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EE 489 Telecommunication Systems Engineering University of Alberta

EE 489 Telecommunication Systems Engineering University of Alberta Dept. of Electrical and Computer Engineering Slides prepared by: Robert Novak (Ph.D. Candidate – 2002). Wireless pre-cellular systems.

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EE 489 Telecommunication Systems Engineering University of Alberta

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  1. EE 489 Telecommunication Systems Engineering University of Alberta Dept. of Electrical and Computer Engineering Slides prepared by: Robert Novak (Ph.D. Candidate – 2002)

  2. Wireless pre-cellular systems • Wireless transmission was originally shown as a method to remain in continuous contact with ships (Marconi in 1897) • In 1946, FM consumer mobile phone systems were introduced. • A group of frequencies allocated to a large geographic zone • when moving to a new zone, calls had to be reinitiated • 120 kHz per channel (voice transmission was only 3 Hz) due to poor filter technology • half-duplex system (only one person could speak at a time) • most users not connected to PSTN • Later progressed to GMTS (general mobile telephone system) • By the 1960’s, IMTS (improved mobile telephone system) • 30 kHz channels in the 450 MHz range • Only 12 channels in NYC in 1976 • poor service due to call blocking and usage over a few channels • still in use in the U.S. in 1995 !

  3. MSC (MTSO) PSTN Advent of Cellular • A geographic area can be divided into several hexagonal areas, or cells, with an transmitter (base station) located in the centre of each cell. • Allows for frequency reuse as two cells in the same geographic area can use the same frequency • Increases the spectral efficiency (channels can be reused in different cells) of the system, but increases infrastructure expense • technology to implement cellular telephony was available only in the late 1970’s • Base station provides access between mobile users and the mobile switching centre (MSC) aka: MTSO= mobile telephone switching office

  4. 2 1 3 2 3 1 2 1 2 1 2 4 3 2 3 2 1 3 3 3 2 4 1 3 4 1 4 1 1 2 5 3 3 3 1 2 4 1 1 1 6 7 2 1 5 1 2 4 1 3 6 2 7 4 5 1 7 6 3 Frequency reuse patterns • Reuse allows a small set of frequencies, K to service a large area (numbers refer to transmitters with same frequencies) • Reuse patterns are designed to minimize co-channel interference (interference from other base stations using the same frequency) • a larger reuse pattern (e.g. K=7) results in a larger distance between base stations that use the same frequency K= 3 K= 4 D R K= 7

  5. 3 2 1 3 2 3 1 3 1 2 1 2 3 1 3 1 2 1 2 1 3 2 2 3 2 2 1 2 3 1 3 3 1 1 2 1 3 3 1 2 3 1 1 1 3 RF=Rsin(60o) R Co-channel interference reduction • We define the the co-channel interference reduction factor by • Note that the radius R, is the distance from the centre of the cell (base station) to an outer corner. • The distance D, is the distance from one transmitter(base station, or centre of the cell) to the next transmitter of the same frequency. • In most cases, only the first tier of interfering cells (on the same frequency) are considered Third tier Second tier First tier

  6. Path Loss (general approximation) • Power radiates from the transmitter antenna in a spherical manner (inversely related to the surface area of the sphere) • the power at d metres away from the transmitter is given in reference to the power Po , some do metres away. • In general, and terrain characteristics can result in a different power law for the previous equation. We generalize the power law by the coefficient n, where n usually ranges from 2 to 4.

  7. C/I ratio • The Carrier-to-interference ratio, C /I, of the signal at the mobile from the transmitter in a given cell, can be found in an approximate manner by summation of interference from all base stations using the same frequency. Usually expressed in dB. • If we assume all base stations are identically spaced, and are at the centres of their cells, we have the C/I approximation of: • M is the number of interfering base stations in the first tier (this is always M=6 for hexagonal cells with the standard reuse patterns K=3,4,7,12,19... )

  8. Fading During transmission from the base station to the mobile, the received power fluctuates. We can generalize the factors that affect the received power level into 3 main groups. • Path Loss (does not change in time) • changes only with distance from transmitter • there are also losses associated with the frequency of transmission, size/height of transmit/receive antenna, etc. • Long-term fading or Shadowing • caused by buildings or tunnels ‘shadowing’ transmission from BS • changes with mobile position (log-normal distribution) • Short-term fading (or small scale fading) • due to multiple paths of transmission (reflections) arriving at the mobile at the same time (flat fading) • If there are other paths that arrive with some delay, it is called multi-path fading

  9. Small scale fading • The signal may take different ‘paths’ to the mobile customer • Reflections off moving objects, cause a shift in frequency in the signal. Each path also arrives at a various angles and phases. • Some paths arrive at the mobile at the same time. Other paths arrive much later do to a longer distance traveled. WDG addition (2008): Calgary noted for long-delay reflection back from the mountains ! Transmitter Mobile customer

  10. Flat fading • Derivation of fading based on electromagnetic fields. • Several paths arrive at the receiver at nearly the same instant. • Each path has been shifted in frequency due to the relative motion of mobile (relative to the transmitter and surrounding objects) • called Doppler shift • The maximum Doppler shift is given by fm=v/, where v is the mobile velocity (metres/sec), and  is the carrier wavelength • =c/fc, where the speed of light is c=3x108 m/s and the carrier frequency is fc. (WDG: Class Q. When does maximum arise? ) • The paths interfere constructively and destructively causing the received power at the mobile to vary with time. • The coherence time is the time over which the received power does not change significantly. A reasonable ‘ball park’ estimate is:

  11. Flat fading (2) • The received envelope fluctuates due to interfering waves. • ‘Flat’ fading is called so because the gain across the signal bandwidth is ~constant, or flat. Tc=0.018 sec ~ 6 km/h in PCS band (~2 GHz)

  12. Multi-path fading • If if one or more paths arrives somewhat later than the first group of paths, the gain over the transmission bandwidth will not be constant. • Intuitive example: consider 2 cosine waves, one arriving 1 s later than the other at the same amplitude. At the instant t =0: • at 800 MHz: cos(2 800t )+ cos(2 800(t+1 ))= 2 volts • at 800.4 MHz: cos(2 800.4t )+ cos(2 800.4(t+1 ))= 0.191 volts • again, note that if there is no time offset (or relatively small), all frequencies would give the same answer - Flat fading • The coherence bandwidth is the bandwidth over which the channel response somewhat flat. This is approximately: where t is the rms. delay spread, a measure of the channel time dispersion (t = 0.5 s in above e.g.)

  13. Multi-path fading (2) • The formula for the coherence bandwidth is strictly a ‘ball park’ estimate. • If a channel bandwidth B, is less than Bc (e.g. a factor of 2 less), we may say the channel is relatively flat, or constant gain. • But on a wider bandwidth view the channel is frequency selective, and not flat. • In multi-path fading, each path undergoes fading, so that the frequency response will change in time Bc=2 MHz Indoor channel

  14. MSC PSTN Cellular structure • Remember: • MSC - mobile switching centre (also called MTSO – mobile telephone switching office) • PSTN - public switched telephone network

  15. Registration • Registration is the process of notifying the network that a phone is active on the system. • When a phone is switched on, it registers by signalling to the MSC via the base station on a set-up or control channel. • Periodic registration is when the phone announces itself on a regular basis. • Forced registration is when the phone monitors a control channel which provides information including the cell identification (i.e. which cell are we in?). • If the channel strength fades below a threshold, the phone selects another channel. • If the new channel has a new cell ID, then the phone reregisters.

  16. Roaming • Roaming is when a phone is outside its home area or local region. • If the phone registers outside its home area, the MSC contacts the phone’s home area and confirms that the phone is OK. • MSC then notifies home area of the phones current location and provides instructions for routing incoming calls to the phone (and billing information, etc.).

  17. Call initiation • Call initiated from PSTN • MSC receives call from PSTN, sends requested MIN (mobile identification number) to all BS’s (base stations). • BS transmits page (MIN) for user • Mobile receives, and confirms MIN match with ESN (electronic serial number). • BS relays info to MSC • MSC verifies ESN/MIN pair • MSC tell BS to select used voice channel pair; BS selects voice channel pair and informs mobile to move to those channels. • MSC connect BS with PSTN to begin call. IMPORTANT NOTE: This initiation is usually done on specific ‘call control’ channels, or paging channels. These are different from the voice or information channels used for actual call transmission.

  18. Call initiation (2) • Call initiated from Mobile • mobile sends MIN and ESN • BS passes to MSC; MSC verifies ESN and MIN pair • MSC tell BS to select used voice channel pair • BS selects voice channel pair; pages mobile and informs to move to those channels. • Mobile receives page, verifies MIN, and moves to specified channels • MSC connects PSTN with mobile (through BS). IMPORTANT NOTE #2: the voice channels selected for call remain dedicated to that mobile in that specific cell (base station area). The channels are released only when the call is terminated, or the mobile moves to another cell area.

  19. Common terms and concepts • Forward or downlink • transmission from the base station to the mobile • C/I calculations from Lecture 13 were for the downlink • Reverse or uplink • transmission from the mobile to the base station • hand-off • (hard) as a mobile moves into a new cell, transmission to and from the current base station ceases, and communication with the new base station begins • soft : as the mobile moves into a new cell area, the mobile is in contact with 2 or more base stations • this increases performance of the mobile in transition, but reduces overall capacity as channels from multiple cells are allocated to a single mobile for a brief period.

  20. Common terms and concepts (2) • Multi-access (MA) method • the manner in which radio resources are allocated into voice channels • FDMA (frequency division) - each voice channel is assigned a separate frequency • TDMA (time division) - each voice channel is assigned segments of time (slots). Mobiles are commonly severed in a round-robin fashion. • CDMA (code division) -each voice channel is assigned a specific code. At the receiver, the voice channels can be separated with minimal interference. • Duplexing method • FDD (frequency division duplex) - voice channels for the downlink and uplink are separate in frequency (common) • TDD (time division duplex) - the downlink and uplink transmission alternate in time over the same channel

  21. Common terms and concepts (3) • Omni-directional antenna • transmits in all directions (360o) serving the whole cell • Sectorization • dividing the cell into 3 or more radial sections (norm. is 3 or 6) • e.g. in the case of 3 sectors/cell, three 120o antennas are used at the base station to service the cell • As the antennas are directional, the interference from other cells is dramatically reduced at the expense of increased antennas per cell. (Note: the three antennas are part of a single BS) • common for CDMA systems • Frequency equalization (relates to lecture 13) • if the transmission bandwidth of the channel is (much) greater then the approximate coherence bandwidth of the channel, frequency equalization may be required at the receiver

  22. Systems: AMPS • Advanced mobile phone system (First generation system) • Still used in Alberta (the only service in rural areas) and across North America • analog system (FM) developed in 1977, introduced in 1983 • FDMA system as ‘pairs’ of frequencies assigned. Each pair of frequencies is dedicated to a call in each cell. • Downlink (MHz) • Block A: non-wireline companies (Cantel etc.) • Block B: wireline (Telus) • FDD: uplink frequencies are the same, but 45 MHz less for the same channel 869 870 880 890 891.5 894 A A B A B Ch. No. 991 1023 / 1 333 666 716 799

  23. Systems: AMPS (2) • Since 1988, bandwidth increased from 666, to 832 channels. • 30 kHz channels • Channel 1: downlink=870.03 MHz, uplink =825.03 MHz • 416 channels/system (or per block) • 21 control channels / 395 voice channels per system • C/I=18 dB generally found to be acceptable for voice quality • generally has a frequency reuse pattern K>=3.

  24. Systems: Digital AMPS • Digital system in AMPS bands (Advanced digital cellular (ADC), IS-54) (2nd generation) • the first digital to arrive here ( “WOW, digital cell phones…”) • digital (/4-DQPSK modulation, 41.15 s symbol duration) • Intended in to backwards compatible with AMPS • FDD and same frequency band as AMPS and • 30 kHz channels, 832 frequency channels in 800 MHz band • In this manner, the MSC and many components of the BS are the same • TDMA method - each frequency channel is divided into 3 time channels • total channels = 832 x 3 • total voice channels per system =395 x 3 • so 3 times the number of channels as AMPS in the same bandwidth

  25. Systems: GSM (European Specs.) • Originally, Groupe Special mobile, until 1992. Then Global System for Mobile Communications. (2nd generation) • European digital standard, later brought to Canada by Microcell (Montreal) and later, Rogers. • In Europe, usually used in 800/900 MHz band (in PCS as well) • TDMA system, that ‘hops’ from one frequency channel to the next to avoid being in a frequency-selective fade for a long period of time (slow frequency hopping [SFN], ~217 hops/sec) • Digital (GMSK, ~3.69 s symbol duration)

  26. Systems: GSM (European Specs.) (2) • 8 time channels (slots) per frequency channel • Total channels =125 x 8 • 200 kHz bandwidth • frequency equalization is usually required • FDD system • C/I=9 to 12 dB is acceptable. Frequency reuse patterns of K>=3 are generally required • Roger’s has GSM phones here (?in PCS band?) - can I use it in Europe? • Only with a special SIM chip (this will be eliminated..eventually) • general packet radio service (GPRS) up and running

  27. Systems: CDMA (IS-95) • A 2-2.5 generation standard (there is also an IS-95b standard). • A mobile is assigned a channel code. • Transmissions occur at the same time and over the same frequency band • bandwidth of channel is 1.25 MHz in PCS band (~1850-2000 MHz) • FDD system • uses fast power control on uplink (800 times per second) • helps to prevent deep ‘fades’ (troughs) in received power

  28. CDMA (2) • The codes, are referred to as spreading codes • called spreading codes because a narrowband data stream is spread to a wideband width (1.25 MHz) • each user in the cell has a different code

  29. CDMA(3) • after despreading and integration, the desired signal will be about L times greater than the power of another user (L is the number of code ‘chips’ per data bit; L=8 in figure)

  30. CDMA (4) • Frequency equalization is generally not required if each received path differs by a chip duration or more. • This is because a delayed sequence (time shifted by more than one chip) appears like a completely different sequence • a RAKE receiver can despread each path separately • each ‘finger’ of rake receiver captures a path with minimal interference (~1/L) from the other paths • Since each finger only receives one path, each finger sees only flat fading ( ideally) • the information from each finger is then combined to form the signal. • IS-95 system uses 1.2288 Mchips/s, and a 3-4 finger RAKE • there are 64 codes (L=64), frequency reuse every sector • symbol length ( L chips/ 1.2288 Mchips/s) = ~ 52.1 s • 61 for voice/data channels, 3 for paging etc.. • The amount of interference increases with the number of users • Full capacity in every sector is unlikely due to interference

  31. Coverage • Telus coverage map • from web site AMPS 800 MHz Digital 800 MHz CDMA 1.9 GHz

  32. Third generation and notes • Full 3G systems to be available in Alberta in this year (or next) • group of systems: cdma2000, WCDMA, etc, some TDD versions • 3G set of systems are expected to allow roaming (moving into different coverage areas) across the world • Packet transmission will be implemented for CDMA systems • receive and send packets from mobile handset • As only a very brief introduction to cellular systems and the wireless channel model has been discussed here, you may wish to consult the follow reference items for further insight Rappaport, T.S., Wireless communications: principles and practice, Prentice Hall, 1996 -highly recommended Peterson, R.L., Ziemer, R.E., Borth, D.E., Introduction to spread spectrum communications, Prentice Hall,1995. Eds. Holma, H. and Toskala, A., WCMDA for UMTS: Radio access for third generation mobile communications, Wiley, 2001

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