COMM 907: Spread Spectrum Communications

Lecture 4 3-Cellular Code Division Multiple Access

Idea of CDMA Transmitter for user #1

Receiver for user #1

Transmitter for user #n

Idea of CDMA (Cont.) • In a CDMA system different users can utilize the same time and frequency channel to transmit data. • The capacity of this system depends on the amount of interference from the other users since the fundamental problem of CDMA is that, each user causes multiple access interference (MAI) affecting all the other users. • The receiver then uses a correlator to “despread” the wanted signal, which is passed through a narrow bandpass filter, while unwanted signals will not be “despread” and will not pass through the filter.

Remember

FDMA, TDMA and CDMA

(a) FDMA, (b) TDMA, and (c) CDMA

• In FDMA the frequency band is divided such that each user has its own band, with guard band between each two users. • The disadvantage of this system is the limitation of the users due to the limitation in the frequency band. • The advantage of this system is that all the channel is available for all the users all the time.

TDMA Time division multiple access (TDMA) allows several users to share the same frequency channel by dividing the signal into different time slots. The users transmit in rapid succession, one after the other, each using its own time slot. This allows multiple stations to share the same transmission medium (e.g. radio frequency channel). TDMA is used in the digital cellular systems such as Global System for Mobile Communications (GSM) TDMA is a type of time division multiplexing, with the special point that instead of having one transmitter connected to one receiver, there are multiple transmitters.

The disadvantage of this system is the delay caused especially in case of large number of users. The advantage of this system is: multiple users share the same radio frequency channel

TDMA

FDMA, TDMA and CDMA (Cont.)

In CDMA all the frequency band and all the time domain are available for all the users. This system overcomes the disadvantages of the other two systems. CDMA has improved capacity over FDMA and TDMA.

Benefits of Cellular CDMA System The Cellular CDMA System spread spectrum technology has several benefits such as:

(1) Avoiding interception: They are difficult to intercept by an

unauthorized person i.e. it has low probability of intercept. Spreading the transmitted power over a wide band, lowers the power spectral density and then hides the transmitted information in the background noise.

(2) Privacy of transmission: the transmitted information over the

spread spectrum system can not be recovered without knowledge of the spreading code sequence. Thus the privacy of individual communications is protected in presence of other users.

Benefits of Cellular CDMA System (3)

Resistance to fading: Fading causes the receiver gets multiple copies of the transmitted signal. These signal components often interfere with each other. CDMA system provides reduction of the effect of these interference because the DSSSS is immune against interference.

(4)

Random Access possibilities: Users can start their transmission at any arbitrary time.

(5) Capacity: The capacity of CDMA systems is good compared to other systems such as FDMA, TDMA.

(6) Frequency Reuse

Cellular CDMA System •

Spread-spectrum was proposed for original cellular telephone standard (Cooper 1978).

•

QUALCOMM has proposed a spread-spectrum standard for digital cellular telephone:

•

With powerful coding, single cell CDMA can support as many users as FDMA or TDMA:

CDMA can support 12-16 times as many users than FDMA or TDMA. Gilhousen, et al., “On the capacity of a cellular CDMA system,” IEEE Trans. on Vehicular Technology, May1991

Frequency Allocation in FDMA & TDMA • In FDMA or TDMA, radio resource is allocated not to interfere among neighbor cells. • Neighbor cells cannot use the same (identical) frequency band. • The figure shows the simple cell allocation with seven bands of frequency. In actual situation, because of complicated radio propagation and irregular cell allocation, it is not easy to allocate frequency appropriately.

Frequency Allocation in CDMA • In CDMA, identical radio resource can be used among all cells, because CDMA channels use same frequency simultaneously. - Frequency allocation in CDMA is not necessary. - In this sense, CDMA cellular system is easy to be designed.

Near-Far Problem • A phenomenon that degrades the service quality of a wireless network is the Near–Far effect which occurs when the interference from a user transmitting near the base station overpowers the weaker signal received from a distant user. This is known as the Near–Far (N-F) effect.

Base Receivers have much more power from Y than from X. This leads to desired signal power is smaller than the interfered power.

• Near–Far (N-F) effect tends to degrade system capacity.

Solution: Power Control: Each user at the base station has the same power

How to Combat Near-Far Problem? • In order to combat the N-F effect, cellular wireless systems must control the transmitted power from each mobile unit as well as from the base station. • Furthermore, an efficient power control scheme will also reduce the multiple access interference and then optimize system capacity. • To account for the rapid changes in path loss and user movement, power control schemes have to be adaptive but fast and accurate. Remember Path loss (or path attenuation) is the reduction in power density (attenuation) of an electromagnetic wave as it propagates through space

Power Control Schemes •

There are two methods to manage user transmit power in the up link. These methods are based on:

(1) (2)

open-loop and closed-loop control schemes.

•

In the open-loop power control method, the mobile unit uses the strength of the received pilot on the down link to determine what adjustment of the transmitter power on the up link is needed to achieve the required power at the base station receiver.

•

The mobile controls its transmitter power by applying Automatic Gain Control (AGC) measurements.

•

The weakness of this design is that the down and up links may be subject to different levels of path loss.

Power Control Schemes •

In the closed-loop power control method, the base station provides each mobile unit with continuous information about the power of their signal to enable frequent adjustments of up-link power.

•

The power received on the up-link at the base station is compared to a desired level and the power difference is used to obtain a power command bit that is transmitted to the user on the down link.

•

The power command changes the user transmitter power by a fixed step.

•

To reduce delay in the power control, the command bit is transmitted on the down link unprotected, so it is exposed to channel errors.

Practical Effect of Near-Far Problem • Example: • IS-95 CDMA system employs both open loop and closed loop power control.

Handoff When?? • When the phone is moving away from the area covered by one cell and entering the area covered by another cell, the call is transferred to the second cell in order to avoid call termination when the phone gets outside the range of the first cell; • When the capacity for connecting new calls of a given cell (cell1)is used up and an existing or new call from a phone, which is located in an area overlapped by another cell (cell2), is transferred to that cell (cell2) in order to free-up some capacity in the first cell (cell1) for other users who can only be connected to that cell (cell1).

• Handoff in CDMA

Hard Handoff Soft Handoff

(1) Hard Handoff • Hard handoff occurs when the air-link connection between the mobile and its initially-serving base station are momentarily disconnected before reconnecting with a new base station.

(2) Soft Handoff (unique feature of CDMA Mobile) • Soft handoff, in which two base stations are briefly simultaneously connected via the air-link with a mobile during the handoff. • As soon as the mobile's RF link with the new base station is acceptable, the initially-serving base station is disconnected from the mobile.

• Same frequency assignment between old and new BS • Diversity techniques are employed at both ends of the radio link to ensure a smooth handoff, which is largely undetectable to the humans affected.

Soft Handoff- A unique feature of CDMA Mobile Advantages: • Contact with new base station is made before the call is switched. • Diversity combining is used between multiple cell sites: – additional resistance to fading – Enhancing the performance

• Neither the mobile nor the base station is required to change frequency 21

CDMA Wireless Communication Standard •

IS-95 CDMA:

• IS-95 : is a digital scheme uses code division multiple access. The current mobile technologies, the wideband CDMA and the CDMA 2000 standard are the natural migrants from IS-95 system technologies. • It uses Direct Sequence Spread Spectrum Signaling on up and down Links Up Link

Down Link

45 MHz 847.74 MHz •

Each channel occupies 1.25 MHz

•

Fixed chip rate 1.2288 Mcps

892.74 MHz

IS - 95 • Base stations are synchronized • Forward link and reverse link each occupy 1.25 MHz • Separation between the two links in 45 MHz • A long code system

• Chip rate - 1.2288 Mcps (128 times the maximum data rate 9.6 Kbps) • Variable data rate - depends on voice activity : 1.2, 2.4, 4.8

or 9.6 Kbps • Power control - 800 times per second on the reverse link. • Convolutional code for error correction

• Also known as cdma one.

Down Link in (IS – 95) : • Pilot Channel

- Provides phase reference for coherent demodulation - Signal strength measurement for handoffs - Unmodulated (all 0s) - Walsh code No. 0

• Sync Channel - Broadcasts system timing messages - Walsh code No. 32 • Paging Channel - 7 Channels (Walsh code No. 1 - 7) - for paging and control messages • Traffic Channel

- 55 channels (Walsh code No. 8-31, 33-63)

Revision on Diversity

Diversity Techniques • Diversity techniques are considered one of most powerful techniques to overcome the fading effect. • The basic concept of diversity is to transmit the signal via several independent diversity branches to get independent signal copies.

• The main types of diversity techniques: - frequency diversity - time diversity - space diversity 6

Frequency Diversity

• The same information is transmitted on more than one carrier frequency. • Each carrier should be separated from the others by at least the coherence bandwidth of the channel so that different copies of the signal have independent fading

Time Diversity

The same information is transmitted on two or more different times. At a certain time, the received signal might be in a deep fade while at a later time it is not. 28

Space Diversity

Transmitter

Use more than one antenna to receive the signal.

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Receiver

Selection combining r1 (t ) r2 (t )

rL (t )

1

2

Determine the branch with the maximum instantaneous SNR

 l  A Eb N 0 30

Receiver

L

rl (t )  l s(t )  wl (t ) ; 2 l

s(t )

y (t )

l  1, 2,...,L

is the instantaneous SNR of the l-th branch

y(t )  A e j s(t )  w(t )

where

A  max A1 , A2 , ..., AL 

Maximal ratio combiner r1 (t )

branch 1

A1e  j1

r2 (t )

branch 2

s(t )

y (t ) +

Receiver

A2 e  j2

rL (t )

branch L

AL e

L

y (t )   Al e  jl rl (t )

 jL

l 1

L

  Al2 s(t )  N m L

where N m   Al e 31

l 1

 jl

l 1

wl (t )

Equal gain combiner (EGC) r1 (t )

branch 1

e  j1

r2 (t )

branch 2

s(t )

y (t ) +

Receiver

e  j2

rL (t )

branch L L

y (t )   e  jl rl (t )

e  jL

l 1

L

L

where 32

Ne   e l 1

  Al s(t )  N e  jl

l 1

wl (t )

Generalized selection combining (GSC) - Selects the best K diversity branches out of the total number of branches depending on the SNR. -Combines the selected branches using MRC.

-Provides a tradeoff between SC and MRC.

Conventional GSC

r1 (t )

r2 (t ) rL (t )

Order the instantan eous signal to noise ratios of the branches

branch 1

Select the first K branches

A1e  j1

s(t )

y (t ) Receiver

A2 e  j2

 1   2  ..   L

branch K

AK e  jK

- In the conventional GSC schemes the number of combined diversity branches is fixed. - Combining a fixed number of branches has a shortcoming: - Strong branches could be discarded. - Weak branches could be combined. - To overcome this shortcoming we use Threshold GSC Schemes.

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Threshold GSC Schemes (T-GSC) In threshold GSC schemes (T-GSC), the instantaneous SNRs of all branches are compared with a fixed threshold. The branches that exceed the threshold are combined and the rest of branches are discarded. Examples: 1- Absolute threshold GSC (AT-GSC) 2-Normalized threshold GSC (NT-GSC) 18

1-Absolute threshold GSC (AT-GSC) r1 (t ) Threshold comparison

branch 1

 1   th

r2 (t ) Threshold comparison  2   th

Select the branches that exceed the threshold

A1e  j1

s (t )

y (t )

Receiver A2 e  j2

rL (t )

Threshold comparison  L   th

branch K

AK e  jK

The output SNR: L

 t    l l 1

where

 l ,   l   0 

 l   th 0   l   th

2-Normalized threshold GSC (NT-GSC) • Each branch SNR is normalized with respect to the maximum SNR and compared with a fixed threshold value between 0 and 1

 th  th  max, 0  th  1  max  max l  l Where

 th

is the branch threshold

 th is a fixed fraction

Performance of different diversity techniques 0

10

MRC EGC SC AT-GSC NT-GSC ST-GSC

-1

10

-2

Pe

10

-3

10

-4

10

-5

10

-15

-10

-5

0 SNR (dB)

5

10

15

Probability of Error versus SNR for ideal channel

39

•

40

T-GSC schemes use fixed threshold which sometimes may result in a severe performance degradation when all the branches fall below the threshold. To overcome this problem we can use Adaptive GSC Receiver