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  • *** Audio Notes (if present) are shown on lower left or right of slide (see icon) depending on slide layout. You must click the icon for it to play! Audio Notes are essentially the same as the written notes on the slides Last update: 14 Nov 2009 13:30
  • Mobility is something we greatly desire, and the availability of small lightweight powerful electronics now enables us to build devices that have high-speed capabilities in small form factors. While there are many different mobile communications technologies, one of the most important and ubiquitous are cellular-based systems.
  • This is THE key concept; signals attenuate and therefore a given frequency band can be reused if the transmitter’s are separated by enough geographic distance. By carefully controlling power levels and using a coordinated set of frequencies with appropriate separation the cellular concept allows comprehensive coverage. Note that the seven frequency set is one common option; as the text explains there are other frequency reuse patterns and cell shapes that can be used.
  • Cells are shared resources and at some point there will be too many subscribers attempting to use the cell’s communications capacity. This can mean busy signals for voice calls and delays for packetized communications (text messages, TCP/IP-based communications, etc.). Once that happens some technique needs to be applied to allow more capacity. Cell splitting is a very common way in operational systems; transmit power levels are adjusted downward to create smaller cells.
  • Cell sectoring is also common, microcells are common in areas with the densest number of subscribers – urban subway stations and sports arenas are two examples of where microcells are implemented to ensure adequate coverage and service levels for crowds of subscribers.
  • The most obvious component in cellular systems are the cell towers that are seen along the highways; these towers (and other structures) hold the antennas used to connect mobile subscribers to the cellular system. Look around a cell tower and you will typically see a small building that houses equipment: in many systems these just house the base station (RF) equipment and the transmission link back to a switching office similar to that used in the PSTN. In other systems switching equipment is co-located with the transmitters at the cell site proper.
  • Many times one or more T-1/DS-1 or DS-3 circuits are used to connect the base station back to a MTSO/MSC. In many of the newer systems these switched TDM links are now carrying packetized voice data (either VoIP or a similar scheme); this is more efficient and lends itself to convergence and servicing of the rapidly growing number of data users.
  • Note how all cellular and land-line (e.g – AT&T, Verizon, etc.) interconnect both their SS7 networks (for call control) as well as voice links for the subscriber talk path and data services. To the end user the system usually works remarkably well, especially from the control and voice perspective. As a user you rarely have to concern yourself with what carrier the subscriber you are calling is actually on.
  • This sequence of slides runs through the basic operational components of a cellular call. Movement and changes in signal strength and noise make what looks like a straightforward process very challenging from a communications engineering standpoint. Movement between cells and among different channels in a cell should be seamless and un-noticed to the subscriber. Note that in most cases the control/signaling plane is separate from the voice and/or data path (traffice channels).
  • Though the format and details are different at a high level the call setup messages used are similar in purpose and layout to ISDN call setup and control messages. Obviously, there has to be additional functionality for RF and handoff management. The “forward” channel is BS -> mobile and the “reverse” channel is mobile -> BS.
  • In some respects this process is like a “broadcast”; all mobile units hear it but the message is only intended for the mobile user that is the recipient of the call (think about the supermarket or other store with overhead speakers where a manager is paged… everybody hears but only the manager is to respond as requested).
  • Traffic channels must be assigned to support a voice path. Some called are mobile-to-land, so this process only happens on one ‘side’ of the connection – in mobile-to-mobile calls this process actually happens on both sides of the connection.
  • User should not know a handoff occurred. Signaling messages between base stations and from BS->mobile allow a coordinated handoff from one BS to another.
  • Fading can be considered fast (rapid fluctuations) or slow; flat (across a wide spectral range) or selective (a relatively limited frequency band).
  • Multipath is arguably the biggest contributor to mobile fading (atmospheric effects do play a role as in all RF systems, but the mobility is ‘harder’ to cope with)
  • Multipath can cause the received signals to cancel each other out or add together similar to the waves on a lake or the ocean. How they interact depends on each received signal component’s phase and delay.
  • As discussed in earlier classes FEC works by adding sufficient redundancy to the signal to have an acceptably high probability of receiving the data even if part of the signal is corrupted. Of course, this adds overhead that reduces efficiency and in itself can increase the BER (bit error rate) because of the additional bits added to the message.
  • Space diversity is used in the new IEEE 802.11n WLAN standard to increase throughput; in this application this is called MIMO (multiple-in, multiple-out – as in antennas).
  • *** We are now roughly in the 2.5G arena; there are “3G” systems deployed but whether they truly meet the subscriber data rate/capacity requirements is debatable. “IS” stands for “Interim Standard” – I am not sure how specifically this was coined but most of these are ANSI or EIA/TIA standards with different designation numbers. However, these designators are in wide use. 2G E-TDMA => IS-54, basic 2G CDMA => IS-95, IS-95B adds data capabilities up to 64kbps
  • These are full-duplex “channels”; half go to each provider. Channels in this context are a pair of frequency bands; one uplink (mobile -> BS) and one downlink (BS -> mobile).
  • Rare to see use of in-band control messages now – though the same can be said about seeing an AMPS system nowadays.
  • [Photo courtesy of Clay] The scary thought is I remember getting to use a cellular phone that looked like that… I believe it was Motorola and it cost somewhere around $2000.
  • Though AMPS provided a popular new service, like analog telephone systems in general it had significant limitations in evolving to support new multimedia requirements. Digital systems could provide: Highly efficient digitized voice Encryption More sophisticated error detection/correction Robust sophisticated multiplexing for more capacity and higher potential data rates GSM was very common in Europe, now being upgraded to and/or replaced with 3G systems.
  • A SIM card looks like a tiny “chip” that fits in a phone and contains the subscriber info needed for activation, feature selection, and billing.
  • Other Standards IMT-TC or TD-CDMA combination of W-CDMA AND TDMA for GSM systems CDMA2000 1x (also called 1xRTT) 1x uses one 1.25MHz band to potentially get 614-kbps CDMA2000 EV-DO is an evolution from 1x allowing 3.75 (3x) channels and multiplexing of the larger channels for higher data rates Verizon currently offers this service in the DC metro area CDMA2000 1xEV-DV 1xEV combines packet and voice on same carriers Potential data rates to 3.09-Mbps Wideband CDMA Uses expanded 5-MHz bandwidth for multi-megabit data rates
  • Note the asymmetric operation upstream and downstream; like cable modems the ‘one-to-many’ downstream or forward channel presents different RF and media access challenges than the upstream or reverse channel. Upstream there is a ‘many-to-one’ relationship; operation in this environment required different mechanisms than in the other direction.
  • If one can build and deploy the advanced signal processing capabilities economically, CDMA has a number of operational advantages. However, as detailed on the next slide, there are two potential engineering problems that must be considered and mitigated for effective operation.
  • **** No Audio This Slide **** Reference: http://www.cdmaonline.com/members/2ginteractive/3000/A27.htm
  • This common cellular receiver tries to optimize performance by using multipath to its advantage instead of fighting it. By adaptively weighting and delaying different copies of the signal suitable to the multiple available paths, the receiver can then attempt to combine them into a ‘better’ received signal than if just one primary path was used and all the other paths were rejected.
  • 635_411 Class 12 Fal..

    1. 1. Fundamentals of Networking & Telecommunications Class # 12 Mobile & Cellular Communications
    2. 2. Mobile & Cellular Communications <ul><li>This area has become very important with the explosive growth in demand for mobility </li></ul><ul><li>Different types of mobile communications </li></ul><ul><ul><li>Satellite </li></ul></ul><ul><ul><li>Paging </li></ul></ul><ul><ul><li>Cordless Telephony </li></ul></ul><ul><ul><li>Mobile & Trunked Radio </li></ul></ul><ul><ul><li>Enterprise Wireless </li></ul></ul><ul><ul><li>Cellular/PCS </li></ul></ul><ul><li>While these can (and do) overlap, we will concentrate on Cellular systems </li></ul>635.411 – Class #12 Fall 2009
    3. 3. The Basic Cellular Concept <ul><li>Because of attenuation frequencies can be reused </li></ul><ul><li>Break areas up into cells; assign seven frequencies to a cluster of cells </li></ul><ul><li>Each cell representing a geographic area gets one of the frequencies at low power </li></ul><ul><li>Frequencies are only re-used two or more cells away </li></ul><ul><li>Capacity can be increased by adding more towers and decreasing power </li></ul><ul><li>Variable power levels allowed cell sites to be sized based on usage. (cell splitting) </li></ul>635.411 – Class #12 Fall 2009 1 4 7 6 5 3 2 1 4 7 6 5 3 2 1 4 7 6 5 3 2 1 4 7 6 5 3 2 1 4 7 6 5 3 2 1 4 7 6 5 3 2 1 4 7 6 5 3 2
    4. 4. Increasing Capacity (1) <ul><li>Add new channels </li></ul><ul><ul><li>Not all channels used to start with </li></ul></ul><ul><li>Frequency borrowing </li></ul><ul><ul><li>Taken from adjacent cells by congested cells </li></ul></ul><ul><ul><li>Or assign frequencies dynamically </li></ul></ul>635.411 – Class #12 Fall 2009 <ul><li>Cell splitting </li></ul><ul><ul><li>Non-uniform distribution of topography and traffic </li></ul></ul><ul><ul><li>Smaller cells in high use areas </li></ul></ul><ul><ul><ul><li>Original cells 6.5 – 13 km </li></ul></ul></ul><ul><ul><ul><li>1.5 km limit in general </li></ul></ul></ul><ul><ul><ul><li>More frequent handoff </li></ul></ul></ul><ul><ul><ul><li>More base stations </li></ul></ul></ul>
    5. 5. Increasing Capacity (2) <ul><li>Cell Sectoring </li></ul><ul><ul><li>Cell divided into wedge shaped sectors </li></ul></ul><ul><ul><li>3 – 6 sectors per cell </li></ul></ul><ul><ul><li>Each with own channel set </li></ul></ul><ul><ul><ul><li>Subsets of cell’s channels </li></ul></ul></ul><ul><ul><li>Done with directional antennas or adaptive antenna arrays </li></ul></ul><ul><li>Microcells </li></ul><ul><ul><li>Move antennas from tops of hills and large buildings to tops of small buildings and sides of large buildings </li></ul></ul><ul><ul><li>Reduced power </li></ul></ul><ul><ul><li>Good for city streets, along roads and inside large buildings </li></ul></ul>635.411 – Class #12 Fall 2009
    6. 6. Cellular System Components <ul><li>Mobile Telecommunications Switching Office (MTSO) </li></ul><ul><ul><li>Or Mobile Switching Center (MSC) </li></ul></ul><ul><li>Base Station </li></ul><ul><li>Mobile Station </li></ul><ul><li>PSTN </li></ul>635.411 – Class #12 Fall 2009 PSTN Mobile Telecommunications Switching Office Control Channel Traffic Channel
    7. 7. Operation of Cellular Systems <ul><li>Base station (BS) at center of each cell </li></ul><ul><ul><li>Antenna, controller, transceivers </li></ul></ul><ul><ul><li>Controller handles call processing between mobile unit and the rest of the network </li></ul></ul><ul><ul><ul><li>Number of mobile units may in use at a time </li></ul></ul></ul><ul><li>BS connect s to a MTSO/MSC </li></ul><ul><ul><li>One MTSO serves multiple BS </li></ul></ul><ul><ul><li>MTSO to BS link by wire or wireless </li></ul></ul><ul><li>MTSO/MSC F unctions </li></ul><ul><ul><li>Connects calls between mobile units and from mobile to fixed telecommunications network (PSTN) </li></ul></ul><ul><ul><li>Assigns voice channels </li></ul></ul><ul><ul><li>Performs handoffs </li></ul></ul><ul><ul><li>Monitors calls (billing) </li></ul></ul>635.411 – Class #12 Fall 2009
    8. 8. A More Detailed Diagram 635.411 – Class #12 Fall 2009
    9. 9. Typical Call in a Single MTSO (1) <ul><li>Mobile unit initialization </li></ul><ul><ul><li>Scan and select strongest set up control channel </li></ul></ul><ul><ul><li>Automatically selected BS antenna of cell </li></ul></ul><ul><ul><ul><li>Usually but not always nearest (propagation anomalies) </li></ul></ul></ul><ul><ul><li>Handshake to identify user and register location </li></ul></ul><ul><ul><li>Scan repeated to allow for movement </li></ul></ul><ul><ul><ul><li>Change of cell if needed </li></ul></ul></ul><ul><ul><li>Mobile unit monitors for pages (see below) </li></ul></ul>635.411 – Class #12 Fall 2009
    10. 10. Typical Call in a Single MTSO ( 2 ) <ul><li>Mobile originated call </li></ul><ul><ul><li>Check to see i f set up channel is free </li></ul></ul><ul><ul><ul><li>Monitor forward channel (from BS) and wait for idle </li></ul></ul></ul><ul><ul><li>Send number on pre-selected channel </li></ul></ul><ul><ul><li>BS sends request to MTSO </li></ul></ul>635.411 – Class #12 Fall 2009
    11. 11. Typical Call in a Single MTSO ( 3 ) <ul><li>Paging </li></ul><ul><ul><li>MTSO attempts to connect to the CALLED mobile unit </li></ul></ul><ul><ul><li>Paging message sent to Base Stations depending on called mobile number </li></ul></ul><ul><ul><li>Paging signal transmitted on set up channel </li></ul></ul><ul><ul><li>Some phones save power by shutting down all receiving circuitry except for paging during “sleep” mode </li></ul></ul>635.411 – Class #12 Fall 2009
    12. 12. Typical Call in a Single MTSO ( 4 ) <ul><li>Call accepted </li></ul><ul><ul><li>Mobile unit recognizes number on set up channel </li></ul></ul><ul><ul><li>Responds to BS which sends response to MTSO </li></ul></ul><ul><ul><li>MTSO sets up circuit between calling and called BSs </li></ul></ul><ul><ul><li>MTSO selects available traffic channel within cells and notifies BSs </li></ul></ul><ul><ul><li>BSs notify mobile unit of channel </li></ul></ul>635.411 – Class #12 Fall 2009
    13. 13. Typical Call in a Single MTSO ( 5 ) <ul><li>Ongoing call </li></ul><ul><ul><li>Voice/data exchanged through respective BSs and MTSO </li></ul></ul><ul><li>Handoff </li></ul><ul><ul><li>Mobile unit moves out of range of cell into range of another cell </li></ul></ul><ul><ul><li>Traffic channel changes to one assigned to new BS </li></ul></ul><ul><ul><ul><li>Without interruption of service to user </li></ul></ul></ul>635.411 – Class #12 Fall 2009
    14. 14. Mobile Radio Propagation Effects <ul><li>Two issues that must be dealt with! </li></ul><ul><li>Signal strength </li></ul><ul><ul><li>Must be strong enough between BS and mobile unit to maintain signal quality at the receiver </li></ul></ul><ul><ul><li>But not so strong it creates too much co-channel interference </li></ul></ul><ul><ul><li>Varies as function of distance from BS (which changes) </li></ul></ul><ul><ul><li>Noise constantly varies </li></ul></ul><ul><ul><ul><li>Automobile ignition noise greater in city than in suburbs </li></ul></ul></ul><ul><ul><ul><li>Other signal sources vary </li></ul></ul></ul><ul><li>Fading: The variation of a received signals power over time caused by changes in the medium and/or path </li></ul><ul><ul><li>Even if signal strength is good, this can cause disruption </li></ul></ul><ul><ul><li>Example: atmospheric conditions (rain) </li></ul></ul><ul><ul><li>Movement of (mobile) antennas significantly complicates this </li></ul></ul>635.411 – Class #12 Fall 2009
    15. 15. Multipath Propagation <ul><li>Reflection </li></ul><ul><ul><li>Surface large relative to signal wavelength </li></ul></ul><ul><ul><li>May have phase shift from original </li></ul></ul><ul><ul><li>May cancel out original or increase it </li></ul></ul><ul><li>Diffraction </li></ul><ul><ul><li>Edge of impenetrable body that is large relative to wavelength </li></ul></ul><ul><ul><li>Edge of body becomes source of signal </li></ul></ul><ul><ul><li>May receive signal even if no line of sight (LOS) to transmitter </li></ul></ul><ul><li>Scattering </li></ul><ul><ul><li>Obstacle size on order of wavelength or less </li></ul></ul><ul><ul><li>Breaks signal into several weaker signals </li></ul></ul><ul><li>If LOS, diffracted and scattered signals not significant </li></ul><ul><ul><li>Though reflected signals may be! </li></ul></ul><ul><li>If no LOS, diffraction and scattering are primary means of reception </li></ul>635.411 – Class #12 Fall 2009
    16. 16. Effects of Multipath Propagation <ul><li>Signals may cancel out due to phase differences </li></ul><ul><li>Intersymbol Interference (ISI) </li></ul><ul><ul><li>Send pulse at given frequency between fixed antenna & mobile unit </li></ul></ul><ul><ul><li>Channel may deliver multiple copies at different times </li></ul></ul><ul><ul><li>Delayed pulses act as noise making recovery of bit information difficult </li></ul></ul><ul><ul><li>Timing changes as mobile unit moves </li></ul></ul><ul><ul><ul><li>Harder to design signal processing to filter out multipath effects </li></ul></ul></ul>635.411 – Class #12 Fall 2009
    17. 17. Error Compensation Mechanisms (1) <ul><li>Forward error correction </li></ul><ul><ul><li>Applicable in digital transmission applications </li></ul></ul><ul><ul><li>Typically, ratio of total bits sent to data bits between 2 and 3 </li></ul></ul><ul><ul><li>There is a big overhead penalty </li></ul></ul><ul><ul><ul><li>Capacity one-half or one-third </li></ul></ul></ul><ul><ul><ul><li>Reflects difficulty of mobile wireless environment </li></ul></ul></ul><ul><li>Adaptive equalization </li></ul><ul><ul><li>Used to combat inter-symbol interference </li></ul></ul><ul><ul><li>Gathering the dispersed symbol energy back together into its original time interval </li></ul></ul>635.411 – Class #12 Fall 2009
    18. 18. Error Compensation Mechanisms (2) <ul><li>Diversity </li></ul><ul><ul><li>Based on fact that individual channels experience independent fading events </li></ul></ul><ul><ul><li>Provide multiple logical channels between transmitter and receiver </li></ul></ul><ul><ul><li>Send part of signal over each channel </li></ul></ul><ul><ul><li>Doesn’t eliminate errors but reduce error rate </li></ul></ul><ul><ul><li>Space diversity </li></ul></ul><ul><ul><ul><li>Multiple nearby antennas receive message or collocated multiple directional antennas </li></ul></ul></ul><ul><ul><li>Frequency Diversity </li></ul></ul><ul><ul><ul><li>Signal is spread out over a larger frequency bandwidth or carried on multiple frequency carriers </li></ul></ul></ul><ul><ul><ul><li>E.g. spread spectrum </li></ul></ul></ul>635.411 – Class #12 Fall 2009
    19. 19. Evolution of Cellular Systems <ul><li>Generations of Cellular Systems </li></ul><ul><ul><li>1G = Analog Systems (AMPS) </li></ul></ul><ul><ul><li>2G = Digital System (GSM, TDMA, CDMA) </li></ul></ul><ul><ul><li>2.5G = Bridge to 3G (GSM/GPRS/EDGE, IS-95B) </li></ul></ul><ul><ul><li>3G = Broadband Capability (W-CDMA) </li></ul></ul><ul><ul><ul><li>Full packet-based infrastructure; voice, high-speed data, & multi-media apps equally supported </li></ul></ul></ul><ul><ul><li>4G = All IP Packet Switched (Stupid Fast!) </li></ul></ul><ul><ul><ul><li>Wi-Fi and/or Wi-MAX integration? </li></ul></ul></ul>635.411 – Class #12 Fall 2009
    20. 20. 1G - Advanced Mobile Phone Service (AMPS) (1) <ul><li>First standard cellular system </li></ul><ul><ul><li>Developed by Bell Labs in the late 1970s </li></ul></ul><ul><ul><li>First deployment in the U.S. in 1983 </li></ul></ul><ul><li>Band is split in two to encourage competition (one given to the local carrier in each market region) </li></ul><ul><li>Original Advanced Mobile Phones Service (AMPS) standard (1983) </li></ul><ul><ul><li>40MHz of RF spectrum allocated (two 20MHz bands) </li></ul></ul><ul><ul><li>30kHz channels used for FDMA </li></ul></ul><ul><ul><li>Total of 666 Channels (624 for analog voice, 42 for control) </li></ul></ul><ul><li>Standard extended in 1986 (E-AMPS) </li></ul><ul><ul><li>RF spectrum expanded to 50MHz (two 25MHz bands) </li></ul></ul><ul><ul><ul><li>Downlink 824MHz  849MHz </li></ul></ul></ul><ul><ul><ul><li>Uplink 869MHz  894MHz </li></ul></ul></ul><ul><ul><li>832 Total Channels (790 for voice, 42 for control) </li></ul></ul>635.411 – Class #12 Fall 2009
    21. 21. 1G - Advanced Mobile Phone Service (AMPS) (2) <ul><li>Narrowband AMPS (N-AMPS) implemented in 1991 </li></ul><ul><ul><li>Uses 10KHz analog voice channels </li></ul></ul><ul><ul><li>Tripled cluster capacity to 2,496 channels (2370 voice, 126 control) </li></ul></ul><ul><li>Full-duplex control channels </li></ul><ul><ul><li>30-kHz (AMPS) or 10-kHz (N-AMPS) </li></ul></ul><ul><ul><li>Transmit digital data using FSK </li></ul></ul><ul><ul><li>Data transmitted in frames </li></ul></ul><ul><li>Control information can be transmitted over voice channel during conversation </li></ul><ul><ul><li>Mobile unit or the base station inserts burst of data </li></ul></ul><ul><ul><ul><li>Turn off voice FM transmission for about 100 ms </li></ul></ul></ul><ul><ul><ul><li>Replacing it with an FSK-encoded message </li></ul></ul></ul><ul><ul><li>Used to exchange urgent messages </li></ul></ul><ul><ul><ul><li>Change power level </li></ul></ul></ul><ul><ul><ul><li>Initiate Handoff </li></ul></ul></ul>635.411 – Class #12 Fall 2009
    22. 22. 1G - Advanced Mobile Phone Service (AMPS) (3) <ul><li>AMPS is a good solid standard but as the demand for mobile voice and data grew its deficiencies became harder to live with </li></ul><ul><ul><li>Did not accommodate digital voice </li></ul></ul>635.411 – Class #12 Fall 2009 <ul><ul><li>Could not support data or converged services easily </li></ul></ul><ul><ul><li>Had security issues and difficultly providing enhanced services </li></ul></ul>1G-AMPS = GrAMPS!
    23. 23. 2G - Global System for Mobile Communication (GSM) (1) <ul><li>Developed & first deployed in Europe (1991) – now under deployment in U.S. (e.g. – AT&T) </li></ul><ul><li>Uses a combination of FDMA and TDMA </li></ul><ul><ul><li>200kHz channels divided into 8 time slots </li></ul></ul><ul><ul><li>Time slots can be used for ‘traffic’ (user voice or data) or control signaling </li></ul></ul><ul><ul><li>Voice digitized at either 8-kbps or 13-kbps </li></ul></ul><ul><li>Spectrum </li></ul><ul><ul><li>900MHz (50-MHz of bandwidth available) </li></ul></ul><ul><ul><li>1800MHz (150-MHz of bandwidth available) </li></ul></ul><ul><ul><li>1900MHz (120-MHz of bandwidth available) – used in the U.S. </li></ul></ul>635.411 – Class #12 Fall 2009
    24. 24. 2G - Global System for Mobile Communication (GSM) (2) <ul><li>GSM is more than a radio interface standard for cellular voice </li></ul><ul><li>Other new features/services </li></ul><ul><ul><li>SIM: subscriber identity module </li></ul></ul><ul><ul><li>Data Services </li></ul></ul><ul><ul><ul><li>SMS (Short Message Service): one and two-way paging </li></ul></ul></ul><ul><ul><ul><li>Bearer Data Services: low-speed circuit switched data channel </li></ul></ul></ul><ul><ul><li>Authentication & Encryption Services </li></ul></ul><ul><ul><li>MAP (Mobile Access Protocol) </li></ul></ul><ul><ul><ul><li>Provide roaming & location management (similar to IS-41 standard used in AMPS and TDMA for the same purpose) </li></ul></ul></ul>635.411 – Class #12 Fall 2009
    25. 25. Code Division Multiple Access (1) <ul><li>Spans both the 2G and 3G “Generations” </li></ul><ul><ul><li>Developed by Qualcomm </li></ul></ul><ul><ul><li>“ Narrowband” CDMA used in many 2G deployments </li></ul></ul><ul><ul><li>Becoming the worldwide standard for cellular modulation -- all versions of 3G utilize some version of CDMA </li></ul></ul><ul><li>Basic Characteristics </li></ul><ul><ul><li>Users share a common frequency channel (½ upstream, ½ downstream) </li></ul></ul><ul><ul><li>Each user gets a unique Pseudo-random </li></ul></ul><ul><ul><li>Supports digital voice and data </li></ul></ul><ul><ul><li>Utilizes Direct Sequence Spread Spectrum (DS-SS) </li></ul></ul><ul><li>Current CDMA Based Standards </li></ul><ul><ul><li>2G: IS-95 in North America (800-MHz & 1900-MHz PCS) </li></ul></ul><ul><ul><li>3G: ‘pure’ standard is called “wideband” CDMA </li></ul></ul><ul><ul><li>3G variants: cdma2000 & TD-CDMA </li></ul></ul>635.411 – Class #12 Fall 2009
    26. 26. Code Division Multiple Access (2) <ul><li>Important IS-95 Characteristics </li></ul><ul><ul><li>Digital Voice coding at ≈ 9600-bps; data up to 14.4-kbps </li></ul></ul><ul><ul><li>Uses 1.228-MHz upstream & downstream channels – specific operation is different in each direction </li></ul></ul><ul><li>Forward Channel </li></ul><ul><ul><li>64 logical channels (pilot, synch, paging, user traffic) </li></ul></ul><ul><ul><li>55 user channels (coding, FEC, control bring channel rate to 19.2-kbps </li></ul></ul><ul><ul><li>QPSK modulation of spread stream onto channel </li></ul></ul><ul><li>Reverse Channel </li></ul><ul><ul><li>94 logical channels (32 access, 64 user traffic) </li></ul></ul><ul><ul><li>Orthogonal coding used to improve reception, but not for chipping </li></ul></ul><ul><ul><li>The mobile unit’s 42-bit SN is used to generate the chipping code </li></ul></ul><ul><li>“ Wideband” CDMA Advancements </li></ul><ul><ul><li>Move to 5-MHz channel; get data rates up to 2+ Mbps </li></ul></ul><ul><ul><li>Multi-rate capabilities (TDMA ‘on top’ of CDMA) </li></ul></ul>635.411 – Class #12 Fall 2009
    27. 27. CDMA Advantages <ul><li>Frequency diversity </li></ul><ul><ul><li>Frequency-dependent transmission impairments (noise bursts, selective fading) have less effect </li></ul></ul><ul><li>Multipath resistance </li></ul><ul><ul><li>DSSS overcomes multipath fading by frequency diversity </li></ul></ul><ul><ul><li>Less Intersymbol Interference </li></ul></ul><ul><li>Privacy </li></ul><ul><ul><li>Inherited from spread spectrum </li></ul></ul><ul><li>Graceful degradation </li></ul><ul><ul><li>With FDMA or TDMA, fixed number of users can access system simultaneously </li></ul></ul><ul><ul><li>With CDMA, as more users access the system simultaneously, noise level and hence error rate increases </li></ul></ul><ul><ul><li>Gradually system degrades </li></ul></ul>635.411 – Class #12 Fall 2009
    28. 28. CDMA Drawbacks <ul><li>Self-jamming </li></ul><ul><ul><li>Unless all mobile users are perfectly synchronized, arriving transmissions from multiple users will not be perfectly aligned on chip boundaries </li></ul></ul><ul><ul><li>Spreading sequences of different users not orthogonal </li></ul></ul><ul><ul><ul><li>Causing signal degradation </li></ul></ul></ul><ul><ul><li>Distinct from either TDMA or FDMA </li></ul></ul><ul><ul><ul><li>Reasonable time or frequency guardbands provide sufficient orthogonality </li></ul></ul></ul><ul><li>Near-far problem </li></ul><ul><ul><li>Signals closer to receiver are received with less attenuation than signals farther away </li></ul></ul><ul><ul><li>A “near” phone can act as a jammer to a “far” phone </li></ul></ul><ul><ul><li>Can be controlled by adjusting power of phones such that received power at the BS is the same </li></ul></ul>635.411 – Class #12 Fall 2009
    29. 29. Principle of RAKE Receiver 635.411 – Class #12 Fall 2009 <ul><li>Receiver attempts to recover signals from multiple paths and combine them, with suitable delays </li></ul><ul><li>Used to overcome multipath issues in CDMA </li></ul>
    30. 30. Reading & Homework <ul><li>Covered this Class: </li></ul><ul><ul><li>Cellular Wireless Networks (Chapter 14) </li></ul></ul><ul><li>Next Class: ATM & Congestion Control </li></ul><ul><li>REMINDER: Research Paper is due in one week! </li></ul><ul><ul><li>PLEASE try to have someone review it before you turn it in. </li></ul></ul><ul><ul><li>Remember to use proper citations/references </li></ul></ul><ul><ul><li>Make it representative of your best work! Would you turn this in to your executive leadership if you wanted a promotion? </li></ul></ul>635.411 – Class #12 Fall 2009
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