoffs • Sectors improve reuse • Every 3 cells possible
First Generation • Advanced Mobile Phone Service (AMPS) • US trials 1978; deployed in Japan (’79) & US (’83) • 800 MHz band — two 20 MHz bands • TIA-553 • Still widely used in US and many parts of the world • Nordic Mobile Telephony (NMT) • Sweden, Norway, Demark & Finland • Launched 1981; now largely retired • 450 MHz; later at 900 MHz (NMT900) • Total Access Communications System (TACS) • British design; similar to AMPS; deployed 1985 • Some TACS-900 systems still in use in Europe
Second Generation — 2G • Digital systems • Leverage technology to increase capacity • Speech compression; digital signal processing • Utilize/extend “Intelligent Network” concepts • Improve fraud prevention • Add new services • There are a wide diversity of 2G systems • IS-54/ IS-136 North American TDMA; PDC (Japan) • iDEN • DECT and PHS • IS-95 CDMA (cdmaOne) • GSM
D-AMPS/ TDMA & PDC • Speech coded as digital bit stream • Compression plus error protection bits • Aggressive compression limits voice quality • Time division multiple access (TDMA) • 3 calls per radio channel using repeating time slices • Deployed 1993 (PDC 1994) • Development through 1980s; bakeoff 1987 • IS-54 / IS-136 standards in US TIA • ATT Wireless & Cingular use IS-136 today • Plan to migrate to GSM and then to W-CDMA • PDC dominant cellular system in Japan today • NTT DoCoMo has largest PDC network
iDEN • Used by Nextel • Motorola proprietary system • Time division multiple access technology • Based on GSM architecture • 800 MHz private mobile radio (PMR) spectrum • Just below 800 MHz cellular band • Special protocol supports fast “Push-to-Talk” • Digital replacement for old PMR services • Nextel has highest APRU in US market due to “Direct Connect” push-to-talk service
DECT and PHS • Also based on time division multiple access • Digital European Cordless Telephony • Focus on business use, i.e. wireless PBX • Very small cells; In building propagation issues • Wide bandwidth (32 kbps channels) • High-quality voice and/or ISDN data • Personal Handiphone Service • Similar performance (32 kbps channels) • Deployed across Japanese cities (high pop. density) • 4 channel base station uses one ISDN BRI line • Base stations on top of phone booths • Legacy in Japan; new deployments in China today
North American CDMA (cdmaOne) • Code Division Multiple Access • All users share same frequency band • Discussed in detail later as CDMA is basis for 3G • Qualcomm demo in 1989 • Claimed improved capacity & simplified planning • First deployment in Hong Kong late 1994 • Major success in Korea (1M subs by 1996) • Used by Verizon and Sprint in US • Simplest 3G migration story today
cdmaOne — IS-95 • TIA standard IS-95 (ANSI-95) in 1993 • IS-95 deployed in the 800 MHz cellular band • J-STD-08 variant deployed in 1900 MHz US “PCS” band • Evolution fixes bugs and adds data • IS-95A provides data rates up to 14.4 kbps • IS-95B provides rates up to 64 kbps (2.5G) • Both A and B are compatible with J-STD-08 • All variants designed for TIA IS-41 core networks (ANSI 41)