Lecture 4: Cellular Fundamentals

1. Lecture 4: Cellular Fundamentals Chapter 3 - Continued

2. I. Adjacent Channel Interference • Two major types of system-generated interference: • Co-Channel Interference (CCI) – discussed in last lecture • Adjacent Channel Interference (ACI) • Adjacent Channel Interference (ACI) • Imperfect Rx filters allow energy from adjacent channels to leak into the passband of other channels

3. actual filter response • desired filter response

4. This affects both forward & reverse links • Forward Link → base-to-mobile • interference @ mobile Rx from a ______ Tx (another mobile or another base station that is not the one the mobile is listening to) when mobile Rx is ___ away from base station. • signal from base station is weak and others are somewhat strong. • Reverse Link → mobile-to-base • interference @ base station Rx from nearby mobile Tx when desired mobile Tx is far away from base station

5. Near/Far Effect • interfering source is near some Rx when desired source is far away • ACI is primarily from mobiles in the same cell • some cell-to-cell ACI does occur as well → but a secondary source • Control of ACI • don’t allocate channels within a given cell from a contiguous band of frequencies • for example, use channels 1, 4, 7, and 10 for a cell. • no channels next to each other

6. maximize channel separation • separation of as many as N channel bandwidths • some schemes also seek to minimize ACI from neighboring cells by not assigning adjacent channels in neighboring cells

8. Originally 666 channels, then 10 MHz of spectrum was added 666+166 = 832 channels • 395 VC plus 21 CC per service provider (providers A & B) 395*2 = 790, plus 42 control channels • Provider A is a company that has not traditionally provided telephone service • Provider B is a traditional wireline operator • 21 VC groups with ≈ 19 channels/group • at least 21 channel separation for each group

9. for N = 7 → 3 VC groups/cell • For example, choose groups 1A, 1B, and 1C for a cell – so channels 1, 8, 15, 22, 29, 36, etc. are used. • ∴ ≈ 57 channels/cell • at least 7 channel separation for each cell group • to have high quality on control channels, 21 cell reuse is used for CC’s • instead of reusing a CC every 7 cells, as for VC’s, reuse every 21 cells (after every three clusters) • greater distance between control channels, so less CCI

10. use high quality filters in base stations • better filters are possible in base stations since they are not constrained by physical size and power as much as in the mobile Rx • makes reverse link ACI less of a concern than forward link ACI • also true because of power control (discussed below) • choice of modulation schemes • different modulation schemes provide less or more energy outside their passband.

11. Power Control • technique to minimize ACI • base station & MSC constantly monitor mobile received signal strength • mobile Tx power varied (controlled) so that smallest Tx power necessary for a quality reverse link signal is used (lower power for the closer the mobile is to the base station) • also helps battery life on mobile

12. dramatically improves adjacent channel S / I ratio, since mobiles in other cells only transmit at high enough power as transmitter controls (not at full power) • most beneficial for ACI on reverse link • will see later that this is especially important for CDMA systems

13. III. Trunking & Grade of Service (GOS) • Trunked radio system: radio system where a large # of users share a pool of channels • channel allocated on demand & returned to channel pool upon call termination • exploit statistical (random) behavior of users so that fixed # of channels can accommodate large # of users • Trade-off between the number of available channels that are provided and the likelihood of a particular user finding no channels available during the busy hour of the day.

14. trunking theory is used by telephone companies to allocate limited # of voice circuits for large # of telephone lines • efficient use of equipment resources → savings • disadvantage is that some probability exists that mobile user will be denied access to a channel • blocked call : access denied → “blocked call cleared” • delayed call : access delayed by call being put into holding queue for specified amount of time

15. GOS : measure of the ability of user access to a trunked system during the _______ hour • specified as probability (Pr) that call is blocked or delayed • designed to handle the busiest hour → typically ______ • Erlang : unitless measure of traffic intensity • e.g. 0.5 erlangs = 1 channel occupied 30 minutes during 1 hour • Table 3.3, pg. 78 → trunking theory definitions

16. “Offered” Traffic Intensity (A) • Offered? → not necessarily carried by system (some is blocked or delayed) • each user Au=λH Erlangs (also called ρ in queueing theory) • λ = traffic intensity (average arrival rate of new calls, in new requests per time unit, say calls/min). • H = average duration of a call (also called 1/ µ in queueing theory) • system with U users → A = UAu = UλH Erlangs • capacity = maximum carried traffic = C Erlangs = (equal to total # of available channels that are busy all the time)

17. Erlang B formula • Calls are either admitted or blocked • A = total offered traffic • C = # channels in trunking pool (e.g. a cell) • AMPS designed for GOS of 2% • blocked call cleared (denied) → BCC

18. capacities to support various GOS values • Note that twice the capacity can support much more than twice the load (twice the number of Erlangs).

19. Erlang C formulas • blocked call delayed → BCD → put into holding queue • GOS is probability that a call will still be blocked even if it spends time in a queue and waits for up to t seconds • equations (3.17) to (3.19) in book

20. Graphical form of Erlang B formulas

21. Graphical form of Erlang C formulas

22. Example: Find how many users can be supported in a cell containing 50 channels for a 2% GOS (Blocked Calls Cleared) if the average user calls twice/hr with an average call duration of 5 minutes. • What is the corresponding C from the figure? • What is A (Traffic Intensity) from the figure? • So, how many users can be supported?

23. Trunking Efficiency • measure of the # of users supported by a specific configuration of fixed channels, efficiency in terms of users per available channel = U / C • Table 3.4, pg. 79 → assume 1% GOS • Assume Au = 0.2 • 1 group of 20 channels: • 2 groups of 10 channels, with equal number of users per group:

24. the allocation of channel groups can substantially change the # of users supported by trunked system • The larger the trunking pool, the better the trunking efficiency. • as trunking pool size ↓ then trunking efficiency ↓ • What is the relationship between trunking pool size, trunking efficiency, received signal quality, and cluster size? • As cluster size decreases…

25. Note: Trunking efficiency is an issue both in FDMA/TDMA systems and in CDMA systems (where the capacity limit is the number of possible codes and the interference levels).

36. IV. Improving Cellular System Capacity • A cellular design eventually (hopefully!) becomes insufficient to support the growing number of users. • Need to provide more channels per unit coverage area • Would like to have orderly growth • Would like to upgrade the system instead of rebuild • Would like to use existing towers as much as possible

37. Cell Splitting • subdivide congested cell into several smaller cells • increases number of times channels are reused in an area • must decrease antenna height & Tx power • so smaller coverage per cell results • and the co-channel interference level is held constant

38. each smaller cell keeps ≈ same # of channels as the larger cell, since each new smaller cell uses the same number of frequencies • this means that we keep that same cluster size • capacity ↑ because channel reuse ↑ per unit area • smaller cells → “micro-cells”

39. Illustration is for towers at the corners

40. advantages include: • only needed for cells that reach max. capacity → not all cells • implement when Pr [blocked call] > acceptable GOS • system capacity can gradually expand as demand ↑ • disadvantages include: • # handoffs/unit area increases • umbrella cell for high velocity traffic may be needed • more base stations → $$ for real estate, towers, etc.

41. complicated design process • new base stations use lower power and antenna height • What about existing base stations? • If kept at the same power, they would overpower new microcells. • If reduced in power, they would not cover their own cells. • One solution: Use separate groups of channels. • One group at the original power and another group at the lower power. • New microcells only use lower power channels. • As load growth continues, more and more channels are moved to lower power.

44. Sectoring • cell splitting keeps D / R unchanged (same cluster size and CCI) but increases frequency reuse/area • alternate way to ↑ capacity is to _____ CCI (increase S / I ratio)

45. replace omni-directional antennas at base station with several directional antennas • 3 sectors → 3 120° antennas • 6 sectors → 6 60° antennas

46. cell channels broken down into sectored groups • CCI reduced because only some of neighboring co-channel cells radiate energy in direction of main cell • center cell labeled "5" has all co-channel cells illustrated • only 2 co-channel cells will interfere if all are using 120° sectoring • only 1 co-channel cell would interfere when using 60° sectoring • If the S/I was 17 dB for N = 7 and n = 4, what is the S / I now with 120° sectoring? • 24.2 dB

48. How is capacity increased? • sectoring only improves S/I which increases voice quality, beyond what is really necessary • by reducing CCI, the cell system designer can choose smaller cluster size (N ↓) for acceptable voice quality • smaller N → greater frequency reuse → larger system capacity • What would the system capacity, Cnew, now be when 120° using sectoring, as compared to the old capacity, Cold?