Chapter 5 North American Cellular System Based on Time Division Multiple Access

Background and Goals Cellular phones entered commercial service in North America in 1983 AMPS: the first generation of cellular system Three ways to expand the capacity of a cellular system: Move into new spectrum bands Split existing cells into smaller cells Propose new technology to make more efficient use of existing bandwidth and base stations IS-54 (since 1992) was created after GSM, it support dual mode access capability NA-TDMA is specified in IS-136 (since 1996) 2018/11/18 Peter Yeh

Architecture NA-TDMA is an extension of AMPS IS-136 systems are capable of operating with AMPS terminals, dual mode terminals, and all-digital terminals NA-TDMA specifies three types of external network: Public systems: terminal as a cellular phone Residential systems: terminal as a cordless phone Private systems: terminal as a business phone 2018/11/18 Peter Yeh

2018/11/18 Peter Yeh

Architecture NA-TDMA defines a large number of ID codes including all of the AMPS codes: 64-bit Encryption Key (A-Key) 12-bit location area identifier (LOCAID): the system can divide its service area into clusters of cells referred to as location areas MIN: mobile ID 34 bits … 2018/11/18 Peter Yeh

2018/11/18 Peter Yeh

Radio Transmission IS-136 specifies dual mode NA-TDMA/AMPS operation in the AMPS frequency bands (Fig. 3.2) Each band of NA-TDMA specifies carriers spaced at 30 kHz. Each pair of NA-TDMA carriers corresponds to an AMPS channel The access technology conforms to the hybrid FDMA/TDMA (Fig. 5.3) Each frame contains six time slots and the frame duration is 40 ms The length of each time slot is 6.67 ms 2018/11/18 Peter Yeh

2018/11/18 Peter Yeh

2018/11/18 Peter Yeh

2018/11/18 Peter Yeh

Radio Transmission Physical Channels: Each time slot carries 324 bits, so that the data rate per carrier is 324 x 6 / 40 ms = 48.6 kb/s A full-rate channel can occupy 2 slots (slots 1 and 4, slots 2 and 5, slots 3 and 6) and the bit rate is 16.2 kb/s Half-rate channels (8.1kb/s) Double full-rate channels (32.4kb/s) Triple full-rate channels (48.6 kb/s) In contrast to AMPS, NA-TDMA has no fixed assignment of physical channels to digital control channels 2018/11/18 Peter Yeh

2018/11/18 Peter Yeh

2018/11/18 Peter Yeh

2018/11/18 Peter Yeh

2018/11/18 Peter Yeh

Radio Transmission Modulation: The modulation format for the 48.6 kb/s is /4 shifted DQPSK (differential quaternary phase shift keying) DQPSK is a four level modulation scheme, each transmitted signal referred to as a channel symbol, carries 2 bits to receiver For each symbol, the possible phase changes are odd multiples of  /4 The modulation efficiency is 48.6 kb/s / 30 kHz = 1.62b/s/Hz 2018/11/18 Peter Yeh

Modulation Techniques (1) 在通信系統中，將需要傳遞的資訊轉換成適合該系統使用的信號，稱之為「調變」(Modulation)；而在接收端把上述已調變的信號轉變成原來資訊的過程，稱為「解調」(Demodulation)。將資訊負載在電磁波上的方式，基本的有三種方式，為(1)變化振幅、(2)變化相位以及(3)變化頻率，分別稱為： (1)振幅調變(AM: Amplitude Modulation) (2)相位調變(PM: Phase Modulation) (3)頻率調變(FM: Frequency Modulation) 2018/11/18 Peter Yeh

Modulation Techniques (2) 在數位信號調變時，又特別稱為 ASK(Amplitude Shift Keying) PSK(Phase Shift Keying) FSK(Frequency Shift Keying) 其中，如果PSK採用每個相位相差90°稱４相PSK，又稱QPSK。以上所述都是以載波所具有的三個參數之一(振幅、相位、頻率)作調變，若使用兩個參數調變則可傳送更多資訊，如由振幅與相位合用發展出的正交振幅調變。如45°相位的OA可視為兩個相位相差90°的AM波合成(OP+OQ)，這種正交合成AM波稱為「正交振幅調變」(QAM: Quadrature Amplitude Modulation)。DQPSK的訊號完全取決於相鄰兩個符號間的相位差。 2018/11/18 Peter Yeh

Carrier Phase Shifts Corresponding to Various Input Bits Pairs DQPSK Information bits                   Phase shift         11 π/4 01 3π/4 00 -3π/4 10 -π/4     Carrier Phase Shifts Corresponding to Various Input Bits Pairs 2018/11/18 Peter Yeh

2018/11/18 Peter Yeh

Radio Transmission Radiated Power Spectrum Efficiency NA-TDMA specifies 11 power levels for terminals The highest power level is 4W(6 dBW) and level differ by increments of 4 dB ranging to a low of –34 dBW (0.25 mW) Spectrum Efficiency An all-digital network occupying 25 MHz has 416 carriers and 3 x 416 = 1248 full rate physical channel Assume the reuse factor is 7 and antenna sector is 3 then the traffic channel is 1248 – 3 x 7 = 1227 the spectrum efficiency is E = 1227/7/25 = 7.01 conversation/cell/MHz 2018/11/18 Peter Yeh

2018/11/18 Peter Yeh

2018/11/18 Peter Yeh

Logical Channel NA-TDMA supports all of the AMPS logical channel in addition to the digital control and traffic channels specified in IS-136 A Digital Traffic Channel (DTCH) transmit information in six formats in the forward direction and 5 formats in the reverse direction as shown in Fig. 5.5 Forward Digital Control Channel (DCCH) multiplex information in nine distinct formats, including 3 broadcast control channels and 3 point-to-point channels Reverse DCCH: A Random Access Channel from terminal to a BS 2018/11/18 Peter Yeh

2018/11/18 Peter Yeh

Digital Traffic Channel (1) Digital Traffic Channel (DTCH) Fig. 5.6 displays the contents of each time slot in a DTCH A 6-bit guard time (G): to prevent the signal interference between slots A 6-bit ramp time (R): to come up the power level to its full radiated power level 2018/11/18 Peter Yeh

2018/11/18 Peter Yeh

Digital Traffic Channel (2) Synchronization Bits (28 bits) It serves two purposes: to synchronize and to train an adaptive equalizer Digital Verification Color Code (DVCC) The DVCC verifies mobile and BSs that they are receiving signals which is not sent from another cell using the same physical channel Each DVCC is represented by an 8-bit word with (12, 8: 3) error correcting block code 2018/11/18 Peter Yeh

Digital Traffic Channel (3) Slow Associated Control Channel (SACCH) the bit rate is 2 x 12 / 0.04 = 600 b/s in a full-rate physical channel 132 bits (11 time slots) comprise a code word The code word contains a 50-bit network control message protected by a 16 bit CRC Use a diagonal interleave to spread the 132 bits over 12 transmission time slots 2018/11/18 Peter Yeh

2018/11/18 Peter Yeh

. . Example : 32-bit CRC 2018/11/18 Peter Yeh = AND = XOR = NOT A X 1 X 2 X 3 X 4 X 5 X 6 X 7 X 8 X 9 X 10 X 11 X 12 X 13 X 14 X 15 X 16 X 17 X 18 X 19 X 20 X 21 X 22 X 23 X 24 X 25 X 26 X 27 X 28 X 29 X 30 X 31 . = AND = XOR = NOT A . O = A, C = 0 MUX MUX 輸出 C O = B, C = 1 控制訊號 B 輸入 2018/11/18 Peter Yeh

Example : rate 1/3 Convolutional Code V1 = R1 V2 = R1  R2  R3 V3 = R1  R3 2018/11/18 Peter Yeh

Example : Interleaving WHAT I TELL YOU THREE TIMES IS TRUE If there are four consecutive errors in the middle, the result is WHAT I TELL YBVOXHREE TIMES IS TRUE Alternatively, it is possible to interleave the symbol using a 5 x 7 interleaving matrix (Ref. pp. 364-365) WHOT I XELL YOU THREE TIMEB IS VRUE 2018/11/18 Peter Yeh

Digital Traffic Channel (4) Digital Control Channel Locator (DL) The 11-bit DL field contains a 7-bit digital locator value protected by an (11,7;3) error-correcting code Fast Associated Control Channel (FACCH) the transmission time on an SACCH with a full rate traffic is 240 ms and for half-rate is 480 ms for some control function this delay is unacceptable NA-TDMA also incorporates an in-band signaling channel it transmits a 260-bit code word with 49 bits message 2018/11/18 Peter Yeh

Digital Traffic Channel (5) The transmission time is 40 ms on a full-rate channel 2018/11/18 Peter Yeh

Digital Control Channel (1) Digital Control Channel (DCCH) Block = 3 Slots = 20 ms Superframe = 32 blocks = 0.64 sec Hyperframe = 2 superframes The structure of each DCCH time slot In the reverse time slot, there are 40 bits of additional sync information relative to the DTCH 16-bit preamble that replace the 16 data bits of DTCH 24 bits SYNC+ replaces the DVCC and SACCH of a reverse DTCH 2018/11/18 Peter Yeh

2018/11/18 Peter Yeh

Digital Control Channel (2) In the forward time slot 12-bit SFP (Superframe phase): inform terminals of the location of the current block SFP codeword is a (12, 8, :3) error-correcting code The first three bits of the 8-bit word are 0 22-bit SCF (Shared Channel Feedback): A busy/reserved/idle (BRI) indication (6 bits) informs terminals whether the current slot is being used by a random access channel 2018/11/18 Peter Yeh

Digital Control Channel (3) A received/not-received (R/N) indication (5 bits) informs terminals of whether the BS has successfully decoded the information transmitted on the reverse DCCH A coded partial echo (CPE, 11 bits) ack receipt of information on the reverse DCCH. It carries the least significant 7 bits of the directory number (MIN or IMSI). An (11,7:3) block code protects this information 2018/11/18 Peter Yeh

Digital Control Channel (4) Multiplexed Logical Channels on the Forward DCCH Fast broadcast control channel (F-BCCH) Extended broadcast control channel (E-BCCH) Short message service broadcast control channel (S-BCCH) Short message service, paging, and access response channel (SPACH) Each superframe begins with the F-BCCH and ends with SPACH 2018/11/18 Peter Yeh

Digital Control Channel (5) 2018/11/18 Peter Yeh

Digital Control Channel (6) Paging Channel Operation, Sleep Mode Paging consumes the terminal power NA-TDMA makes it possible for terminal to recognize paging in sleep mode Paging message are always transmitted twice, once in each superframe of a hyperframe Paging message arrive in the SPACH blocks of each superframe the number of paging subchannels is a parameter, PEN, range from 1 to 96 2018/11/18 Peter Yeh

Digital Control Channel (7) With PEN = 1, there is only one paging subchannel that occupies all hyperframes With PEN =96, a subchannel appears once in 96 hyperframes Each terminal listen to a specific paging subchannel, it sleep PEN – 1 paging subchannel There is a hyperframe counter in the BCCH that informs terminals of the current paging subchannel Each terminal listen to an assigned subchannel which is determined by a hashing function with MIN 2018/11/18 Peter Yeh

Network Operations The principal network management innovations in NA-TDMA : Authentication and Privacy A-key, SSD (Shard Secret Data) CAVE (Cellular Authentication and Voice Encryption) Mobile-Assisted Handoff (MAHO) Call Management Mobility Management 2018/11/18 Peter Yeh

Authentication and Privacy (1) Encipher Plaintext Key Ciphertext Decipher 2018/11/18 Peter Yeh

Authentication and Privacy (2) 2018/11/18 Peter Yeh

Authentication and Privacy (3) 2018/11/18 Peter Yeh

Authentication and Privacy (4) 2018/11/18 Peter Yeh

Mobile-Assisted Handoff (MAHO) MAHO monitors the quality of the signal received at the terminal as well as at the base station. MAHO responds more promptly to channel-quality problems. MAHO provides BER estimates as well as signal-strength estimates. MAHO moves some of the information processing necessary for network control from switched to base stations and terminals. 2018/11/18 Peter Yeh

Review Exercises Why is there a guard time in reverse link NA-TDMA transmissions but not in forward link transmissions? Explain the sleep mode operation in NA-TDMA. What are the advantage and disadvantage of having a large number of paging sub-channels (PFN)? What is the advantage of using shared secret data as an encryption key relative to using the A-key? 2018/11/18 Peter Yeh