cdma and fdma

Seminar Seminar on on

CDMA and FDMACDMA and FDMA

By:By: sradha guptasradha gupta Branch: eteBranch: ete Regd no:0701216249Regd no:0701216249 College: P.c.eCollege: P.c.e

Multiple AccessMultiple Access

Base station

mobile1

mobile2

mobile3

ChannelizationChannelization

It is a multiple-access method in It is a multiple-access method in which the available bandwidth of a which the available bandwidth of a link is shared in time,frequency or link is shared in time,frequency or through code between different through code between different stations.stations.

There are three channelization There are three channelization protocols:protocols:

1)FDMA1)FDMA 2)CDMA2)CDMA 3)TDMA3)TDMA

For wireless communications, For wireless communications, multiple access can be carried out multiple access can be carried out in 4 dimensions:in 4 dimensions:

1) space1) space 2)time2)time 3)frequency3)frequency 4) code4) code The goal is to assign a channel The goal is to assign a channel

with minimum interference.with minimum interference.

Freq. Division Multiple Access Freq. Division Multiple Access (FDMA)(FDMA)

Dividing the Dividing the available available frequency spectrumfrequency spectrum into into some frequency some frequency channels.channels.

Each channel occupies a Each channel occupies a portion of total available portion of total available bandwidth and is given to bandwidth and is given to a single user.a single user.

Multiple users using Multiple users using separate frequency separate frequency channels could access channels could access the same system.the same system.


Advantages:Advantages:

If channel is not in use, it sits idle.If channel is not in use, it sits idle. Channel bandwidth is relatively narrow (30kHz)Channel bandwidth is relatively narrow (30kHz) Simple algorithms.Simple algorithms. Fairly efficient when the number of stations is small and Fairly efficient when the number of stations is small and

the traffic is uniformly constant.the traffic is uniformly constant. Capacity increase can be obtained by reducing the Capacity increase can be obtained by reducing the

information bit rate and using efficient digital codeinformation bit rate and using efficient digital code No need for network timing.No need for network timing. No restriction regarding the type of baseband or type of No restriction regarding the type of baseband or type of

modulation.modulation.


Disadvantages:Disadvantages:

The presence of guard bands.The presence of guard bands. Requires right RF filtering to minimize adjacent Requires right RF filtering to minimize adjacent

channel interference.channel interference. Maximum bit rate per channel is fixed. Maximum bit rate per channel is fixed. Small inhibiting flexibility in bit rate capability.Small inhibiting flexibility in bit rate capability. Does not differ significantly from analog system.Does not differ significantly from analog system.

Code Division Multiple Access Code Division Multiple Access (CDMA)(CDMA)

In both FDMA and TDMA techniques the In both FDMA and TDMA techniques the number of channels or time slots is fixed.number of channels or time slots is fixed.By increasing the number of services from By increasing the number of services from

simple voice to more burst data simple voice to more burst data transmissions, it lack of efficiency in utilizing transmissions, it lack of efficiency in utilizing the scarce spectrum.the scarce spectrum.

Hence, CDMA schemes started to come into Hence, CDMA schemes started to come into commercial systems.commercial systems.

Code Division Multiple Access Code Division Multiple Access (CDMA)(CDMA)

In a CDMA systemIn a CDMA system…… Each user is assigned a Each user is assigned a

spreading codespreading code.. UserUser’’s information is s information is

spread into a much wider spread into a much wider spectrum using a high spectrum using a high clock (clock (chipchip) rate.) rate.

It is possible to send It is possible to send multiple usersmultiple users’’ information information on the same frequency on the same frequency spectrum.spectrum.

CDMA is an MA scheme CDMA is an MA scheme that has no fixed that has no fixed maximum number of maximum number of users. (in theory)users. (in theory)

Spreading Codes

The spreading code Ck must be unique for each user. Ideally, they are orthogonal to one another, i.e. <Ci, Ck > = 0, unless i = k <Ci, Ck > = J, if i = k Example: Walsh Codes For a spreading factor J=4,there are 4 Walsh codes In general there are always JWalsh codes, as long as J = 2, 4, 8, 16, 32, 64, 128, ...

CDMA MultiplexerCDMA Multiplexer+1,+1,+1,+1

-1,-1,-1,-1

+1,-1,+1,-1

-1,+1,-1,+1

+1,+1,-1,-1

0,0,0,0

-1,-1,-1,+1

+1,-1,-1,+1

-1,,-1,-3,+1

-1

-1

0

+1

Station1 bit0

Station2 bit0

Station3 silent

Station4 bit1

A

B

C

D

Spread Spectrum Multiple Access Spread Spectrum Multiple Access TechnologiesTechnologies

Advantages of CDMAAdvantages of CDMA Many users of CDMA use the same frequency.Many users of CDMA use the same frequency. No absolute limit on the number of users.No absolute limit on the number of users. Easy addition of more users.Easy addition of more users. Multipath fading may be substantially reduced because of large Multipath fading may be substantially reduced because of large

signal bandwidth.signal bandwidth. Impossible for hackers to decipher the code sentImpossible for hackers to decipher the code sent Better signal quality.Better signal quality. Flexible network planning (planning is no longer needed).Flexible network planning (planning is no longer needed). Greater coverage (larger area for a given amount of power ).Greater coverage (larger area for a given amount of power ). High capacity (greater coverage capacity).High capacity (greater coverage capacity). Cost (larger profit for providers due to increased capacity, less Cost (larger profit for providers due to increased capacity, less

infrastructure).infrastructure). Customer satisfaction (privacy, better call quality longer battery life Customer satisfaction (privacy, better call quality longer battery life

due to less power consumption, prevent cross talks).due to less power consumption, prevent cross talks). Compatibility (dual mode analog and digital).Compatibility (dual mode analog and digital).

Disadvantages of using CDMADisadvantages of using CDMA

SynchronizationSynchronizationDifficulty to satisfy synchronization requirements.Difficulty to satisfy synchronization requirements.

Self jammingSelf jammingSelf jamming is a steep deterioration of performance as a result of Self jamming is a steep deterioration of performance as a result of poor synchronization. Poor synchronization causes partial-poor synchronization. Poor synchronization causes partial-correlation with the codes of other users and the result will be a vast correlation with the codes of other users and the result will be a vast increase of the interference.increase of the interference.

Near-far problemNear-far problem

power control is necessary for mitigating the Near-far problem. power control is necessary for mitigating the Near-far problem. There are some factors for imperfect power control such as: There are some factors for imperfect power control such as: feedback delays, imperfect power estimates, traffic conditions, feedback delays, imperfect power estimates, traffic conditions, errors in the feedback channel.errors in the feedback channel.

Network complexityNetwork complexity Complex network support is needed for implementing soft handoff, Complex network support is needed for implementing soft handoff,

and also for countering multipath and fading effects.and also for countering multipath and fading effects. ThroughputThroughput

Low throughput efficiency for large number of users.Low throughput efficiency for large number of users.

Difference between Difference between CDMA and FDMACDMA and FDMA

Interference Averagingof CDMA and FDMA

• •

•CDMA is better suited for multipath fading channels as additional paths “combined” to result in path diversity.In FDMA/TDMA, additional paths not able to be combined and results in link degradation. • CDMA reuses same frequencies in adjacent cells (unlike TMDA/FDMA).

•Soft Handoff gains Resistant to narrowband interference CDMA inherently spread spectrum.

• Inactivity Gain (such as pauses in speech) can be translated to capacity gain. This is not possible with FDMA and difficult with TDMA.

Voice Activitycomparison of CDMA and FDMA

In TDMA and FDMA systems:In TDMA and FDMA systems: If a user doesn’t have anything to send, the If a user doesn’t have anything to send, the

time/frequency slot allocated to them is wasted.time/frequency slot allocated to them is wasted. It is typically very difficult to dynamically allocate time It is typically very difficult to dynamically allocate time

and frequency slots.and frequency slots.

In CDMA systems:In CDMA systems: If a user doesn’t have anything to send, it causes less If a user doesn’t have anything to send, it causes less

interference to other users of the system.interference to other users of the system. Typically, each user needs to transmit less than half the Typically, each user needs to transmit less than half the

timetime Since interference-limited, this doubles the capacity.Since interference-limited, this doubles the capacity.

Sectorized Antennas

• • Cells can use directional Cells can use directional antennas to “sectorize” the antennas to “sectorize” the cell.cell.

• • At 120 degree antennas At 120 degree antennas create 3-sector cells – very create 3-sector cells – very Common.Common.

• • For CDMA, this reduces the For CDMA, this reduces the interference by a factor of interference by a factor of three. Capacity is increased three. Capacity is increased by a factor of three.by a factor of three.

• • FDMA/TDMA also use FDMA/TDMA also use sectored antennas, but just sectored antennas, but just to decrease the distance.to decrease the distance.

• • Comparing the capacity of TDMA/FDMA/CDMA is very Comparing the capacity of TDMA/FDMA/CDMA is very controversial.controversial.

• • In 1991, a famous Qualcomm paper claimed that due to In 1991, a famous Qualcomm paper claimed that due to voice activity, frequency reuse,and sectorization, CDMA voice activity, frequency reuse,and sectorization, CDMA has increased capacity by:has increased capacity by:

Factor of 18 relative to AMPSFactor of 18 relative to AMPS Factor of 6 relative to US TDMA (and similar for GSM)Factor of 6 relative to US TDMA (and similar for GSM)• • This turned out to be optimistic, about 1/3 of this gain This turned out to be optimistic, about 1/3 of this gain

actually happened .actually happened .• • All 3G systems use CDMA for multiple access.All 3G systems use CDMA for multiple access.

Capacity Comparisonof CDMA and FDMA

conclusionconclusion From the comparisons made above we cannot say that

FDMA is better than CDMA or vice versa. The main advantage of the CDMA is that, in the single detection method it is more flexible than FDMA/TDMA or joint detection. CDMA is said to have higher capacity than TDMA. But in the future GSM can be extended by an optional CDMA component in order to further increase the capacity. Finally, it does not matter whether which one is better CDMA or TDMA right now. It can be only found out with the evolution of these technologies.

When going for a cell phone the user should choose the technology according to where they use it. For users who travel abroad it is better to go with GSM handsets. For the users in United States CDMA is better than TDMA because of the coverage we can get at rural areas where digital signals cannot be transmitted.

cdma and fdma

Documents