T325: Technologies for digital media

T325: Technologies for digital media Block III – Part 4: Better and Beyond

Arab Open University - Spring 2012 1


Radio propagation – Simplest scenario

• An antenna radiating uniformly in all directions in free space

• Signal strength is independent of the direction from the transmitter

• Attenuation is independent of the transmission frequency• Attenuation follows an inverse square law; that is, signal

strength at a radius r from the transmitter is inversely proportional to 1/r2

In the mobile world, regrettably, none of these apply !!


Radio propagation – Real world conditions

• Attenuation is not independent of the direction from the transmitter• Random pattern of obstacles between the user and the base

station at any given time

• Attenuation depends on frequency: the higher the frequency the greater the attenuation.


Radio propagation – Real world conditions

• The total signal received at the mobile is the sum of the two rays • If the mobile is sufficiently far away from the base station, the

signal strength diminishes as the fourth power of distance from the base station (1 / r4)

Fading

• Fading is signal attenuation which occurs in addition to the fourth-power law attenuation, and which varies with distance from the base station

• There are two basic types of fading• Slow fading: relatively large changes of distance are

required to produce significant variations of received signal strength.

• Fast fading: variations happen over a much shorter range relatively small changes of distance produce significant variations of signal strength.

• Slow and Fast refer to the relationship between fading and change of distance from the base station, not with time.


Fading

Slow Fading Fast fading

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Arab Open University - Spring 2012


Fading

• To model the effects of slow fading, the placing of obstacles and reflecting surfaces between the base station and the mobile can be taken as essentially random in most environments

• The received power varies with distance randomly relative to the expected value predicted by the fourth-power model, following a normal (Gaussian or ‘bell curve’) statistical distribution referred to as log-normal fading

Fading

• For slow fading, the signal typically varies from a maximum to a minimum over a distance of 2-3 m or more


Fading

• Fast fading is caused by multiple paths between the transmitter and the receiver

• The attenuation will vary from a minimum to a maximum over distances of order one-half of a wavelength

• Unlike slow fading, fast fading can also occur when one of the paths is the direct line of sight from the transmitter to the receiver


• In fast fading, the received signal amplitude (rather than the power) follows a statistical distribution known as a Ricean distribution

• In the special case where there is no line of sight, the received signal follows the Rayleigh distribution

Gaussian distributionArab Open University - Spring 2012 10

Ricean distributionArab Open University - Spring 2012 11

Rayleigh distributionArab Open University - Spring 2012 12

Activity 4.2

• Given a downlink frequency centered on 2142.5MHz, calculate the distance over which you would expect fast fading to change from a minimum to a maximum.

(Take the speed of light to be 3*108m*s-1)



Propagation issues – Doppler Shift

• Doppler Shift: comes into play as the mobile speed increases

• The speed v is taken to be positive if the mobile receiving device is moving towards the source (giving an increase in frequency) and negative if it is moving away (giving a decrease in frequency).


Propagation issues – Activity

• A mobile device is being used by someone sitting on a high-speed train moving at 300 km/h and the carrier frequency is nominally 2142.5 MHz.

• Calculate the Doppler shift in frequency, both as a frequency shift and as a percentage of fc, for the case where the mobile is moving directly towards the base station from which it is receiving a signal.

Overcoming radio propagation issues

• Power control to combat attenuation and fading• Signal processing techniques to combat

channel limitations• Coding and interleaving to detect and

correct errors introduced during transmission


Overcoming radio propagation issues:Power control mechanism

• In 3G W-CDMA systems, each user appears as noise to the other users

• A fast power control mechanism is required• On the uplink : to ensure that the received power at the Node

B is approximately the same for all mobiles • On the downlink: to keep the power received at the mobile

constant as the mobile moves out towards the edge of the cell

• The power control mechanisms adjust the power 1500 times per second, which is sufficiently fast to track and compensate for fast fading at low mobile speeds.

• At higher mobile speeds, coding and interleaving mechanisms are used to detect and correct burst errors arising from fast fading.


Overcoming radio propagation issues: Signal processing techniques

• The equalization technique is based on the use of digital signal processing to compensate for distortions introduced by the channel.1. Transmitter sends a known signal (pilot channel) through

the radio channel alongside the signaling and user traffic

2. The receiver can recover this signal and compare it with a stored copy of the signal before transmission.

3. The receiver can then configure a local filter, called an equaliser, apply it to the known signal and adjust it until it cancels out the distortion introduced by the radio channel.

4. Filter can then be applied to the user and signaling channels to cancel out, or at least mitigate, distortion from the radio channel.


Overcoming radio propagation issues: Coding and interleaving

• Applying coding techniques (of the error-correcting kind) to correct as many errors as possible, even in the presence of fading

• Interleaving used to handle burst errors


Quality of service and scheduling

• Importance of delay factor in certain communication services (telephony and video)

• Delays arise because packets can pass through several nodes on their journey, and at each node the packet is likely to join a queue

• Scheduling algorithms are used to determine which packet is sent next


Quality of service and scheduling

• The simplest scheduling method is the so-called round-robin method.• Offers the communication resource to each contender in turn. • It is fair, but does not make optimal use of the system’s

capacity. • Many more sophisticated algorithms have been devised to

optimize the use of the system capacity, or to privilege certain types of traffic over others.

• The term quality of service (QoS) refers to factors such as: • delay time• variability of delay time and • packet loss in the delivery of packets


QoS differentiation

• When the system load gets higher, it becomes important to prioritize the different services according to their requirements.

• QoS differentiation: how to make applications work in a loaded multiservice system

• Four traffic classes are defined in 3GPP• Conversational• Streaming• Interactive• Background


QoS differentiationArab Open University - Spring 2012 23

URL: http://www.umtsworld.com/technology/qos.htm

BEYOND 3G: LTE, WIMAX AND THE ALL-IP CORE


From Circuit Switching to IP

Circuit switching

+ Network can guarantee data delivery within a specified time period

- Inflexible and wasteful of bandwidth: 50% of the reserved bandwidth is typically wasted in a voice call (people don’t speak at the same time)

Packet switching

+ more efficient and flexible in the use of bandwidth

- It is not possible to control the packet transmission delay: less time critical bursty streams can occupy network bandwidth at the expense of time critical data streams


From Circuit Switching to IP

• Overcoming packet switching problems through• IP traffic engineering • reserve bandwidth for time-critical flows • prevent these from being disrupted by bursty traffic

• Real-Time Protocol (RTP)• Runs over IP • Can timestamp packets at source so that when they arrive

at the destination they can be reassembled into a smooth data flow without variable delays, or jitter, between the packets.

• Particularly useful for voice or video traffic


From Circuit switching to IP

• Mobile networks have been quietly migrating to an all-IP Core network Signalling and user traffic are carried over IP

• The standards bodies have been making determined efforts to redesign the radio network in order to move to a fully packet-based architecture which makes full use of IP throughout.


3G LTE and WiMAX

• The system known as 3G LTE is envisaged as the ‘beyond 3G’ mobile technology for cellular operators for voice and data.

• Mobile WiMAX is a competitor of 3G LTE• WiMAX exists in two incompatible standards: one for fixed

or nomadic applications and another for mobile.

• The WiMAX standard has emerged from a consortium of companies that is more associated with Wi-Fi and nomadic wireless communications than with mobile.

• 3G LTE is intended to be an all-IP network in which every element has an IP address.


3G LTE Requirements

• Instantaneous downlink peak data rate of 100Mbit/s within a 20MHz downlink spectrum allocation (5bit/s per Hz)

• Instantaneous uplink peak data rate of 50 Mbit/s within a 20MHz uplink spectrum allocation (2.5 bit/s per Hz)

• ‘wake-up’ time for a mobile in idle mode to start of data transmission to be less than 100ms

• At least 200 users per cell to be supported in the active state for spectrum allocations up to 5MHz

• Maximum mobile speed to be 350 km/h (500 km/h for certain frequency bands)

• Network to be capable of operating in spectrum allocations of different sizes, including 1.25, 1.6, 2.5, 5, 10, 15 and 20 MHz in both the uplink and downlink

• Network to be able to coexist and interwork with 3G radio technology in the same geographical area, and coexist with GSM and GPRS radio systems in the same geographical area.


3G LTE Architecture

• 3G LTE vs. 3G (release 99)


3G vs. 3G LTE : Terminology

3G architecture

• UTRAN• Core Network

• Node B

3G LTE architecture

• Evolved UTRAN (eUTRAN)• Evolved Packet Core (EPC) or

System Architecture Evolution (SAE)

• Evolved Node B (eNode B)


Radio Access Network

• 3G LTE does not support circuit-switched technology. • Architecture is simplified considerably in comparison with

the original 3G design.• With circuit-switched technology based on time-division

multiplexing links and time slots, the key constraint is that the MSC is the only network element capable of switching between links and time slots.

• This imposes an architecture in which lines fan out from the MSC with no possibility of direct links between the base stations or between base station controllers.

• The use of ATM in 3G brought the ability to establish direct links between the radio network controllers, but it was not cost effective to do this at the base station level.



• With IP-based packet switching, any network element can communicate with any other element in a flexible way without the need to set up any form of connection.

• Reduction in the cost of complex electronics over the years now makes it feasible to carry out much more processing in the base station than when the first version of 3G was designed.



• E-UTRAN is drastically simplified in comparison with the 3G UTRAN

• It contains only one type of element: evolved NodeB (eNodeB), otherwise known as the radio base station.

• The eNode Bs can talk to each other on a peer-to-peer basis and the RNC has disappeared

• Most of the RNC’s functions, including handling the radio access network protocol stack, have been transferred to the eNode B, with the rest being handed over to the core network

• Mobile WiMAX has a similar architecture, in which the base station is known, rather less mysteriously, as the ‘base station’.



• The radio interface for LTE is based on OFDMA technology.

• On the downlink, data streams are allocated to sets of subcarriers

• The uplink uses an approach closely related to OFDMA, called single carrier frequency-division multiple access (SC-FDMA) to reduce the level of instantaneous variation in transmitter power levels encountered with plain OFDMA, which would otherwise increase the cost of the handset and reduce the battery life.


OFDMA and SCFDMA

• In OFDM systems• Data streams on subcarriers are not correlated in general• The combined output can vary from low values (when data

streams cancel) to high values (when data streams add up)• Peak power requirement can be high relative to the average

power requirement requires an expensive high dynamic range linear amplifier to handle the full signal range.

• In SC-FDMA• Set of data streams is transformed at the transmitter to

spread the data across a group of subcarriers rather than being allocated to individual subcarriers

• Reducing the peak to average power ratio to the same level as if the data were transmitted on a single carrier.



• WiMAX also uses OFDMA as the transmission technology, but in this case it was decided to live with OFDMA for the uplink rather than use SC-FDMA.

• There are many similarities between the 3G LTE and WiMAX approaches

• There have been calls for the two sets of standards to be merged into one global standard in the interests of economies of scale.


Core Network

• The Evolved Packet Core (EPC) network is also simplified relative to the original 3G design

• The circuit-switched part has disappeared completely (appears only in the interfaces to legacy circuit-switched networks)

• Voice calls and data transfers are now handled in the same way, as specific instances of a more generalized idea called a multimedia session.

• Multimedia sessions are set up by a new part of the network called the integrated multimedia subsystem (IMS)


Core Network

• The principal elements of the EPC are:• Serving SAE Gateway (analogous to the

SGSN)• Packet Data Network (PDN) SAE Gateway

(analogous to the GGSN)• Mobility Management Entity (MME) • Home Subscriber Server (HSS)


Core Network

• The PDN SAE gateway is the fixed node in the core network which is used to interface to external data networks, in a similar way to the GGSN in GPRS and 3G.


Core Network

• The serving SAE gateway is used to track the mobile as it moves around the radio network, as with the SGSN in GRPS and 3G. • User sessions are handed over from one serving SAE

gateway to another as the mobile moves into the area of coverage of a new serving SAE gateway


Core Network

• The HSS has evolved from the HLR (home location register) and is used to provide similar functions in the new all-IP network.


Core Network

• The MME is used to handle authentication on behalf of the HSS, to handle mobile to network sessions, and to maintain a record of the location of the mobile when it is in idle mode (that is, when not sending or receiving data).


Key protocols

• Key signaling protocols for LTE mobility management


Key protocols

• Same protocol stack between the user equipment and the radio access network as in 3G W-CDMA.

• Following the disappearance of the RNC, the end point for the RRC/RLC/MAC layers is now the eNode B

• The GPRS mobility management (GMM) protocol in the 3G non-access stratum (NAS) layer is replaced by the EPS mobility management (EMM) protocol, which performs similar functions to GMM.

• The end point of the EMM protocol for mobility management purposes is now the mobility management entity.



3G LTE

3G

Key protocols

• The main difference on the core network side is that ATM has now disappeared. • All devices have an IP address and are connected to a

standard fixed IP network.

• Signalling messages are transported over the core IP network using a new protocol called the stream control transport protocol (SCTP). • Alternative to TCP• Provides reliable delivery of data streams across an IP

network. • Capable of handling multiple data streams in parallel,

whereas TCP can handle only one data flow per session.


Integrated Multimedia Subsystem (IMS)

• The integrated multimedia subsystem (IMS) has been developed separately from 3G LTE.

• It can be used with• the packet domain of a 3G network• the 3G LTE network and • And even a fixed IP-based network

• IMS is an entirely IP-based system: all the network elements have IP addresses.

• In the IMS world, everything is a multimedia session, even a voice call.

• Sessions are set up using Session Initiation Protocol (SIP).



• Key elements of the IMS with interface to LTE



• Setting up sessions in IMS is the primary function of the call/session control functions.

• The call/session control functions interact with the HSS, which, has evolved from the HLR.

• In addition, SIP application servers are used to provide support services such as announcements and voice messaging.



• The key advantage of using IMS is that all services can be handled in the same way, as specific instances of multimedia sessions, which promises to simplify the process of introducing new services, cutting costs and time to market.

• Because IMS is IP-based, however, it is not able to interwork directly with circuit-switched networks.

• The inability of IMS to interwork directly with circuit-switched networks means that it is necessary to provide an interface to circuit-switched networks: media gateway.



• Media Gateway terminates incoming circuit-switched calls and converts them into SIP-based multimedia sessions, and carries out the reverse operation in the other direction.



• The combination of the E-UTRAN, the EPC and the IMS provides an end-to-end IP-based network which will • Make much more efficient use of bandwidth• Eliminate the need to run circuit-switched and packet domains

side by side• Streamline the process of introducing new services.

• All this can be expected to: • Reduce significantly the cost of providing a mobile network

while enabling mobile operators to provide a better service to their customers.



Limits to mobile broadband

• Shannon’s equation sets a limit for the rate at which user data can be transmitted for a given signal bandwidth and signal-to-noise ratio.• Given a particular amount of spectrum and

a signal-to-noise ratio, any wireless system is subject to the same limitation on its maximum useful data rate.

Arab Open University - Spring 2012

Limits to mobile broadband

• Table 4.2 is derived from Shannon’s equation. It shows, for a range of signal-to-noise ratios (in the left and middle columns), the maximum useful data rate that can be transmitted per megahertz of bandwidth in the right-hand column.

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Limits to mobile broadband - Activity

• For a signal-to-noise ratio of 10000:1, what is the theoretical channel capacity per megahertz?

• Hence, for a bandwidth of 20MHz, what is the highest theoretical data rate if the signal-to-noise ratio is 10000:1?

• WiMAX and LTE can both use a maximum of 20 MHz of spectrum, so the answer is 260 Mbit/s is the upper limit for both systems for the quoted signal-to-noise ratio of 10000:1.

• However, typical rates for users could be expected to be significantly less than this.


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T325: Technologies for digital media