Cellular Concepts and Design Fundamentals Rappaport

Cellular Concepts and Design Fundamentals

Rappaport: Chapter 1 & 2 &3Lin: Chapter 1

Wireless Information Networking Group (WING)

Introduction

• Earliest wireless communications system: as early as the occurrence of vocal interaction--the very existence of living creature

• Example: – Given more than one communication entity, build a

communication system among them

– Need something M meaningful to all (signals)

– Need something C to carry M (transmission media)

– Any one can understand what is being delivered (protocols)


Wireless • Transmission media

– Light (e.g., smoke, flashlight, infrared)

– Radio (e.g., AM, FM, microwave, …)

– Sound (our daily routine)

• Transceiver--Transmitter and Receiver (e.g., mouth and ear)

• Power is limited: communication range is then limited

• Bandwidth is limited: capacity is bounded

• Media is noisy (due to fading or interference): need more processing


Wireless Applications• Broadcast: AM & FM radio, TV • Point-to-Point: Garage door opener, remote

control for entertainment equipment, Walkie-Talkie, remote controlled toys, cordless phone

• Point-to-multi-point: pager (beeper) • Applications with more functionality

– paging– cordless phones: home use, public places – cellular phones: voice only, voice with some data,

multimedia (voice, data with flexible rate, auido & video, images, …)

– iPhones, iPad, …


Mobile Radiotelephone in U.S. • First natural application was phone service • Bell Telephone Lab developed AMPS (Advanced

Mobile Phone Systems): frequency as channel – uplink: 824-849 MHz (BW=25 MHz)– downlink: 869-894 Mhz (BW=25 MHz)

• IS-54/GSM (TDMA based 2G): time slot as channel • IS-95 (CDMA-based 2G): code as channel• Personal Communications Systems (PCS): the PCS

band: 2GHz band and 2.4GHz band


Mobile Radiotelephone in World

• 1G – E-TACS (UK): 900MHz

– NMT-450 (Nordic countries): 450-470MHz

– NMT-900

– C-450 (Germany)

– JTACS (Japan) : 900MHz

– NTT: 400/800 MHz

• Everybody followed U.S. – USA was indeed the leader!


Mobile Radiotelephone in World • 2G

– GSM: the European standard, took the leadership from US

– DCS-1800: PCS counterpart – PDC: Japan --> famous DoCoMo I-mode

• Cordless: CT, CT-2, PHS, DECT • European dominated!• 2.5G: GPRS and EDGE• 3G has the similar infrastructure except some new

technologies in improving data rate, service integration, inter-operability

• 4G: originally integration of cellular and wi-fi, then integration and harmonization data rate


Mobile Cellular Radio Systems• Terminology

– mobile: any radio terminal that could be moved during operation. In most literature, the following can be interchangeably used: mobile host, mobile terminal, portable, subscriber or subscriber unit, users or mobiles

– base station: a fixed transmitting and receiving facility connected to commercial power source and fixed backbone network (such as Public Switched Telephone Network --PSTN)

– MSC: Mobile Switching Center, the router, the switching facility connected to PSTN and base stations


Mobile Cellular Radio Systems

•


Cellular Infrastructure

• Service coverage is divided into smaller area, called cell

• Each cell is equipped with fixed transmitting and receiving devices (transceivers), called base station (BS)

• Each base station is connected a switching center, called mobile switching center (MSC)

• Each MSC is directly connected to public infrastructure (such as PSTN)


Base Station

• CDMA (MSC)


MSC

• CDMA (Nortel)


PCS Architecture


Call Origination (Call Out)• A mobile station (MS) contact a BS which is strongest in

power (serving base station)

• MS transmits its Mobile Identification Number (MIN), its Electronic Serial Number (ESN) and the called phone number

• BS sends these data to the MSC via wireline connection

• MSC finds the terminating switch (EO for wired or MSC for wireless) of the called party via connection finding operation in PSTN

• Terminating switch finds the called party via paging

• Connection is established


Call Termination (Call In)

• Caller’s terminating switch finds the MSC of the callee using MIN,ESN,callee’s phone number

• Upon receiving these numbers, MSC will page its charging area (send paging message to all BSs) and BSs will send a message on their paging channels for the callee

• The MS being paged will respond to BS, then MSC

• Connection is then established


Cellular Concept• Beauty of Nature: one’s meat is another poison! • What if there is no power degradation for a

transmitted signal? Impossible to communicate! • Transmission range is LIMITED: the possibility

of cellular concept! • Examples:

– Daily conversation: by lowering voice, a room could accommodate more simultaneous conversations

– AM & FM radios, TV: radios or TV at different places far apart can provide different programs on the same channel


Power Propagation


Signal Detection

• A signal at base station in a cell can be detected and recovered if SIR >= SIR_th


Frequency Reuse

• Idea: the same frequency channels can be reused in cells far apart (so long as the interference within the tolerable margin)

• Cellular structure: overall frequency spectrum are sliced into channels, neighbouring cells or cells “close to’’ each others are using different channels, which do not interfere with each other, each base station is equipped with a group of distinct channels

• Frequency reuse or frequency planning or frequency allocation


Frequency Reuse

• Footprint: the actual coverage of a base station

• Cell shape: irregular in nature due to geography, propagation environment, transmitting equipment, … triangle, square, circle etc. For analysis, hexagon is used

• Position of base station: (hexagonal layout) – Center: omni-directional (center-excited cell)

– On three of the six cell vertices: directional (edge-excited cell)


Hexagonal Layout

• Seven-cell frequency reuse scheme

•


Frequency Planning • Consider S total duplex channels in the system,

which are divided into N groups, each group of k channels is allocated to a cell – S = k N

• Cluster: N cells which collectively use the complete set of available frequency channels

• If a service area has M replications of cluster of N cells, the total number of duplex channels which can be used in the whole system: – C = M (kN) = MS

i.e., each channel will be reused M times


Frequency Planning • Frequency reuse factor: 1/N

• Cluster size: N -- N-cell reuse

• N only assumes certain values:

• Interpretation: To find the co-channel cells, move i cells along any chain of hexagons, then turn 60 degree counter-clockwise and move j cells


Finding Co-channels

•


Example

• Spectrum: 33 MHz, use FDD, 25 kHz simplex channel (50 kHz duplex channel). Compute the number of channels available per cell if a system uses (a) 4-cell reuse, (b) 7-cell reuse, (c) 12-cell reuse.

• Solution: S=33 MHz/50 kHz=660, k=S/N so – (a). 660/4=165

– (b). 660/7=95

– (c). 660/12=55


Design Issues

• Channel assignment• Mobility management

– Location management– Handoff strategies

• Call admission control (Resource allocation) • Interference and system capacity

– Power control

• Trunking and grade of service (GoS) • Capacity improvement

– Cell splitting – Sectorization – Power control


Channel Assignment

• Fixed – Each cell is allocated a predetermined set of voice

channels

– Channel borrowing schemes supervised by MSC

• Dynamic– Channels are not allocated to different cells

permanently, each call requests channels from MSC

– Require the MSC to collect real-time data on • channel occupancy

• traffic distribution

• RSSI: radio signal strength indications of all channels

• mobile speed and direction


Handoff

• Handover (European usage)

• Definition: a process of transfer one base station or a channel to another

• Necessity: when a mobile moves from a cell to another, power from serving base station in the old cell may become weak, the base station in the new cell has stronger power in serving the call – Identifying the new serving base station

– Voice and control signaling at the new BS


Handoff

• Decide when to hand off: too early may lead to too many handoffs, too late may lead to call dropping

• Measurement of received signals: may use the difference between the received signal power and the minimum required signal power

• Handoff area: the boundary area between cells, where handoff may be necessary

• Moving speed: useful in handoff decision

• Cell residence time (dwell time): the time spent by a mobile in a cell, useful for GoS design


Handoff


Handoff Strategies • Mobile-initiated handoff strategies

– mobile makes a handoff decision based on its power measurement

• Network-initiated – BS monitors the signal power on the reverse voice

channels – Locator receiver is controlled by MSC and monitors

the signal strengths of mobiles in neighboring cells – Handoff decision is made by MSC

• Combined handoff schemes – Mobile-Assisted Handoff (MAHO) – Inter-system handoff


Handoff Strategies • Handoff types

– hard handoff: a call served by one BS at any time

– soft handoff: can be simultaneously served by multiple BSs

• Hierarchical handoff strategy – microcell and macrocell concept: PCS cell as microcell

while AMPS (high tower BS) as macrocell, satellite as macrocell etc

– slower mobile is served by microcell while fast mobile is served by macrocell---reduce handoff rate

– handoff prediction may be useful!


Call Admission Control (CAC) and Handoff Prioritization

• Decide whether a new call is accepted and how handoff calls are handled

• Guard channel scheme: a number of channels is set aside for handoff calls – new calls are accepted only when the number of busy

channels is less than a threshold

• Queueing priority schemes– queueing handoff requests while blocking new calls

• Objective: minimizes the call dropping while keeps the call blocking under control


Interference

• Adjacent channel interference – Out-of-band user interference

– Receiver imperfection

• Co-channel interference – Frequency reuse leads to co-channel interference

– CDMA uses the same frequency band (channel)!

– Downlink interference is more serious problem

– Major bottleneck in increasing system capacity

– May lead to dropped calls

– Co-channel cells: the ones using the same channel


System Capacity

• Cellular system is interference-limited: increasing one’s transmitting power may increase interference to others (e.g., at party)

• System capacity: maximum total number of customers can be supported in the whole system (will be clear in the future lecture)

• Limitation on system capacity: – interference!

– minimum SIR: a minimum required SIR for reasonable voice conversation


System Capacity

• Simplified analysis – Assume the same cell size, same transmitting power

from each BS

– R: cell radius, D: frequency reuse distance (the distance between the centers of co-channel cells)

– Co-channel reuse ratio Q=D/R, where


System Capacity

• Let i0 denote the number of co-channel interfering cell


System Capacity

• Smaller value of Q provides larger capacity

• Larger value of Q improves transmission quality (less interference)

• Tradeoff is needed!

• Example 3.2 in Rappaport’s book


Power Control

• In TDMA, co-channel interference is controlled by power control – MS power is also controlled for ongoing calls

– BS interference seems to be more severe!

• In CDMA, MS transmission powers are controlled: all mobile use the same channel – need to control the interference

– need to control the near-far effect: nobody should be power-dominating


Trunking

• Trunking: a concept from POTS, a kind of multiplexing or resource sharing, a mathod allowing a large number of users to share a relatively small number of channels

• In cellular systems, channels at BS are shared by any user in the cell on a per call basis: a user is granted a channel (if available) upon request, after the call termination, the channel will be returned to the channel pool at BS

• Will use the statistical behavior of mobile users

• Queueing may be used for requests


Grade of Service (GoS)

• A Quality of Service (QoS) parameter: for voice calls only

• Trunking theory (and Queueing Theory)

• GoS: a measure of the ability of user to access a trunked system during the busiest hour. It is typically given as the blocking probability or the probability of a call experiencing a delay greater than a certain tolerable queueing time

• Will be discussed in more detail in future


Grade of Service (GoS)

• Blocking probability (blocked-call-clear policy)


GoS

• GoS with queueing policy


GoS

• Trunking efficiency: a measure of number of users which can be offered a particular GoS with a particular configuration of fixed channels, which can be found by the ratio of total traffic supported with the GoS to total number of channels

• Examples: (in Rappaport) – Example 3.4

– Example 3.6

• Self-study: Example 3.5, 3.7


Capacity Improvements

• Cell-splitting: raising your voice does not help, but lowering your voice does

• Sectoring: directing your voice also helps (use your hand when you talk to your neighbors)


Cell Splitting

• Cell splitting: a process of subdividing a congested area into smaller cells, each with its own BS of lower antenna and lower transmitting powermicrocells

• PCS cells can be regarded as the consequence of AMPS cell splitting – MS shrinks in size

– less interference

– lower power consumption

• Rescaling the system: decreasing R and keeping the Q unchanged


Cell Splitting

• BSs on the corners

• Antenna downtilting


Sectoring

• Keeping cell radius R unchanged and decreasing frequency reuse factor Q=D/R or reducing the number of interfering cells (co-channel cells)

• Using directional antenna! --sectorization


Sectorization


Sectorization

• The number of interfering BS will decrease: 120 degree sectoring reduces from 6 to 2

• SIR can be increased significantly

• Disadvantage: no pain, no gain – handoff rate increases: sector to sector

– the number of antennas increases

– trunking efficiency decreases

• Adaptive sectorization is possible: adapt to change of traffic


Reading

• Chapter 1, 2 and 3 in Rappaport’s book

• Websites of the big players: Nortel, Qualcomm, Nokia, Ericsson, Sprint PCS, AT&T, Motorola,MCI /Worldcom etc

• Search for 3GPP and 3GPP2 for standard harmonization

Cellular Concepts and Design Fundamentals Rappaport

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