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Chapter 3. The Cellular Concept - System Design Fundamentals. I. Introduction. Goals of a Cellular System High capacity Large coverage area Efficient use of limited spectrum Large coverage area - Bell system in New York City had early mobile radio Single Tx, high power, and tall tower

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Chapter 3

Chapter 3

The Cellular Concept - System DesignFundamentals

I introduction
I. Introduction

  • Goals of a Cellular System

    • High capacity

    • Large coverage area

    • Efficient use of limited spectrum

  • Large coverage area - Bell system in New York City had early mobile radio

    • Single Tx, high power, and tall tower

    • Low cost

    • Large coverage area - Bell system in New York City had 12 simultaneous channels for 1000 square miles

    • Small # users

    • Poor spectrum utilization

  • What are possible ways we could increase the number of channels available in a cellular system?

  • Cells labeled with the same letter use the same group of channels.

  • Cell Cluster: group of N cells using complete set of available channels

  • Many base stations, lower power, and shorter towers

  • Small coverage areas called “cells”

  • Each cell allocated a % of the total number of available channels

  • Nearby (adjacent) cells assigned different channel groups

    • to prevent interference between neighboring base stations and mobile users

  • Same frequency channels may be reused by cells a “reasonable” distance away

    • reused many times as long as interference between same channel (co-channel) cells is < acceptable level

  • As frequency reuse↑ → # possible simultaneous users↑→ # subscribers ↑→ but system cost ↑ (more towers)

  • To increase number of users without increasing radio frequency allocation, reduce cell sizes (more base stations) ↑→ # possible simultaneous users ↑

  • The cellular concept allows all mobiles to be manufactured to use the same set of freqencies

  • *** A fixed # of channels serves a large # of users by reusing channels in a coverage area ***

Ii frequency reuse planning
II. Frequency Reuse/Planning “reasonable” distance away

  • Design process of selecting & allocating channel groups of cellular base stations

  • Two competing/conflicting objectives:

    1) maximize frequency reuse in specified area

    2) minimize interference between cells

  • Cells “reasonable” distance away

    • base station antennas designed to cover specific cell area

    • hexagonal cell shape assumed for planning

      • simple model for easy analysis → circles leave gaps

      • actual cell “footprint” is amorphous (no specific shape)

        • where Tx successfully serves mobile unit

    • base station location

      • cell center → omni-directional antenna (360° coverage)

        • not necessarily in the exact center (can be up to R/4 from the ideal location)

  • cell corners “reasonable” distance away→ sectored or directional antennas on 3 corners with 120° coverage.

    • very commom

    • Note that what is defined as a “corner” is somewhat flexible → a sectored antenna covers 120° of a hexagonal cell.

    • So one can define a cell as having three antennas in the center or antennas at 3 corners.

Iii system capacity
III. System Capacity “reasonable” distance away

  • S : total # of duplex channels available for use in a given area; determined by:

    • amount of allocated spectrum

    • channel BW → modulation format and/or standard specs. (e.g. AMPS)

  • k : number of channels for each cell (k < S)

  • N : cluster size → # of cells forming cluster

  • S = k N

  • M “reasonable” distance away: # of times a cluster is replicated over a geographic coverage area

  • System Capacity = Total # Duplex Channels = C

    C = M S = M k N

    (assuming exactly MN cells will cover the area)

  • If cluster size (N) is reduced and the geographic area for each cell is kept constant:

    • The geographic area covered by each cluster is smaller, so M must ↑ to cover the entire coverage area (more clusters needed).

    • S remains constant.

    • So C ↑

    • The smallest possible value of N is desirable to maximize system capacity.

  • Cluster size “reasonable” distance awayN determines:

    • distance between co-channel cells (D)

    • level of co-channel interference

    • A mobile or base station can only tolerate so much interference from other cells using the same frequency and maintain sufficient quality.

    • large N → large D → low interference → but small M and low C !

    • Tradeoff in quality and cluster size.

    • The larger the capacity for a given geographic area, the poorer the quality.

  • Frequency reuse factor “reasonable” distance away= 1 / N

    • each frequency is reused every N cells

    • each cell assigned k ≒ S / N

  • N cells/cluster

    • connect without gaps

    • specific values are required for hexagonal geometry

      • N = i2 + i j + j2 where i, j ≧ 1

      • Typical N values → 3, 4, 7, 12; (i, j) = (1,1), (2,0), (2,1), (2,2)

Iv channel assignment strategies
IV. Channel Assignment Strategies cell

  • Goal is to minimize interference & maximize use of capacity

    • lower interference allows smaller N to be used → greater frequency reuse → larger C

  • Two main strategies: Fixed or Dynamic

  • Fixed

    • each cell allocated a pre-determined set of voice channels

      • calls within cell only served by unused cell channels

      • all channels used → blocked call → no service

    • several variations

      • MSC allows cell to borrow a VC (that is to say, a FVC/RVC pair) from an adjacent cell

      • donor cell must have an available VC to give

  • Dynamic cell

    • channels NOT allocated permanently

    • call request → goes to serving base station → goes to MSC

    • MSC allocates channel “on the fly”

      • allocation strategy considers:

        • likelihood of future call blocking in the cell

        • reuse distance (interference potential with other cells that are using the same frequency)

        • channel frequency

    • All frequencies in a market are available to be used

  • Advantage: reduces call blocking (that is to say, it increases the trunking capacity), and increases voice quality

  • Disadvantage: increases storage & computational load @ MSC

    • requires real-time data from entire network related to:

      • channel occupancy

      • traffic distribution

      • Radio Signal Strength Indications (RSSI's) from all channels

V handoff strategies
V. Handoff Strategies increases the trunking capacity), and increases voice quality

  • Handoff: when a mobile unit moves from one cell to another while a call is in progress, the MSC must transfer (handoff) the call to a new channel belonging to a new base station

    • new voice and control channel frequencies

    • very important task → often given higher priority than new call

      • It is worse to drop an in-progress call than to deny a new one

  • Minimum useable signal level increases the trunking capacity), and increases voice quality

    • lowest acceptable voice quality

    • call is dropped if below this level

    • specified by system designers

    • typical values → -90 to -100 dBm

Quick review: Decibels increases the trunking capacity), and increases voice quality

S = Signal power in Watts

Power of a signal in decibels (dBW) is Psignal = 10 log10(S)

Remember dB is used for ratios (like S/N)

dBW is used for Watts

dBm = dB for power in milliwatts = 10 log10(S x 103)

dBm = 10 log10(S) + 10 log10(103) = dBW + 30

-90 dBm = 10 log10(S x 103)

10-9 = S x 103

S = 10-12 Watts = 10-9 milliwatts

-90 dBm = -120 dBW

Signal-to-noise ratio:

N = Noise power in Watts

S/N = 10 log10(S/N) dB (unitless raio)

  • Handoff Margin △ level)

    • △ = Phandoff threshold - Pminimumusable signaldB

    • carefully selected

    • △ too large → unnecessary handoff → MSC loaded down

    • △ too small → not enough time to transfer → call dropped!

  • A dropped handoff can be caused by two factors

    • not enough time to perform handoff

      • delay by MSC in assigning handoff

      • high traffic conditions and high computational load on MSC can cause excessive delay by the MSC

    • no channels available in new cell

  • Handoff Decision level)

    • signal level decreases due to

      • signal fading → don’t handoff

      • mobile moving away from base station → handoff

    • must monitor received signal strength over a period of time → moving average

    • time allowed to complete handoff depends on mobile speed

      • large negative received signal strength (RSS) slope → high speed → quick handoff

    • statistics of the fading signal are important to making appropriate handoff decisions → Chapters 4 and 5

  • 1st Generation Cellular (Analog FM level)→ AMPS)

    • Received signal strength (RSS) of RVC measured at base station & monitored by MSC

    • A spare Rx in base station (locator Rx) monitors RSS of RVC's in neighboring cells

      • Tells Mobile Switching Center about these mobiles and their channels

    • Locator Rx can see if signal to this base station is significantly better than to the host base station

    • MSC monitors RSS from all base stations & decides on handoff

  • 2nd Generation Cellular w/ digital TDMA (GSM, IS-136) level)

    • Mobile Assisted HandOffs (MAHO)

      • important advancement

      • The mobilemeasures the RSS of the FCC’s from adjacent base stations & reports back to serving base station

      • if Rx power from new base station > Rx power from serving (current) base station by pre-determined margin for a long enough time period → handoff initiated by MSC

  • A mobile may move into a different system controlled by a different MSC

    • Called an intersystem handoff

    • What issues would be involved here?

  • Prioritizing Handoffs

    • Issue: Perceived Grade of Service (GOS) – service quality as viewed by users

      • “quality” in terms of dropped or blocked calls (not voice quality)

      • assign higher priority to handoff vs. new call request

      • a dropped call is more aggravating than an occasional blocked call

  • Guard Channels different MSC

    • % of total available cell channels exclusively set aside for handoff requests

    • makes fewer channels available for new call requests

    • a good strategy is dynamic channel allocation (not fixed)

      • adjust number of guard channels as needed by demand

      • so channels are not wasted in cells with low traffic

  • Queuing Handoff Requests different MSC

    • use time delay between handoff threshold and minimum useable signal level to place a blocked handoff request in queue

    • a handoff request can "keep trying" during that time period, instead of having a single block/no block decision

    • prioritize requests (based on mobile speed) and handoff as needed

    • calls will still be dropped if time period expires

Vi practical handoff considerations
VI. Practical Handoff Considerations different MSC

  • Problems occur because of a large range of mobile velocities

    • pedestrian vs. vehicle user

  • Small cell sizes and/or micro-cells → larger # handoffs

  • MSC load is heavy when high speed users are passed between very small cells

  • Umbrella Cells different MSC

    • Fig. 3.4, pg. 67

    • use different antenna heights and Tx power levels to provide large and small cell coverage

    • multiple antennas & Tx can be co-located at single location if necessary (saves on obtaining new tower licenses)

    • large cell → high speed traffic → fewer handoffs

    • small cell → low speed traffic

    • example areas: interstate highway passing thru urban center, office park, or nearby shopping mall

  • Cell Dragging different MSC

    • low speed user w/ line of sight to base station (very strong signal)

    • strong signal changing slowly

    • user moves into the area of an adjacent cell without handoff

    • causes interference with adjacent cells and other cells

      • Remember: handoffs help all users, not just the one which is handed off.

      • If this mobile is closer to a reused channel → interference ­ for the other user using the same frequency

      • So this mobile needs to hand off anyway, so other users benefit because that mobile stays far away from them.

  • Typical handoff parameters different MSC

    • Analog cellular (1st generation)

      • threshold margin △ ≈ 6 to 12 dB

      • total time to complete handoff ≈ 8 to 10 sec

    • Digital cellular (2nd generation)

      • total time to complete handoff ≈ 1 to 2 sec

      • lower necessary threshold margin △ ≈ 0 to 6 dB

      • enabled by mobile assisted handoff

  • benefits of small handoff time different MSC

    • greater flexibility in handling high/low speed users

    • queuing handoffs & prioritizing

    • more time to “rescue” calls needing urgent handoff

    • fewer dropped calls → GOS increased

  • can make decisions based on a wide range of metrics other than signal strength

    • such as also measure interference levels

    • can have a multidimensional algorithm for making decisions

  • Soft vs. Hard Handoffs different MSC

    • Hard handoff: different radio channels assigned when moving from cell to cell

      • all analog (AMPS) & digital TDMA systems (IS-136, GSM, etc.)

    • Many spread spectrum users share the same frequency in every cell

      • CDMA → IS-95

      • Since a mobile uses the same frequency in every cell, it can also be assigned the same code for multiple cells when it is near the boundary of multiple cells.

      • The MSC simultaneously monitors reverse link signal at several base stations

  • MSC dynamically decides which signal is best and then listens to that one

    • Soft Handoff

    • passes data from that base station on to the PSTN

  • This choice of best signal can keep changing.

  • Mobile user does nothing for handoffs except just transmit, MSC does all the work

  • Advantage unique to CDMA systems

    • As long as there are enough codes available.

Vii co channel interference
VII. Co-Channel Interference listens to that one

  • Interference is the limiting factor in performance of all cellular radio systems

  • What are the sources of interference for a mobile receiver?

  • Interference is in both

    • voice channels

    • control channels

  • Two major types of system-generated interference:

    1) Co-Channel Interference (CCI)

    2) Adjacent Channel Interference (ACI)

  • First we look at CCI listens to that one

  • Frequency Reuse

    • Many cells in a given coverage area use the same set of channel frequencies to increase system capacity (C)

    • Co-channel cells → cells that share the same set of frequencies

    • VC & CC traffic in co-channel cells is an interfering source to mobiles in Several different cells

  • Possible Solutions? listens to that one

    1) Increase base station Tx power to improve radio signal reception? __

    • this will also increase interference from co-channel cells by the same amount

    • no net improvement

      2) Separate co-channel cells by some minimum distance to provide sufficient isolation from propagation of radio signals?

    • if all cell sizes, transmit powers, and coverage patterns ≈ same → co-channel interference is independent of Tx power

  • co-channel interference depends on: listens to that one

    • R : cell radius

    • D : distance to base station of nearest co-channel cell

  • if D / R ↑ then spatial separation relative to cell coverage area ↑

    • improved isolation from co-channel RF energy

  • Q = D / R : co-channel reuse ratio

    • hexagonal cells →Q = D/R =

  • Fundamental tradeoff in cellular system design: listens to that one

    • small Q → small cluster size → more frequency reuse → larger system capacity great

    • But also: small Q → small cell separation → increased co-channel interference (CCI) → reduced voice quality → not so great

    • Tradeoff: Capacity vs. Voice Quality

  • Signal to Interference ratio listens to that one→S / I, ____________

    • S : desired signal power

    • Ii: interference power from ith co-channel cell

    • io: # of co-channel interfering cells

  • n listens to that one: path loss exponent

    • free space or line of sight (LOS) (no obstruction) →n = 2

    • urban cellular →n = 2 to 4, signal decays faster with distance away from the base station

    • having the same n throughout the coverage area means radio propagation properties are roughly the same everywhere

    • if base stations have equal Tx power and n is the same throughout coverage area (not always true) then the above equation (Eq. 3.8) can be used.

  • What determines acceptable co-channel cellsS / I ?

    • voice quality → subjective testing

    • AMPS →S / I ≧18 dB (assumes path loss exponent n = 4)

    • Solving (3.9) for N

    • Most reasonable assumption is io: # of co-channel interfering cells = 6

    • N = 7 (very common choice for AMPS)

  • Many assumptions involved in (3.9) : co-channel cells

    • same Tx power

    • hexagonal geometry

    • n same throughout area

    • Di≈ D (all interfering cells are equidistant from the base station receiver)

    • optimistic result in many cases

    • propagation tools are used to calculate S / I when assumptions aren’t valid

  • S / I co-channel cellsis usually the worst casewhen a mobile is at the cell edge

    • low signal power from its own base station & high interference power from other cells

    • more accurate approximations are necessary in those cases

N co-channel cells=7 and S / I ≈ 17 dB

  • Eq. (3.5), (3.8), and (3.9) are ( co-channel cellsS / I) for forward link only, i.e. the cochannel base Tx interfering with desired base station transmission to mobile unit

    • so this considers interference @ the mobile unit

  • What about reverse link co-channel interference?

    • less important because signals from mobile antennas (near the ground) don’t propagate as well as those from tall base station antennas

    • obstructions near ground level significantly attenuate mobile energy in direction of base station Rx

    • also weaker because mobile Tx power is variable → base stations regulate transmit power of mobiles to be no larger than necessary

  • HW1: co-channel cells

    1-9, 1-11, 1-18, 3-5, 3-7