Some Design Issues in Mobile Cellular Communication  

P. P. BhattacharyaURSI & DST Young Scientist Awardee

Department of Electronics and Communication Engineering, Netaji Subhash Engineering College,

Techno City, Garia, Kolkata – 700 152, IndiaPhone – 91-33-2436 1285, Fax – 91-33-2436 1286

E – mail :partha_p_b@yahoo.com

Partha Pratim Bhattacharya meeting President of India in Rashtrapati Bhavan

Contents –

Selection of cluster size

Channel assignment strategies

Call handover strategies

Some newly proposed call handover strategies

Cellular system capacity

Improving capacity in cellular systems

Radio design for a cellular network

Base station antennas

Cell phone antennas

Second generation (2G) cellular networks

Global System for Mobile (GSM)

Introduction to CDMA digital cellular standard

Evolution for TDMA and CDMA standards

Third generation (3G) cellular networks

Towards 4G

Selection of Cluster Size -

The ratio D/R is called co-channel reuse ratio (Q) and may be approximated as Q = D/R = (3N)1/2

Low N -> Low D/R -> Low D -> More interferenceHigh N-> High D/R -> Low R -> More number of cellsThe carrier to interference ratio can be approximately given as C / I = (D/R)n / 6, n=4 (n can be anything from 2 to 6).

System Minimum C/I D/R Suitable value for N

AMPS 18 dB 4.6 7

GSM 11 dB 3.0 4

IS-54 16 dB 3.9 7

CDMA -15 dB 0.7 1

Channel Assignment Strategies –Fixed Channel Assignment (FCA) Strategy

Channels are preallocated during planning.. If all the channels in a cell are occupied, the call is blocked and the subscriber does not receive service.

Dynamic Channel Assignment (DCA) Strategy In this case, voice channels are not allocated permanently to the cells. Each time a call request is made, the serving BS requests a channel from the MSC. MSC then allocates a channel to the requested cell in fashion which accounts the chance of future blocking within the cell, the frequency of use of the candidate channel, reuse distance of the channel and other cost functions. Therefore, the MSC only allocates a given frequency if the frequency is not presently in use in the cell or any other cell which falls within the minimum restricted distance of frequency reuse to avoid co-channel interference. Dynamic channel allocation reduces the probability of blocking.

DCA scheme performs well under nonuniform and low traffic density.

FCA performs well under high and uniform traffic.

Hybrid Channel Assignment (HCA) - Some channels are permanently assigned to BS as in FCA Other channels are kept in a central pool for borrowing

Borrowing strategy.- Combination of FCA & DCA. In this scheme, a cell is allowed to borrow channels from a neighbouring cell if all of its own channels are already occupied. Better performance than FCA under light and moderate traffic load.

Borrowing with channel ordering (BCO) – First channel has the highest priority to be assigned to the next call and the last channel has the highest priority to be assigned to neighbouring cell.

Borrowing with directional channel locking (BDCL) – After a channel is borrowed it is locked in the cochannel cell within the channel reuse distance of the borrowing cell.

Borrowing without channel locking (BWCL) – Overcomes disadvantages of other schemes. Channel can be borrowed only from adjacent BS and used with reduced transmitted power.

Call Handover Strategies –

Handover procedure involves measurement, decision and execution. Handover may be based on measurement such as –       Signal strength·        Bit error rate·        Traffic load·        Carrier to interference ratio etc. Signal strength based algorithm is simple and effective

1st generation systems – Signal strength was measured by base stations

2nd generation systems – Signal strength is measured by mobile stations (Mobile Assisted Handoff)

* Relative signal strength , the handover will occur at position A.

* Relative signal strength with threshold allows a user to hand over only if the current signal is sufficiently weak (less than a threshold) and the other is the stronger of the two.

The effect of the threshold depends on its value compared to the signal strengths of the two base stations at the point at which they are equal. If the threshold is higher than this value, say T1,

this scheme performs exactly like the relative signal strength scheme, so the handover occurs at position A. If the threshold is lower than this value, say T2, the mobile will delay handover

until the current signal level crosses the threshold at position B. In the case of T3, the delay may be so long that the mobile drifts

far into the new cell. This reduces the quality of the communication link and may result in a dropped call.

* Relative signal strength with hysteresis allows a user to hand over only if the new base station is sufficiently stronger (by a hysteresis margin, h) than the current one. In this case the handover will occur at point C. This technique prevents the so-called ping-pong effect, the repeated handover between two base stations caused by rapid fluctuations in the received signal strengths from both base stations.

* Relative signal strength with hysteresis and threshold hands a user over to a new base only if the current signal level drops below a threshold and the target base station is stronger than the current one by a given hysteresis margin. The handover will occur at point C if the threshold is either T1 or T2, and will occur at point D if the

threshold is T3.

* Prediction techniques base the handover decision on the expected future value of the received signal strength.

Prioritizing handoff –

1) Guard channel concept

2) Queuing of handoff calls 

Fuzzy Logic Based Handover Algorithm

Advantages of Fuzzy Logic –

Multivalued logic, many input parameters can be considered, less number of fluctuations

Input parameters –

1. Distance from base station which is defined as very near, near, medium, far and very far

2. Signal strength difference which is defined as very high, high, medium, low and very low

Handover state - No handover, Wait, Be careful, Handover, Sure handover

Some Newly Proposed Call Handover Strategies -

0

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Distance in meters

Poss

ibility

of ha

ndov

er

Handover response -

 

  

Handover characteristics depend on velocity.

To describe the effect of user mobility a parameter α is considered and defined as

 

where Tm is the mean call duration.

 The interval tmc, between the time a user starts a call in a cell and the time the

user reaches the cell boundary is  

 

where L is the distance which the user transits.

 Thus a handover occurs if tmc < td where td is the call duration time.

 

mTV

R2

V

Ltmc

Characterization of Velocity Dependence of Call Handover -

The probability that a call in the current cell produces a handover towards a neighbouring cell  

      Average number of handovers per call attempt ηh

 

 

where Pba is the blocking probability for new call attempts and Pbh is

the handover failure probability.

dx

x

eeP

x

h 22

]1[1

hbh

hbah PP

PP

)1(1

)1(

)1(1 bhh

bhhdrop PP

PPP

drop

baC P

PP

1

1

Probability that a call is dropped by an unsuccessful handover

where

Pba - Probability of new call blocking

Pdrop - Probability of handover call dropping

1/η - Mean residual time (taken to be 2 minutes)1/μ - Mean call holding time (taken to be 3 minutes)

The probability of call completion or throughput

0

0.1

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10 20 30 40 50 60 70 80 90 100

Velocity of MS (Km/hr)

Prob

abilit

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ando

ver

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10 20 30 40 50 60 70 80 90 100

Velocity of MS (Km/hr)

Aver

age n

umbe

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ver

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0.0005

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0.0025

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0.0035

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0.0045

10 20 30 40 50 60 70 80 90 100

Velocity of MS (Km/hr)

Prob

abilit

y of

cal

l dro

ppin

g

0.989

0.99

0.991

0.992

0.993

0.994

0.995

0.996

0.997

0.998

10 20 30 40 50 60 70 80 90 100

Velocity of MS (Km/hr)

Prob

abilit

y of c

all co

mple

tion

Velocity Dependent Variable Hysteresis Margin based Algorithm -

Recently proposed variable hysteresis margin based scheme – 

h exp [ - / 6],

where is the path loss exponent and varies from 2 to 6.  Velocity dependent variable hysteresis margin based scheme –  

h = H exp [ - / 6] / h

 

where H is a constant hysteresis margin, h is the average

number of handover and depends on mobile velocity.

H = 5 dBm (Optimum response)

200

250

300

350

400

450

500

10 20 30 40 50 60 70 80 90 100

Velocity of MS (Km/hr)

Han

dove

r del

ay (m

eter

s)

Path loss exponent = 2

Path loss exponent = 4

Path loss exponent = 6

Path loss exponent = 8

Handover response -

Umbrella cell approach to accommodate wide range of velocities -

Fuzzy Logic Based Velocity Dependent Call Handover

Algorithm -

Input parameters –

1. Distance from base station which is defined as very near, near, far and very far

2. Signal strength difference which is defined as very high, high, low and very low

3. Velocity of mobile user which is defined as very low, low, high and very high

Handover state - No handover, Wait, Be careful, Handover

Membership function – Bell shaped

0

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1.2

Distance in meters

Po

ssib

ility

of

han

do

ver

Velocity=10 Km/hr

Velocity=100 Km/hr

Handover response -

Cellular System Capacity –

Let, a cellular system has a total of S duplex channels available for use and each cell is allocated a group of K channels. If the S channels are divided among N cells (cluster) such that each cell has the same number of channels, the total number of radio channels can be expressed as

S = KNThe N cells which use the complete set of available frequencies is called a cluster.

If a cluster is replicated M times, then the total number of duplex channels (C) can be used as a measure of capacity and given by

C = MKN = MS

Thus, capacity of a cellular system is directly proportional to the number of times a cluster is repeated in a service area. N is called cluster size and is typically equal to 4, 7 or 12.

Improving Capacity In Cellular Systems –

1) Conventional Microcell Approach –

In this case, much smaller cells compared to the normal cells are used to accommodate more users. It does not provide intelligence because when the cell size becomes smaller, the control of interference among the cells becomes harder. Handoffs may not have enough time to complete.

2) Cell Splitting – Cell splitting is the process of subdividing a congested cell into smaller cells such that each smaller cell has its own base station with reduced antenna height and transmitter power. It increases the capacity of a cellular system since number of times channels are reused increases.

The increased number of cells would increase the number of clusters over the coverage region, which again would increase the number of channels and thus capacity. The distance between co-channel cells also reduces to half as the cell radius is reduced to half. Thus the co-channel reuse ratio remains same.

A

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Fig: Cell splitting

Splitting – Static, Dynamic

3) Sectoring – The co-channel interference in a cellular system may be reduced by replacing a single omni-directional antenna at the base station by several directional antennas radiating within specified sectors. A cell is normally partitioned in three 120 sectors or six 60 sectors. A given cell will receive interference and transmit with only a fraction of the available co-channel cells. In the sectoring scheme, the co-channel interference is reduced and thus system capacity is improved. Co-channel interference is reduced because the number of interferer gets reduced.

1

3

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(a)

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6

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(b)

Fig: (a) 120 sector and (b) 60 sector

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Fig: Only 2 co-channel cells out of 6 interfere with the center cell

4) Novel Microcell Zone Concept –

In case of sectoring, number of handoffs increases which results in an increased load on the system. As a solution of this problem, novel microcell zone concept was proposed in which each cell is divided into three or more zone sites, which are connected to a single base station.

Zone splitter Base

station

Microwave or fiber optic link

Fig: Novel microcell zone concept

Tx / Rx

Tx / Rx

Tx / Rx

Radio design for a cellular network –

1. Radio link design -

BS density and corresponding radio coverage is determined.

Gain of a mobile antenna is assumed to be 0 dBi. In reality, it can be as low as –6 to –8 dBi.

2. Coverage planning –

Propagation model – Large scale model, Small scale model

–20

–30

– 40

– 50

– 60

– 70

– 8050 100 150 200 250 300 350 400

Distance from transmitter (metres)

Fig: Small-scale fading

Small-scale fading

Average signal power

Received power (dBm)

0

Accurate field measurement should be made in urban area and the measured data can be used in the planning process. Loss = Path loss + shadow fading + multipath fading + penetration loss for buildings and vehicles + vegetation loss etc. Path loss exponent - 2 to 6 Shadow fading – 8 to 12 dB Multipath fading – + / - 20 dB Penetration loss – 10 to 30 dB in a building, 3 to 6 dB in a car, 10 to 12 dB in a bus.

Propagation models –

Lee model

Walfish – Ikegami model

Okumara model

Hata model

Walfish – Bertoni model

Some new models –

1. Indoor office environment –

L = 37+30log R+18.3 n[(n+2)/(n+1)-0.46] dB,

where R is the distance (m) and n is the number of floors.

2. Outdoor to outdoor and pedestrian environment –

L = 40 log R + 30 log f + 49 dB,

valid for NLOS link, average building penetration loss of 18 dB, log normal shadowing of 10 – 12 dB.

3. Vehicular environment –

L = 40 (1-0.04h)log R-18log h+21 log f+80 dB,

h is the BS antenna height measured from the rooftop level (m) and building penetration loss of 18 dB.

   

900MHz Horizontally Polarized Omnidirectional Antenna

900MHz HPOL Sector Antenna, 12dB, 120 deg

     

900MHz 6,8,11dBi Vertically Polarized Omni

The Standard Omnis are available in 6dBi , 8dBi and 11dBi models.

   

    

 

900MHz VPOL Sector Antenna

120 Degrees, 13dBi

Base Station Antennas -

Fig: Nonsmart-antenna system

Fig. Beam forming smart antenna

Compared with traditional omni-directional and sectorized antennas, smart-antenna systems can provide:

•Greater coverage area for each cell site

•Better rejection of co-channel interference

•Reduced multipath interference via increased directionality

•Reduced delay spread as fewer scatterers are allowed into the beam

•Increased frequency reuse with fewer base stations

•Higher range in rural areas

•Improved building penetration

•Location information for emergency situations

•Increased data rates and overall system capacity

•Reduction in dropped calls

Cell Phone Antennas –

A cellular handset antenna is the small cylindrical stub sticking out of the top of the hand set case.

In some cases antennas are embedded in the handset case and are not visible to the user.

Since a cell phone user is constantly changing his or her position and moving from cell to cell, the mobile phone requires an antenna that transmits and receives equally well in all directions.

Another major concern in cellular antenna design is the potential radiation hazards posed to humans. As the use of cell phones increases, there are growing concerns about what affect the cell phone’s radio waves have on human health. Various antenna designs have been promoted to minimize the amount of energy that is radiated into the skull of the user.

Embedded antennas may make a handset more durable but having the antenna inside the device can impede performance because other electrical components inside the phone can cause interference.

CDMA handsets tend to have more external antennas than GSM handsets because on CDMA networks operators can squeeze in more users per base station. GSM systems don't have the same advantage.

Today there are four leading antenna architectures that are commonly used in embedded applications: microstrip, patch, Planar Inverted 'F' Antenna (PIFA) and Meander Line Antenna (MLA).

Microstrip lines are an extension of the monopole, only laying it down on a two-dimensional surface. It can be easily fabricated by etching a copper strip of 1/2- or 1/4-wavelength onto the radio circuit board. While very inexpensive to make, its performance is limited to the extent that surrounding metallic sections of the circuit board severely interfere with its radiation efficiency. Furthermore, it is a single-frequency solution and most wireless devices today implement more than one mode of communication, usually in different frequency bands.

Patch antennas have been around for a long time and are a good choice for a system that requires a beam pattern focused in a certain direction. They are typically used in single frequency applications requiring the directed beam pattern, such as a GPS receiver or a wall-mounted access point. The PIFA antenna literally looks like the letter 'F' lying on its side with the two shorter sections providing feed and ground points and the 'tail' providing the radiating surface. PIFAs make good embedded antennas in that they exhibit a somewhat omnidirectional pattern and can be made to radiate in more than one frequency band. But, their efficiency is only average and it can be difficult to properly match the device to the transmitting circuitry at both operating frequencies. The MLA is made from a combination of a loop antenna and frequency tuning meander lines. This is more efficient for its size than many competitive antennas used in wireless applications. In addition, MLAs can be designed to exhibit broadband capabilities that allow operation on several frequency bands, such as AMPS, PCS, and GPS bands simultaneously. However, initial MLA designs are slightly more expensive than the previous antenna options.

Second Generation (2G) Cellular Networks –

Since 1990, most of the cellular networks use second generation or 2G digital technology. Unlike first generation (1G) which relied exclusively on FDMA/FDD and FM, 2G technology use digital modulation formats and TDMA/FDD and CDMA/FDD multiple access techniques.

The most popular second generation TDMA standards include Global System for Mobile Communication (GSM), Interim Standard 136 (IS-136), Pacific Digital Cellular (PDC).

The popular 2G CDMA standard is Interim Standard 95 Code Division Multiple Access (IS-95) also called cdmaone.

BTS

BTS

BTS

BTS

MS

BTS

BTS

BTS

BSC

BSC

OMC

MSC

VLRHLR AUC

PSTN

ISDN

Operation support subsystem (OSS)

Network Switching Subsystem (NSS)

Public networks

Data network

Fig: GSM system architecture

Base Station Subsystem (BSS)

Global System for Mobile (GSM) -

GSM Air Interface Specifications –

Reverse Channel Frequency 890 915 MHzForward Channel Frequency 935 960 MHzARFCN Number 0 to 124 and 975 to 1023TX/RX Frequency spacing 45 MHzModulation data rate 270.833 KbpsFrame period 4.615 msusers per frame 8Time slot period 576.9 μsBit period 3.692 μsModulation 0.3 GMSKARFCN channel spacing 200 KHz

GSM channels - Basically there are two types of GSM logical channels traffic channels (TCH) and control channel (CCH).

GSM Traffic channels –

Full rate TCHThe different full rate speech and data channels are discussed below.a) Full rate speech channel (TCH/FS)The full rate speech channel carries user speech, digitized at a raw data rate of 13 Kbps. With GSM channel coding it becomes 22.8 Kbps.b) Full Rate Data channel for 9.6 Kbps (TCH/F9.6) The full rate traffic data channel carries raw user data, sent at 9.6 Kbps. After forward error connection, it is sent at 22.8 Kbps.c) Full Rate Data channel for 4.8 Kbps (TCH/F4.8) The full rate traffic data channel carries raw user data sent at 4.8 Kbps. The data are ultimately sent at 22.8 Kbps with additional forward error connection.d) Full Rate Data Channel for 2.4 Kbps (TCH/F 2.4)It carries user data sent at 2.4 Kbps which with additional forward error correction is sent at 22.8 Kbps.

Half rate TCHDifferent half rate speech and data channels are discussed below.

a) Half Rate speech channel (TCH/HS) This is used to carry digitised speech which is sampled at a rate half that of the full rate channel (6.5 Kbps). With GSM channel coding added, half rate speech channel carries 11.4 Kbps.b) Half Rate Data channel for 4.8 Kbps (TCH/H 4.8) This half rate data channel carries raw user data sent at 4.8 Kbps. With additional error correction coding it is sent at 11.4 Kbps.c) Half Rate Data channel for 2.4 Kbps (TCH/H2.4)This half rate channel carries user data which is sent at 2.4 Kbps. It is sent at 11.4 Kbps after adding forward error correction as per GSM standard.

Broadcast Control Channel (BCCH)

Broadcast Channel (BCH)

Common Control Channel (CCCH)

GSM Control Channels

Dedicated Control Channel (DCCH)

Frequency Correction Channel (FCCH)

Synchronization Channel (SCH)

Paging Channel (PCH)

Random Access Channel (RACH)

Access Grant Channel (AGCH)

Stand-alone Dedicated Control Channel (SDCCH)

Slow Associated Control Channel (SACCH)

Fast Associated Control Channel (FACCH)

Fig: Different GSM Control Channels

GSM Control Channels -

Introduction to CDMA digital cellular standard –

In code division multiple access (CDMA) systems, the narrowband message signal is multiplied by a very large bandwidth signal called the spreading signal. The spreading signal is a pseudo-noise (PN) code sequence that has a chip rate which is few orders of magnitudes greater than the message data rate. In this system, each cell is a cluster (cluster size N = 1) and shares the same bandwidth. All users use the same carrier frequency and may transmit simultaneously. Each user has its own PN code which is almost orthogonal to all other codewords. The receiver needs to know the code used by the transmitter for detection of the message signal.

IS - 95 uses 824 849 MHz band for reverse channel and 869 894 MHz for forward channel. A forward and reverse channel pair is separated by 45 MHz.

In India, CDMA service provides offer two services – WLL (M) and fixed wireless terminal (FWT).

Particulars GSM CDMA

1. Frequency band of operation 890 915 MHz for reverse link and 935 960 MHz for forward link.

824 849 MHz for reverse link and 869 894 MHz for forward link.

2. Mobility Mobility is better compared to CDMA Mobility not so good.

3. Bandwidth 50 Kbps speed for GPRS 144 Kbps for CDMA 2000

4. Roaming International roaming is easy as many service provides opted GSM

International roaming is a problem.

5. Voice clarity Voice clarity is less Voice clarity is better

6. Spectrum utilization Inefficient frequency spectrum utilization

Efficient frequency spectrum utilization

7. Cell sites More number of cell sites Fewer number of cell sites.

8. Calling capacity Reduced calling capacity due to lower spectral efficiency.

More calling capacity due to better spectral efficiency.

9. Battery life Talk time lower than that of the CDMA Better power management leading to enhanced battery life and longer talk time.

10. Security Less security Increased security

Comparison between GSM and CDMA -

Evolution for TDMA & CDMA standards -Three different upgrade paths have been developed for GSM carriers

– High Speed Circuit Switched Data (HSCSD)General Packet Radio Service (GPRS)Enhanced Data Rates for GSM Evolution (EDGE).

IS 95 GSM IS 136 & PDC

IS 95B HSCSD

GPRS

EDGE

Cdma 2000

3GPP2

WCDMA

TD SCDMA

EDGE

3GPP

Fig: Various upgrade paths for 2G technologies

3G

2.5G

2G

Third Generation (3G) wireless networks - 3G systems which promise multi-megabit internet access, communication using Voice over Internet Protocol (VoIP), voice activated calls, unparallel network capacity and many more. The 3G evolution for CDMA systems leads to cdma 2000. The 3G evolution for GSM, IS-136 and PDC systems lead to Wideband CDMA (W-CDMA), also called Universal Mobile Telecommunications Service (UMTS). CDMA 2000 is based on IS-95 and IS-95B technologies and W-CDMA is based on GSM.International Telecommunications Union (ITU) formulated a plan to implement a global frequency band in 2 GHz range that would support a single, ubiquitous wireless communication standard for all countries, called International Mobile Telephone 2000 (IMT-2000). The ITU IMT-2000 standards organizations are currently separated into two major organizations –(i) 3GPP (3G Partnership Project) for wideband CDMA standards based on backward compatibility with GSM and IS-136/PDC. (ii)3GPP2 for cdma 2000 standards based on backward compatibility with IS-95.

The 3G W-CDMA air interface standard has been designed for “ALWAYS ON’ packet based wireless service so that computers, entertainment devices and telephones may all share the same wireless network and may be connected to the internet, anytime, anywhere. W-CDMA supports data rates up to 2.048 Mbps per user, and allows high quality data, multimedia, streaming audio, streaming video and broadcast type services to consumers. Future versions of W-CDMA will support user data rates in excess of 8 Mbps. With W-CDMA the data is carried on a single W-CDMA 5 MHz radio channel and each channel can support 100 to 350 simultaneous voice calls depending on factors such as antenna sectoring, propagation conditions, user velocity etc. The first 3G CDMA air interface, cdma 2000 1RTT implies that a single 1.25 MHz radio channel is used. 1 implies one time the original cdmaone channel bandwidth. RTT stands for Radio Transmission Technology. Cdma 2000 1 supports an instantaneous data rate of up to 307 Kbps for a user in packet mode and yields typical throughput rates of up to 144 Kbps per user depending on number of users, velocity, propagation condition.

Cdma 2000 1EV is an evolutionary advancement for CDMA originally developed by Qualcomm Inc. as a high data rate packet standard to be overlaid upon existing IS-95, IS-95B and cdma 2000 networks. Cdma 2000 IEV provides CDMA carriers with the option of installing radio channels with data only (cdma 2000 1EV-DO) or with data and voice (cdma 2000 1EV-DV)). Cdma 2000 1EV-DO option supports greater than 2.4 Mbps throughput per user although actual data rates are much lower and dependent upon number of users, propagation conditions and vehicle speed. The cdma 2000 3RTT standard uses three adjacent 1.25 MHz radio channels to provide data throughput in excess of 2 Mbps per user. Cdma 2000 3RTT has a very similar data rate throughput goal like W-CDMA.

Towards 4G –

The 4G will be a fully IP-based integrated system of systems and network of networks achieved after the convergence of wired and wireless networks as well as computer, consumer electronics, communication technology, and several other convergences that will be capable of providing 100 Mbps and 1Gbps, respectively, in outdoor and indoor environments with end-to-end QoS and high security, offering any kind of services anytime, anywhere, at affordable cost.

The Wireless World Research Forum (WWRF) defines 4G as a network that operates on Internet technology, combines it with other applications and technologies such as Wi-Fi and Wimax, and runs at speeds ranging from 100 Mbps (in cell-phone networks) to 1 Gbps (in local Wi-Fi networks).

Towards 4G

No distance too long