Introduction to Mobile Communications

Introduction to Mobile Communications TCOM 552, Lecture #8 Hung Nguyen, Ph.D. 30 October, 2006

Outline • 2G CDMA • Advantages and disadvantages • RAKE Receiver • Logical Channels • Capacity Improvement • 3G CDMA Preliminaries • W-CDMA • CDMA 2000 Hung Nguyen, TCOM 552, Fall 2006

IS-95 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) Hung Nguyen, TCOM 552, Fall 2006

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 version which seems to be the one most of the world is going to Hung Nguyen, TCOM 552, Fall 2006

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 Hung Nguyen, TCOM 552, Fall 2006

General Model of Spread Spectrum Communications Spreading Despreading From Stallings Hung Nguyen, TCOM 552, Fall 2006

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 Hung Nguyen, TCOM 552, Fall 2006

Disadvantages 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 Hung Nguyen, TCOM 552, Fall 2006

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 base stations, 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 Hung Nguyen, TCOM 552, Fall 2006

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 RAKEreceiver path called a finger, includes a correlator Hung Nguyen, TCOM 552, Fall 2006

CDMA Logical Channels Hung Nguyen, TCOM 552, Fall 2006

IS-95 Channel Structure • Forward channel maximum is 64x64 Walsh codes but reverse channels can be more, uses PN codes • All limited by S/I Hung Nguyen, TCOM 552, Fall 2006

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 code), pure sinusoids, level 4-6 dB higher than others, used to acquire frequency & 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 Hung Nguyen, TCOM 552, Fall 2006

More on Forward Channels • SYNC - Message can be long, in multiple frames, each 32 bits • Each 3-frame message could be 1,146 data bits • Message repeats, has header, data, CRC, system time from GPS • Paging uses W1 to W7 for paging MS’s. Messages can be 1,184 bits in timeslots of 80 msec. MS only looks at fraction, e.g., 1 in 16, 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 • Multiple codes inserted - for BS ID, scrambling and spreading Hung Nguyen, TCOM 552, Fall 2006

Obtaining a Traffic Channel in CDMA System Initialization State System Idle State Traffic Termination State Traffic Channel State System Access State Hung Nguyen, TCOM 552, Fall 2006

Forward Link Channel Parameters Hung Nguyen, TCOM 552, Fall 2006

Forward Traffic Channel Processing Steps • Speech is encoded at a rate of ≤ 8.55 Kbps • 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 • 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 64x64 Walsh matrix • Digital bit stream modulated onto the carrier using QPSK modulation scheme Hung Nguyen, TCOM 552, Fall 2006

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 Hung Nguyen, TCOM 552, Fall 2006

Hung Nguyen, TCOM 552, Fall 2006

Reverse Link Channel Parameters Hung Nguyen, TCOM 552, Fall 2006

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, to provide additional orthogonality over long codes. Hung Nguyen, TCOM 552, Fall 2006

Logical Channels and Messages - Some Features • Traffic channels can carry voice/data, or signaling • Speech Vocoder: QCELP, at 8.55, 4, 2, 0.8 kbps • Variable rate - When no or little voice it reduces the output rate - voice activity detection • Signaling with blanking/dimming (stealing bits) • Also power controlled • Messages • Paging and Access are like FOCC and RECC in AMPS, richer • Most messages have CRC and ARQ or selective ARQ, e.g., Paging channels ACK’s messages on Access channels Hung Nguyen, TCOM 552, Fall 2006

Some RRM Network Operations Features • 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 w.r.t. 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 information on traffic channels, as signaling Hung Nguyen, TCOM 552, Fall 2006

Capacity Comparison - Ideal • Assume 12.5 MHz of spectrum each way • Analog: (12.5 MHz)/(30 KHz) = 416.6 channels. Reuse of 7 => 59.5 users/cell • GSM: (12.5 MHz)/(200 KHz/8 time slots) = 500 channels. Reuse of 4 => 125 users/cell • CDMA = (12.5 MHz)/(1.25 MHz) = 10 frequencies. Reuse of 1 but 10x64 orthogonal codes = 640 channels. • Summary: • Analog: 59.9 users/cell • GSM: 125 users/cell • CDMA: 640 users/cell Hung Nguyen, TCOM 552, Fall 2006

2G Basics Hung Nguyen, TCOM 552, Fall 2006

2G Capacity Improvement over 1G Some would say the true numbers are TDMA/GSM 3-4 to 1, CDMA 6-10 to 1 Hung Nguyen, TCOM 552, Fall 2006

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 Hung Nguyen, TCOM 552, Fall 2006

ITU’s Standards for Third-Generation Systems (3G) (cont’d) • 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 Hung Nguyen, TCOM 552, Fall 2006

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 Hung Nguyen, TCOM 552, Fall 2006

3G Evolution (1) Hung Nguyen, TCOM 552, Fall 2006

3G Evolution (2) Hung Nguyen, TCOM 552, Fall 2006

Alternative Interfaces Hung Nguyen, TCOM 552, Fall 2006

cdma2000 Radio Link Parameters (1) Hung Nguyen, TCOM 552, Fall 2006

EV-DO is geared toward the use of IP packets for data transmission and voice traffic can be supported using VoIP Hung Nguyen, TCOM 552, Fall 2006

Introduction to Mobile Communications