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Mobile Computing COE 446

Wireless Multiple AccessTarek Sheltami

KFUPMCCSECOE

http://faculty.kfupm.edu.sa/coe/tarek/coe446.htm

Principles of Wireless Networks

K. Pahlavan and P. Krishnamurth

Outline

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Comparison between FDMA, TDMA and CDMA

Format complexitySystem capacityHandoffPower control

Traffic EngineeringErlang B (blockage probability)Eralng C (delay probability)

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Comparison of CDMA, TDMA and FDMA

Format Flexibility The chief advantage of CDMA relative

to TDMA is its flexibility in timing and quality of transmission

CDMA users are separated by their codes, unaffected by the transmission time relative to other users

Each user is far more liberated from other users

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Comparison of CDMA, TDMA and FDMA.. Performance of Multipath Fading:

Multipath in wireless channels causes frequency selective fading

If BW is narrowband system coincides with the location of fade, no useful signal is received

As we increase the transmission BW, fading occupy only portion of the transmission band, providing an opportunity for a wideband receiver to take advantage of the portion of the transmission band not under fade and a more reliable communication link

The wider the BW, the better is the opportunity for averaging out the faded frequency

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Comparison of CDMA, TDMA and FDMA.. Performance of Multipath Fading:

Sectored antenna and spread spectrum were not used in 1G FDMA systems, because they were analog systems

System Capacity: Depends on a number issues:

The frequency reuse factor Speech coding rate Type of antenna

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FDMA

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TDMA

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16.7 users per cell

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Comparison of CDMA, TDMA and FDMA.. Handoff:

Occurs when a received signal in an MT becomes weak and another BS can provide a stronger signal to the MT

The 1G FDMA cellular systems often used the so-called hard decision handoff in which BS controller monitors the received signal from BS and at the appropriate time switches the connection from BS to another

TDMA systems used the so-called mobile-assisted handoff in which the MT monitors the received signal from available BSs and reports it to the BS controller, which then makes the decision on the handoff

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Comparison of CDMA, TDMA and FDMA.. Handoff:

Because adjacent cells in both FDMA and TDMA are using different frequencies, the MT has to disconnect from and reconnect to the network, which will appear as a click to the user

Signals fluctuate anyway because they are arriving over radio channels

At the edge of the cells, users experience a period of poor signal quality and possibly several clicks during the completion of the handoff process

Because adjacent cells in a CDMA network use the same frequency, an MT moving from one cell to another can make seamless handoff by the use of the signal combining

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Comparison of CDMA, TDMA and FDMA.. Handoff:

When an MT approaches the boundary between cells, it communicates with both

A controller combines the signals from both links to form a better communication link

When a reliable link has been established with new BS, the mobile stops communicating with the previous BS and communication is fully established with the new BS (soft handoff)

Soft handoff provides a dual diversity for the received signal from two links, which improves the quality of reception and eliminates clicking as well as the ping-pong problem

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Comparison of CDMA, TDMA and FDMA.. Power Control:

Power control is necessary for FDMA and TDMA systems to control adjacent channel interference and mitigate the unexpected interference caused by the near-far-problem

In FDMA and TDMA some sort of power control is needed to improve the quality of voice delivered to the user

In CDMA, the capacity of the system depends directly on power control and an accurate power control mechanism is needed for proper operation of the network

CDMA systems adjust the transmitted power more often and with smaller adjustment steps to support a more refined control of power

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Comparison of CDMA, TDMA and FDMA.. Power Control:

Better power control saves on transmitted power of the MT, which increases the life of the battery

The more refined power control in CDMA system, the more power management of the MTs, which is important practical issue for users of the MTs

Implementation Complexity: Spread spectrum requires greater circuit

complexity than conventional modulation This leads to higher electronic power

consumption and larger weight and cost MTs Gradual improvement in battery and IC

technologies have made it transparent to the user

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Performance of Fix-assignment Access Methods In wired network two types of blockage:

1. When the calling number is not available2. When the telephone company out of resources

In most cellular systems:1. Blockage results in a response that is busy tone2. A message “All circuits are busy….”

We will refer to # 2 in the rest of the course Telephone service providers design their network

so that the blockage rate at the peak traffic is always below a certain percentage

Cellular operators try to keep this average below 2%

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Performance of Fix-assignment Access Methods.. The blockage rate is a function of the number of

subscribers, number of initiated calls and length of conversation

In cellular networks, the number of subscribers operation is a cell is a function of time

In downtown areas, everyone uses their cell phones during the day and in the evenings they use them in their residential area which is covered by a different cell

Traffic fluctuations in cellular telephone networks To cope with the limitations, cellular operators use

complex frequency assignment strategies to share the available resources in the optimal manner

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Traffic Engineering Using the Erlang Equations Erlang EQ. is the core in traffic engineering for

telephone application Two basic equations are used for traffic

engineering (Erlang B and Erlang C) Erlang B relates to the probability of blockage B(N,

ρ) to the number of channels N, and the normalized call density in unit channels ρ

Where ρ = λ/μ, λ is Poisson arrival rate and μ is the service rate of the calls

N

Ni

i

/ N!B(N, )

( / i!)

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We want to provide a wireless public phone service with five lines to a ferry crossing between islands carrying 100 passengers where on average each user makes a three-minute telephone call every two hours. What is the probability of a passenger approaching the telephones and none of the five lines are available? Is it acceptable for cellular network?

1* 100*1 5 1/ ,

120 6 3

numberof users numbercallsErland min min

total duration

Example 1 (Capacity using Erlang B formula)

2.5Erlang

55

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0.02

2.5

B(5, 2.5)=0.07

3

: ( , ) 0.07,From the chart B N which is unacceptable for cellular networks

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An IS-136 cellular phone owns 50 cell sites and 19 traffic per cell each with bandwidth of 30 kHz. Assuming each user makes three calls per hour and the average holding time per call is 5 minutes, determine the total number of subscribers that the service provider can support with a blocking probability of 2%.

**

45*60/ 180 /

3*5

numberof users numbercallsholding time

total duration

number of users cell subscriber cell

Example 2 (Capacity using Erlang B formula)

50*180 9000Number of subscribers for the service provider users

( , ) 0.02, 19*3 57 45for B N N channels Erlang

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0.02

55

N=57

45

Back

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Traffic Engineering Using the Erlang Equations.. The Erlang C is formula results in waiting time in a

queue if a cell does not get through, but it is buffered until the channel is available

The probability that a call gets delayed:

Because of the complexity of the calculation, tables and graphs are used to provide values for the probability based on normalized values of ρ

N

kN 1N

k 0

P(delay 0)N!(1 )

N k!

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Traffic Engineering Using the Erlang Equations.. The probability of having a delay that is more than a

time t is given by:

This indicates the exponential of the delay time The average delay is give by the average of the

exponential distribution:

(N ) tP[delay t] P[delay 0]e (1)

1D P[delay 0] (2)

(N )

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For the ferry described in example#1, solve the following questions:a)What is the average delay for a passenger to get access to the telephone?b)What is the probability of having a passenger waiting more than a minute to access the telephone?

Example (Call Delay using Erlang C formula)

1D P[delay 0] 0.17 min utes

(N )

(N ) t, ( t 1)P[delay t] P[delay 0]e 0.0565

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2.5

0.13

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