3g Seminar Topic Printout

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3G Technology

1. Introduction

What is 3G?

3G is a wireless data standard that has been developed and agreed upon by wireless infrastructure and equipment manufacturers. Developing a standard like 3G is no easy task. There are many wireless data technologies out there and there are big dollars chasing these technologies around not making the task any easier. The 3G specs have also been modified over the years and currently we are looking at 3G release 6.

3G encompasses a group of technologies that are all aimed at bringing high speed data transfers to mobile electronic devices. Many of the original 3G technologies have been built on W-CDMA (Wideband Code Division Multiple Access) and TD-CDMA (Time Division Code Division Multiple Access) radio technologies. But, there has been great success using the European Telecommunications Standards Institute (ETSI) standardized, GSM (Global System for Mobile Communication). GSM is used by companies like Cingular and AT&T Wireless. Today, HSDPA is promising to bring broadband speeds to GSM wireless networks with downstream speeds of up to 3.6Mbps and upstream of 384Kbps.

2. Generations and history

3G refers to the third generation of developments in wireless technology, especially mobile communications. The third generation, as its name suggests, follows the first generation (1G) and second generation (2G) in wireless communications.

2.1 1GThe 1G period began in the late 1970s and lasted through the 1980s. These systems featured the first true mobile phone systems, known at first as "cellular mobile radio telephone." These networks used analog voice

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signaling, and were little more sophisticated than the repeater networks used by amateur radio operators.

2.2 2G

The 2G phase began in the 1990s and much of this technology is still in use. The 2G cell phone features digital voice encoding. Examples include CDMA and GSM. Since its inception, 2G technology has steadily improved, with increased bandwidth, packet routing, and the introduction of multimedia.

3. 3G includes capabilities and features such as:

Enhanced multimedia (voice, data, video, and remote control). Usability on all popular modes (cellular telephone, e-mail, paging,

fax, videoconferencing, and Web browsing).

Broad bandwidth and high speed (upwards of 2 Mbps).

Roaming capability throughout Europe, Japan, and North America.

While 3G is generally considered applicable mainly to mobile wireless, it is also relevant to fixed wireless and portable wireless. A 3G system should be operational from any location on, or over, the earth's surface, including use in homes, businesses, government offices, medical establishments, the military, personal and commercial land vehicles, private and commercial watercraft and marine craft, private and commercial aircraft (except where passenger use restrictions apply), portable (pedestrians, hikers, cyclists, campers), and space stations and spacecraft.

3G offers the potential to keep people connected at all times and in all places. Researchers, engineers, and marketers are faced with the challenge of accurately predicting how much technology consumers will actually be willing to pay for. Another challenge faced by 3G services is competition from other high-speed wireless technologies, especially mobile WiMAX, and ability to roam between different kinds of wireless networks.

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The current status of mobile wireless communications, as of July 2007, is a mix of 2nd and 3rd generation technologies.

4. 4G

Although the new, third generation (3G) wireless technology has not yet been implemented, leading companies in the industry are already laying the groundwork for what some are calling fourth generation (4G) technology. For the purposes of this article, 4G will be considered those technologies that are still in the planning stages and will not be deployed within the next five years.

Researchers are continuing their ideas in the development of an undefined wireless world, which could become operational by 2010.

The first generation (1G) and second generation (2G) of mobile telephony were intended primarily for voice transmission.

The third generation of mobile telephony (3G) will serve both voice and data applications.

There really is no clear definition of what 4G will be. It is generally accepted that 4G will be a super-enhanced version of 3G – i.e., an entirely packet switched network with all digital network elements and extremely high available bandwidth. For the most part, it is believed that 4G will bring true multimedia capabilities such as high-speed data access and video conferencing to the handset. It is also envisioned that 4G systems will be deployed with software defined radios, allowing the equipment to be upgraded to new protocols and services via software upgrades.

4G also holds the promise of worldwide roaming using a single handheld device.

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5. Wireless Broadband

Right now, wireless broadband is just becoming a reality here in the United States. Verizon Wireless was the first to roll out their wireless broadband service using the EV-DO (Evolution Data Only) technology. Sprint Nextel also offers an EV-DO system of broadband wireless. Finally, Cingular has announced that they will be hurrying to upgrade their network via HSDPA to provide broadband data speeds to their wireless customers.

5.1 Wireless Generations At-a-Glance

As with all technology progressions, the “next” upgrades must be in planning and development phases while its predecessors are being deployed. This statement holds true with all mobile telecommunications to date. It seems that it will also hold true forthe next generations of wireless networks. ] The original analog cellular systems are considered the first generation of mobile telephony (1G). In the early 1980s, 1G systems were deployed. At the same time, the cellular industry began developing the second generation of mobile telephony (2G). The difference between 1G and 2G is in the signaling techniques used: 1G used analog signaling, 2G used digital signaling. As experience shows, the lead-time for mobile phone systems development is about 10 years. It was not until the early to mid 1990s that 2G was deployed.

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Primary thinking and concept development on 3G generally began around 1991 as 2G systems just started to roll out. Since the general model of 10 years to develop a new mobile system is being followed, that timeline would suggest 4G should be operational some time around 2011.

4G would build on the second phase of 3G, when all networks are expected to embrace Internet protocol (IP) technology. During the last year, companies such as Ericsson, Motorola, Lucent, Nortel and Qualcomm came up with "3G-plus" concepts that would push performance of approved, though still emerging, standards beyond current ones.

6. Interoperability and the Evolution of Network Architectures

One of the most challenging issues facing deployment of 4G technologies ishow to make the network architectures compatible with each other. New signaling techniques are being designed specifically to enhance today's second generation (2G) networks, deliver unprecedented functionality for 3G, and successfully drive the Fourth Generation (4G) of wireless, thus delivering immediate and long-term benefits to carriers.

With the architecture of each generation of wireless devices addressed inthe development of advanced technologies, carriers can easily evolve their systems without additional network modifications, significantly reducing costs and implementation time. Currently, different wireless technologies (e.g., GSM, CDMA,and TDMA1) are used throughout the world for the 2G, 2.5G, and eventually 3G networks. There are two approaches being used to develop 4G access techniques: 3xRTT (currently 1xRTT for 2.5 and 3G) and Wideband CDMA (W-CDMA). These disparate access techniques currently do not interoperate.

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This issue may be solved with software defined radios. Link Air Communications is developing a new access technology called large-area-synchronized code-division multiple access (LAS-CDMA). LASCDMA will be compatible with all current and future standards, and there is a relatively easy transition from existing systems toLAS-CDMA (using software defined radios). Link Air emphasizes that LASCDMA will accommodate all the advanced technologies planned for 4G and that LASCDMA will further enhance either 3xRTT or W-CDMA system’s performance or capacity.

7. Internet Speeds 2.5G is the interim solution for current 2G networks to have 3Gfunctionality. 2.5G networks are being designed such that a smooth transition (software upgrade) to 3G can be realized. 2.5G networks currently offer true data speeds up to 28kbps. In comparison, the theoretical speed of 3G can be up to 2 Mbps,i.e., approximately 200 times faster than previous 2G networks. This added speed and throughput will make it possible to run applications such as streaming video clips. It is anticipated that 4G speeds couldbe as high as 100 Mbps. Thus, 4G will represent another quantum leap in mobile Internet speeds and picture quality. Ericsson confirms that 4G could bring connection speeds of up to 50 times faster than 3G networks and could offer three-dimensional visual experiences for the first time. The following graph represents what has beenthe typical progression of wireless

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1 GSM – Global System for Mobile Communications CDMA – Code Division Multiple Access TDMA – Time Division Multiple Access

8. Quality of Service Challenges

In wireless networks, Quality of Service (QOS) refers to the measure of the performance for a system reflecting its transmission quality and service availability (e.g., 4G is expected to have at least are liability of 99.99%).

Supporting QOS in 4G networks will be a major challenge. When considering QOS, the major hurdles to overcome in 4G include: varying rate channel characteristics, bandwidth allocations, fault tolerance levels, and handoff support among heterogeneousWireless networks.

Fortunately, QOS support can occur at the packet, transaction, circuit, and network levels. QOS will be able to be tweaked at these different operating levels, making the network more flexible and possibly more tolerant to QOS issues.

Varying rate channel characteristics refers to the fact that 4G applications will have varying bandwidth and transition rate requirements. In order to provide solid network access to support the anticipated 4G

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applications, the 4G networks must be designed with both flexibility and scalability. Varying rate channel characteristics must be considered to effectively meet user demand and ensure efficient network management.

Spectrum is a finite resource. Incurrent wireless systems, frequency licensing and efficient spectrum management are key issues. In 4G systems, bandwidth allocations may still be a concern.

Another concern is interoperability between the signaling techniques that are planned to be used in 4G (e.g., 3xRTT, WCDMA).In comparison with current 2G and 2.5G networks, 4G will have more fault tolerance capabilities built-in to avoid unnecessary network failure, poor coverage, and dropped calls. 4G technology promises to enhance QOS by the use of better diagnostic techniques and alarms tools.

4G will have better support of roaming and handoffs across heterogeneous networks. Users, even in today’s wireless market, demand service transparency and roaming. 4G may support interoperability between disparate network technologies by using techniques such as LAS-CDMA signaling. Other solutions such as software defined radios could also support roaming across disparate network technologies in 4G systems.

These major challenges to QOS in 4G networks are currently being studied and solutions are being developed. Developers believe that QOS in 4G will rival that of any current 2G or 2.5G network. It is anticipated that the QOS in 4G networks will closely approximate the QOS requirements in the wire line environment (99.999% reliability).

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The emergence of next generation wireless technologies is going enhance the effectiveness of the existing methods used by public safety...

9. 3G Applications and Their Benefits to Public Safety

One of the most notable advanced applications for 4G systems is location based services. 4G location applications would be based on visualized, virtual navigation schemes that would support a remote database containing graphical representations of streets, buildings, and other physical characteristics of a large metropolitan area.

This database could be accessed by a subscriber in a moving vehicle equipped with the appropriate wireless device, which would provide the platform on which would appear a virtual representation of the environment ahead.

For example, one would be able to see the internal layout of a building during an emergency rescue. This type of applicationis sometimes referred to as "Telegeoprocessing", which is a combination of Geographical Information Systems (GIS) and Global PositioningSystems (GPS) working in concert over a high-capacity wireless mobile system.

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Telegeoprocessing over 4G networks will make it possible for the public safety community to have wireless operational functionality and specialized applications for everyday operations, as well as for crisis management.

The emergence of next generation wireless technologies will enhance the effectiveness of the existing methods used by public safety.

3G technologies and beyond could possibly bring the following new features to public safety:

9.1 Virtual navigation:

As described, a remote database contains the graphicalrepresentation of streets, buildings, and physical characteristics of a largeMetropolis. Blocks of this database are transmitted in rapid sequence to a vehicle, where a rendering program permits the occupants to visualize the environment ahead. They may also "virtually" see the internal layout of buildings to plan an emergency rescue, or to plan to engage hostile elements hidden in the building.

9.2 Tele-medicine:

A paramedic assisting a victim of a traffic accident in a remoteLocation could access medical records (e.g.-rays) and establish a video conference so that a remotely based surgeon could provide“on-scene” assistance.

In such a circumstance, the paramedic could relay the victim's vital information (recorded locally) back to the hospital in real time, for reviewby the surgeon.

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9.3 Crisis-management applications:

These arise, for example, as a result of natural disasters where the entire communications infrastructure is in disarray. In such circumstances, restoring communications quickly is essential.

With wideband wireless mobile communications, both limited and complete communications capabilities, including Internet and video services, could be set up in a matter of hours. In comparison, it may take days or even weeks to re-establish communications capabilities when a wire line network isrendered inoperable.

10. WCDMA – Technology for 3G Cellular Systems

What is 3G Mobile System?

•Broadband Service

•Packet Based Transmission

•Transmission of text, digitized voice, video at data rate higher than 2 Mbps

•Consistent set of services to mobile computer and phone users wherever they are located in the world

10.1 Universal Mobile Telecommunication System

One can directly dive straight into the mobile multimedia wave

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One can directly dive straight into the mobile multimedia wave

10.1.1 UMTS Network

It consists of:

• UMTS Terrestrial Radio Access Network (UTRAN) - WCDMA and FDD are used here

• Core Network

10.2 Spread Spectrum System

• A kind of modulation system in which the modulated (spread spectrum) signal bandwidth is much greater than the message signal bandwidth

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• The spectral spreading is performed by a code that is independent of the message signal. The same code is used at receiver to dispread the received signal and to recover the message signal.

10.2.1 Advantages of Spread Spectrum System

• It is Secure, difficult to intercept.

• A large number of codes can support a Large number of users.

• As a large bandwidth is used the system is less prone to distortion.

• Resistant to jamming.

• Asynchronous multiple access technology.

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10.2.2Different Spread Spectrum System

• Direct Sequence Spread Spectrum

• Frequency Hoping

• Hybrid System

10.3 Code division multiple access

• A multiple access technology using DS/SS

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10.4 Why WCDMA?

• WCDMA is a step further in the CDMA technology. It uses a 5 MHz wide radio signal and a chip rate of 3.84 Mcps, which is about three times higher than the chip rate of CDMA2000 (1.22 Mcps)

• Higher spectrum efficiency thanks to improved trunking efficiency (i.e. a better statistical averaging)

• Higher QoS

WCDMA Protocol Architecture

Three separate channels concepts in the UTRA: logical, transport, and physical channels.

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•Logical channels define what type of data is transferred. •Transport channels define how and with which type of characteristics the data is transferred by the physical layer.

•Physical data define the exact physical characteristics of the radio channel.

10.4.1 WCDMA Protocol Architecture view

Simplified View

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Detailed View

10.5 Physical Layer

Main 3G Requirements

1) High Bit Rates for better transmission.

2) Flexible Variable bit rate both in Uplink and Downlink.

3) Different services have been multiplexed on a single physical connection

4) Support for All-IP RAN

5) High Spectral Efficiency

Main Functions of Physical Layer

1) Error Detection

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2) Multiplexing Demultiplexing

3) Modulation Demodulation

4) Spreading Despreading

5) Synchronization

6) Measurement of Bit Error Rate, TX Power, Signal to Interference ratio

7) Handover

At physical layer

• W= 3.84 Mcps, one time slot 2560 chips

•Physical channel is characterize with frequency, code, duration, and in uplink with phase shift

•1 radio frame (10 ms) includes 15 time slots (one slot equal to power control period, 1/ (10 ms/15) =1500 Hz). Slot structure is just for controlling the physicalChannel and radio performance

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Physical Layer to Transport Layer

•Several transport channels can be multiplexed together by physical layer to form a single Coded Composite Transport Channel (CCTrCh)

•The physical layer combines several TFI information into the TransportFormat Combination Indicator (TFCI), which indicate which transportChannels are active for the current frame.

•Two types of transport channels: dedicated channels and commonChannels.

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Dedicated channel –reserved for a single user only.Support fast power control and soft handover.Common channel – can be used by any user at any time.Don’t support soft handover but some supportFast power control.

•In addition to the physical channels mapped from the transportChannels, there exist physical channels for signaling purposes to carry onlyInformation between network and the terminals.

10.5.1 Transport Channel

•Transport channels contain the data generated at the higher layers, which is carried over the air and are mapped in the physical layer to different physical channels.

• The data is sent by transport block from MAC layer to physical layer and generated by MAC layer every 10 ms.

• The transport format of each transport channel is identified by the Transport Format Indicator (TFI), which is used in the interlayer

• Communication between the MAC layer and physical layer.Different Transport Channels and Corresponding

Physical

•

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RLC Layer

•The Radio Link Control (RLC) protocol (Layer2) operates in one of three modes: transparent, unacknowledged or acknowledged mode

• It performs segmentation/re-assembly functions and, in acknowledged mode, provides an assured mode delivery ser-vice by use of retransmission

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• RLC provides a service both for the RRC signaling (the Signaling Radio Bearer) and for the user data transfer (the Radio Access Bearer).

RRC Layer :

• The Radio Resource Control (RRC) protocol (Layer 3) provides control of the handset from the RNC

• It includes functions to control radio bearers, physical channels, mapping of the different channel types, handover, measurement and other mobility procedures. Because of the flexibility of the WCDMA radio interface, this is a fairly complex protocol

10.6 WCDMA System Overview

RAN Architecture

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• The main purpose of the WCDMA Radio Access Network is to provide a connection between the hand-set and the corenetwork and to isolate all the radio issues from the core network.

•The advantage is one core network supporting multipleaccess technologies.

The WCDMA Radio Access Network consists of twotypes of nodes:

Radio Base Station (Node B)

• The Radio Base Station handles the radio transmission and reception to/from the handset over the radioInterface (Uu). It is controlled from the Radio NetworkController via the Hub interface. One Radio Base StationCan handle one or more cells.

Radio Network Controller (RNC)

• The Radio Network Controller is the node that controlsall WCDMA Radio Access Network functions. It connects

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the WCDMA Radio Access Network to the core network via the Iu Interface.

Pictorial representation of RAN architecture:

11.Overview of the 3GPP Long TermEvolution Physical Layer

Introduction

Long Term Evolution (LTE) is the next step forward in cellular 3G services. Expected in the 2008 time frame, LTE is a 3GPP standard

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that provides for an uplink speed of up to 50 megabits per second (Mbps) and a downlink speed of up to 100 Mbps. LTE will bring many technical benefits to cellular networks. Bandwidth will be scalable from 1.25 MHz to 20MHz. This will suit the needs of different network operators that have different bandwidth allocations, and also allow operators to provide different services based on spectrum. LTE is also expected to improve spectral efficiency in 3G networks, allowing carriers to provide more data and voice services over a given bandwidth. This technical white paper provides an overview of the LTE physical layer (PHY), including technologies that are new to cellular such as Orthogonal Frequency Division Multiplexing (OFDM) and Multiple Input Multiple Output (MIMO) data transmission.

It encompasses high-speed data, multimedia unicast and multimedia broadcast services. Although technical specifications are not yet finalized, significant details are emerging. This paper focuses on the LTE physical layer(PHY). The LTE PHY is a highly efficient means of conveying both data and control information between an enhanced base station (eNodeB) and mobile user equipment (UE). The LTE PHY employs some advanced technologies that are newto cellular applications. These include Orthogonal Frequency Division Multiplexing (OFDM) and Multiple Input MultipleOutput (MIMO) data transmission. In addition, the LTE PHY uses Orthogonal Frequency Division Multiple Access

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11.1 LTE Design Goals

The LTE PHY is designed to meet the following goals [1]:1. Support scalable bandwidths of 1.25, 2.5, 5.0, 10.0 and 20.0 MHz

2. Peak data rate that scales with system bandwidtha. Downlink (2 Ch MIMO) peak rate of 100 Mbps in 20 MHz channelb. Uplink (single Ch Tx) peak rate of 50 Mbps in 20 MHz channel

3. Supported antenna configurationsa. Downlink: 4x2, 2x2, 1x2, 1x1b. Uplink: 1x2, 1x1

4. Spectrum efficiencya. Downlink: 3 to 4 x HSDPA Rel. 6b. Uplink: 2 to 3 x HSUPA Rel. 6

5. Latencya. C-plane: <50 – 100 msec to establish U-planeb. U-plane: <10 msec from UE to server

6. MobilityA. Optimized for low speeds (<15 km/hr)B. High performance at speeds up to 120 km/hrC. Maintain link at speeds up to 350 km/hr

7. Coveragea. Full performance up to 5 kmb. Slight degradation 5 km – 30 kmc. Operation up to 100 km should not be precluded by standard

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11.2 LTE Basic Concepts

Many methods employed in LTE are relatively new in cellular applications. These include OFDM, OFDMA, MIMO and Single Carrier Frequency Div LTE employs OFDM for downlink data transmission and SC-FDMA for uplink transmission. OFDM is a well-known modulation technique, but is rather novel in cellular applications. A brief discussion of the basic properties and advantages of this method is therefore warranted. When information is transmitted over a wireless channel, the signal can be distorted due to multipath. Typically (but not always) there is a line-of-sight path between the transmitter and receiver. In addition, there are many other pathscreated by signal reflection off buildings, vehicles and other obstructions as shown in Figure 2.0-1. Signals travelingalong these paths all reach the receiver, but are shifted in time by an amount corresponding to the differences in thedistance traveled traveled along each path.

OFDM

Unlike single carrier systems described above, OFDM communication systems do not rely on increased symbol rates in order to achieve higher data rates. This makes the task of managing ISI much simpler. OFDM systems break the available bandwidth into many narrower sub-carriers and transmit the data in parallel streams. Each subcarrier is modulated using varying levels

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of QAM modulation, e.g. QPSK, QAM, 64QAM or possibly higher orders depending on signal quality. Each OFDM symbol is therefore a linear combination of the instantaneous signals on each of the sub4 carriers in the channel. Because data is transmitted in parallel rather than serially, OFDM symbols are generally MUCHlonger than symbols on single carrier systems of equivalent data rate.

OFDMA

OFDMA is employed as the multiplexing scheme in the LTE downlink. Perhaps the best way to describe OFDMA is bycontrasting it with a packet-oriented networking scheme such as 802.11a. In 802.11a, Carrier-Sense Multiple Access (CSMA) is the multiplexing method. Downlink and uplink traffic from the fixed access point (AP) to mobile user stations (STAs) is by means of PHY layer packets. As explained below, OFDMA makes much more efficient use of network resources.

SC-FDMA

LTE uplink requirements differ from downlink requirements in several ways. Not surprisingly, power consumption is a key consideration for UE terminals. The high PAPR and related loss of efficiency associated with OFDM signaling are major concerns. As a result, an alternative to OFDM

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was sought for use in the LTE uplink. Single Carrier – Frequency Domain Multiple Access (SC-FDMA) is well suited to the LTE uplink requirements. The basic transmitter and receiver architecture is very similar (nearly identical) to OFDMA, and it offers the same degree ofmultipath protection. Importantly, because the underlying waveform is essentially single-carrier, the PAPR is lower.

SC-FDMA Subcarriers Can be Mapped in Either Localized or Distributed Mode

As SC-FDMA subcarriers can be mapped in one of two ways: localized or distributed. However, the current working assumption is that LTE will use localized Subcarriers mapping. This decision was motivated by the fact that with localized mapping, it is possible to exploit frequency selective gain via channel dependent scheduling (assigning uplink frequencies to UE based on favorable propagation conditions.

11.3 LTE Physical Layer

Physical Channels Three different types of physical channels are defined for the LTE downlink. One common characteristic of physical channels is that they all convey information from higher layers in the LTE stack. This is in contrast to physical signals,

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Which convey information that is used exclusively within the PHY layer.

LTE DL physical channels are:

• Physical Downlink Shared Channel (PDSCH)

• Physical Downlink Control Channel (PDCCH)

• Common Control Physical Channel (CCPCH)

Physical channels are mapped to specific transport channels below. Transport channels are SAPs for higher layers.

Each physical channel has defined algorithms for:

• Bit scrambling• Modulation• Layer mapping• CDD precoding• Resource element assignment

Layer mapping and pre-coding are related to MIMO applications. Basically, a layer corresponds to a spatial multiplexing

Layer mapping and pre-coding are related to MIMO applications. Basically, a layer corresponds to a spatial multiplexing channel. MIMO systems are defined in terms of Ntransmitters x Nreceivers. For LTE, defined configurations are 1x 1, 2 x 2,3 x 2 and 4 x 2.

Note that while there are as many as four transmitting antennas, there are only a maximum of two receivers and thus a maximum of only two spatial multiplexing data streams.

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For a 1 x 1 or a 2 x 2 system, there is a simple 1:1 relationship between layers and transmitting antenna ports. However, for a 3 x 2 and 4 x 2 system, there are still only two spatial multiplexing channels. Therefore, there is redundancy on one or both data streams. Layer mapping specifies exactly how the extra transmitter antennas are employed. Precoding is also used in conjunction with spatial multiplexing. Recall that MIMO exploits multipath to resolveindependent spatial data streams. In other words, MIMO systems require a certain degree of multipath for reliableoperation. In a noise-limited environment with low multipath distortion, MIMO systems can actually become impaired.

Physical Downlink Shared Channel:

The PDSCH is utilized basically for data and multimedia transport. It therefore is designed for very high data rates. Modulation options therefore include QPSK, 16QAM and 64QAM. Spatial multiplexing is also used in the PDSCH. In fact, spatial multiplexing is exclusive to the PDSCH. It is not used on either the PDCCH or the CCPCH.

Physical Downlink Control Channel:

The PDCCH conveys UE-specific control information. Robustness rather than maximum data rate is therefore the chief consideration. QPSK is the only available modulation format. The PDCCH is mapped onto resource elements in up to the first three OFDM symbols in the first slot of a subframe.

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Common Control Physical Channel

The CCPCH carries cell-wide control information. Like the PDCCH, robustness rather than maximum data rate is the chief consideration. QPSK is therefore the only available modulation format. In addition, the CCPCH is transmitted as close to the center frequency as possible. CCPCH is transmitted exclusively on the 72 active subcarriers centered on the DC subcarrier. Control information is mapped to resource elements (k, l) where k refers to the OFDM symbol withinthe slot and l refers to the subcarrier. CCPCH symbols are mapped to resource elements in increasing order of index kfirst, then l.

11.4 Physical Signals

Physical signals use assigned resource elements. However, unlike physical channels, physical signals do not conveyinformation to/from higher layers.

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There are two types of physical signals:

• Reference signals used to determine the channel impulse response (CIR)• Synchronization signals which convey network timing information

Reference Signals

Reference signals are generated as the product of an orthogonal sequence and a pseudo-random numerical (PRN)Sequence. Overall, there are 510 unique reference signals possible. A specified reference signal is assigned to each cell within a network and acts as a cell-specific identifier.

When a reference signal is transmitted from one antenna port, the other antenna ports in the cell are idle.Reference signals are sent on every sixth Subcarriers. CIR estimates for Subcarriers that do not bear reference signals are computed via interpolation. Changing the Subcarriers that bear reference signals by pseudo-random frequency hopping is also under consideration.

Synchronization Signals

Synchronization signals use the same type of pseudo-random orthogonal sequences as reference signals. These are

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classified as primary and secondary synchronization signals, depending how they are used by UE during the cellsearch procedure.

Both primary and secondary synchronization signals are transmitted on the 72 subcarriers centered around the DC subcarrier during the 0th and 10th slots of a frame (recall there are 20 slots within each frame).

11.5 Transport Channels

Transport channels are included in the LTE PHY and act as service access points (SAPs) for higher layers.

Downlink Transport channels are:

•Broadcast Channel (BCH)

•Downlink Shared Channel (DL-SCH)

•Paging Channel (PCH)

•Multicast Channel (MCH)

12. Limitations of 3G

Although the concept of 3G communications shows much promise, there are still limitations that must be addressed.

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One major limitation is operating area. Although 2G networks are becoming more ubiquitous, there are still many areas notserved. Rural areas and many buildings in metropolitan areas are not being served well by existing wireless networks. Thislimitation of today’s networks will carry over into future generations of wireless systems.

The hype that is being created by 3G networks is giving the general public unrealistic expectations of always on,always available, anywhere, anytime communications. The public must realize that although high-speed data communications will be delivered, it will not be equivalent to the wired Internet – at leastnot at first. If measures are not taken now to correct perception issues, when 3G and later 4G services are deployed, there may be agreat deal of disappointment associated with the deployment of the technology, and perceptions could become negative. If thiswere to happen, neither 3G nor 4G may realize its full potential.

Another limitation is cost. The equipment required to implement a next generation network is still very expensive. Carriers and providers have to plan carefully to make sure that expenses are kept realistic.

One technique currently being implemented in Asian networks is a Pay-Per-Use model of services. This model will be difficult toimplement in the United States, where the public is used to a service-for-free model (e.g., Internet).

13. Conclusions

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3G networks may eventually deliver on all the promises. At times, it seems that technological advances are being made on a daily basis. These advances will make high speed data/voice-over-Internet-protocol (VoIP) networks a reality.

In the meantime, it is important for industry to develop a strong 3G offering that is palatable for the general public. Equally as important, industry must ensure that expectations are realistic and that services meet and exceed those expectations. If all goes according to what the industry envisions, it may be sooner, rather than later that we will see wireless communications evolve. This evolution will give the general public as well as the public safety community amazing functionality from the convenience of a single handhelddevice.

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