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ECE 5221 Personal Communication Systems

ECE 5221 Personal Communication Systems. Prepared by: Dr . Ivica Kostanic Lecture 12: Frequency allocation and channelization. Spring 2011. Outline . Duplexing Spectrum allocation in the US Frequency division multiple access (FDMA) Time division multiple access (TDMA) Examples.

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ECE 5221 Personal Communication Systems

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  1. ECE 5221 Personal Communication Systems Prepared by: Dr. Ivica Kostanic Lecture 12: Frequency allocation and channelization Spring 2011

  2. Outline • Duplexing • Spectrum allocation in the US • Frequency division multiple access (FDMA) • Time division multiple access (TDMA) • Examples Important note: Slides present summary of the results. Detailed derivations are given in notes.

  3. Duplexing schemes in cellular networks • Cellular communications is full duplex • Communication from MS to BS -> uplink • Communications from BS to MS -> downlink • Two ways for full duplex • Frequency Division Duplexing (FDD) • Time Division Duplexing (TDD) • Frequency Division Duplexing (FDD) • More common • Uses paired spectrum allocation • One part (usually lower in frequency) – uplink • Second pert (usually higher in frequency) – downlink • Lower frequencies used for UL due to slightly better propagation Note: frequency separation between UL and DL is referred to as the duplexing separation / duplexing space

  4. Case study 1. US cellular frequency band • Spectrum is around 850 MHz • Two licenses A and B (25MHz each) • Initial licenses awarded in 1983 • A – newcomer • B – existing landline carrier • Additional spectrum allocated in 1989 • License A • Downlink: • 869-870MHz (A’’) • 870-880MHz (A) • 890-891.5 (A’) • Uplink: • 824-825MHz (A’’) • 825-835MHz (A) • 845-846.5MHz (A’) • License B • Downlink: • 880-890MHz (B) • 891.5-894MHz (B’) • Uplink • 835-845 MHz (B) • 846.5-849MHz (B’) Spectral plan for US cellular band

  5. Case study 2: US PCS frequency band • Six licenses (A-F) • Large licenses (2 times 15 MHz): A,B,C • Small licenses (2 times 5MHz) D, E, F • Auctioned in 1995 • Duplexing space 80MHz Note: Original rules prevented operators from owning spectrum in both 850 and 1900MHz. This has changed in 2003. PCS spectrum allocation

  6. Time division duplexing • Uses same spectrum for UL and DL • Spectrum access is divided in time between UL and DL • Allocation of time slots may be managed in a dynamic manner – accommodates shift in traffic patterns Time Division Duplexing (TDD) Original spectrum allocation for 3G. Note both paired and unpaired spectrum bands World wide spectrum allocations: http://www.worldtimezone.com/gsm.html

  7. Frequency Division Multiple Access (FDMA) • Allocated spectrum separated in smaller frequency bands • A pair of segments is referred to as the channel (one frequency for UL and one for DL) • Number of available channels depend on the total amount of spectrum and channel bandwidth of the cellular technology • Users that are geographically close operate on different channels Note: The mobiles are co time, but do not interfere due to frequency separation Spectrum channelization

  8. FDMA Example Consider deployment of GSM in A block of PCS band. Calculate the number of channels that can be accommodated. GSM channel is 200KHz wide. Guard band – prevents interference with adjacent bands Number of channels

  9. GSM – Frequency <-> ARFCN calculations ARFCN – Absolute Radio Frequency Channel Number Example – US PCS allocation 2

  10. Time Division Multiple Access (TDMA) • Multiple users share the same frequency channel • The sharing is done in time domain • Only one user access the channel in a given time • Portions of time allocated to the user are called time slots • The number of users sharing the channel is technology dependent (3-16) Note that: Rc = N x Ru

  11. Example. TDMA in GSM • Channel is shared between eight users • User does not transmit and receive at the same time • Transmission occurs in “accumulate and burst” fashion • Has to be digital • Chanel bandwidth larger than the single user rate Note: for good portion of time phone does not transmit or receive

  12. Near-far problem in TDMA systems • In TDMA synchronization between users – vital • Delay of the burst for the further mobiles is larger • Due to different delays – burst may collide • Te remedy the problem, mobiles advance their transmission • The amount of time advancement is determined by base station and communicated to the mobiles

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