2.5.2.1 Error Control Techniques

2.5.2.1 Error Control Techniques

2.5.2.1 Error Control Techniques:

1-Automatic Repeat request (ARQ):

            The transmitter sends a packet of the encoded message and the receiver checks it. If no error detected an acknowledgement(ACK) is sent by the receiver to sent another package. But if an error detected no acknowledgement (NACK) sent by the receiver and the transmitter will retransmit the packet again after a certain period of time.

2-Forward Error Correction(FEC):

            The transmitter encodes the data with an error correcting code (ECC).The receiver here doesn’t send any acknowledgement to the transmitter. But It decodes the received message to the most likely data.

3-Hybrid ARQ (H-ARQ) (ARQ+FEC):

            It is a combination of both the previous techniques. The minor errors are corrected by the ECC without any retransmission request and major errors are requested to be retransmitted again.

2.5.2.2 Error detection schemes:

1-Repetition codes:

            A repetition code is a coding scheme that repeats the bits across a channel to achieve error-free communication. Given a stream of data to be transmitted, the data is divided into blocks of bits. Each block is transmitted some predetermined number of times. If one of the blocks was different  the decoder will detect that an error occurred.

2-Parity bits:

            A parity bit is a bit that is added to a group of source bits to ensure that the number of set bits (i.e., bits with value 1) in the outcome is even or odd. It is a very simple scheme that can be used to detect single or any other odd number of errors in the output. But an even number of flipped bits will make the parity bit appear correct even though the data is erroneous.

3-Checksums:

            A checksum of a message is a modular arithmetic sum of message code words of a fixed word length (e.g., byte values). The sum may be negated by means of a ones'-complement operation prior to transmission to detect errors resulting in all-zero messages.

4-Cyclic redundancy checks (CRCs):

            A cyclic redundancy check (CRC) is a single-burst-error-detecting cyclic code and non-secure hash function designed to detect accidental changes to digital data in computer networks.

            It is characterized by specification of a so-called generator polynomial, which is used as the divisor in a polynomial long division over a finite field, taking the input data as the dividend, and where the remainder becomes the result.

2.5 Important definitions for LTE Communication systems

2.5 Important definitions for LTE Communication systems

2.5 Important definitions for LTE Communication systems:

2.5.1 Channel estimation:

The radio channels in mobile radio systems are usually multipath fading channels, which are causing inter-symbol interference (ISI) in the received signal.

           

            To remove ISI from the signal, many kinds of equalizers can be used. Detection algorithms based on trellis search (like MLSE or MAP) offer a good receiver performance, but still often not too much computation. Therefore, these algorithms are currently quite popular.

           

            However, these detectors require knowledge on the channel impulse response (CIR), which can be provided by a separate channel estimator. Usually the channel estimation is based on the known sequence of bits, which is unique for a certain transmitter and which is repeated in every transmission burst.

            Thus, the channel estimator is able to estimate CIR for each burst separately by exploiting the known transmitted bits and the corresponding received samples.

Preamble & pilot:

There are two ways to transmit training symbols:

1         1)    Preamble                  2) Pilots tones.

            Channel estimation in MIMO-OFDM systems can be performed in a variety of ways , but it is typical to use the preamble for synchronization & initial channel estimation and the pilot tones for tracking the time varying channel in order to maintain accurate channel estimates.

2.5.2 Channel Coding:

            The engineering problem treated by the subject of error-control codes is that of protecting digital data against the errors that occur during transmission or storage. The storage and transmission of digital data lies at the heart of modern computers and telecommunications.

            If data is corrupted in storage or transmission, the consequences can range from mildly annoying to disastrous. Many error-protection techniques have been developed based on a rich mathematical theory, and the rapid advances in digital integrated circuitry have made possible the implementation of these algorithms.

            The channel coding is considered as an important signal processing operation which provides a reliable transmission of digital information over channel.

            It is used mainly to minimize the effect of NOISE by facilitate two basic operations, Error detection and Error correction.

            Coding is achieved by adding properly designed ―controlled‖ redundant bits to each message or make an operation on the message to get it encoded with some methods.

            These redundant bits (digits) are used for detecting and/or correcting transmission errors, in other words for protecting data against channel impairments (e.g., noise, fading, interference).

2.3.3.4 Fourth Generation (4G)

2.3.3.4 Fourth Generation (4G)

 4G is the short name for fourth-generation wireless, the stage of broadband mobile communications that will supercede the third generation (3G ).Carriers that use orthogonal frequency-division multiplexing (OFDM) instead of time division multiple access (TDMA) or code division multiple access (CDMA) are increasingly marketing their services as being 4G, even when their data speeds are not as fast as the International Telecommunication Union (ITU) specifies.

According to the ITU, a 4G network requires a mobile device to be able to exchange data at 100 Mb/sec. A 3G network, on the other hand, can offer data speeds as slow as 3.84 Mb/sec.

2.1 Long Term Evolution (LTE)

2.1 Long Term Evolution (LTE)

2.1 Introduction for Long Term Evolution (LTE):

Long Term Evolution (LTE) is a 4G wireless broadband technology developed by the Third Generation Partnership Project (3GPP), an industry trade group.

3GPP engineers named the technology "Long Term Evolution" because it represents the next step (4G) in a progression from GSM, a 2G standard, to UMTS, the 3G technologies based upon GSM.

LTE provides significantly increased peak data rates, with the potential for 100 Mbps downstream and 30 Mbps upstream, reduced latency, scalable bandwidth capacity, and backwards compatibility with existing GSM and UMTS technology, Future developments to could yield peak throughput on the order of 300 Mbps.

The upper layers of LTE are based upon TCP/IP, which will likely result in an all-IP network similar to the current state of wired communications.

LTE will support mixed data, voice, video and messaging traffic, and it  uses OFDM (Orthogonal Frequency Division Multiplexing) , and in later releases, MIMO (Multiple Input Multiple Output) antenna technology similar to that used in the IEEE 802.11n wireless local area network (WLAN) standard.

The higher signal to noise ratio (SNR) at the receiver enabled by MIMO, along with OFDM, provides improved coverage and throughput, especially in dense urban areas. 

2.2 Why LTE Matters:

Smaller, faster and more portable are persistent trends in the digital world, the computing power that once required a room sized machine can now be found in laptops and handheld devices, and you can put stacks of CDs in your pocket in the form of a portable media player.

The same trends affect the cellular market, and pose some interesting engineering challenges.

Cellular long term evolution (LTE) is the next step forward in cellular 3G services, with an expected market rollout in the 2009 time frame, LTE technology is a based on a 3GPP standard that provides for a downlink speed of up to 100 megabits per second (Mbps) and an uplink speed of up to 50 Mbps.

             

              With multiple antennas, speeds can reach more than 320 Mbps on the downlink. Fixed wireless and wired standards are already approaching or achieving 100 Mbps or faster, and LTE is a way for cellular communications to operate at that high data rate.

2.3 Steps towards LTE :

 The design target of the first version of the LTE system was being finalized in June 2006. It aimed to solve some of the old systems problems and reach more capabilities. The delays have to be reduced in terms of both connection establishment and transmission latency.

The user data rates are also increased. The cell edge data rates have to be increased with more simplification for the hand over problem. Flexible band width has to be valid to improve the spectral efficiency.

The system must be capable with the old systems to simplify the network architecture. Finally it aimed to have lower power consumption for the mobile terminal.

 

2.4 LTE Capabilities

2.4 LTE Capabilities

2.4 System Capabilities: 

Improved system performance compared to existing systems is one of the main requirements from network operators, to ensure the competitiveness of LTE and hence to arouse market interest. In this section, we highlight the main performance metrics used in the definition of the LTE requirements and its performance assessment.

            Table summarizes the main performance requirements to which the first release of LTE was designed.

Down link	Absolute requirements	Comment
Peak transmission rate	>100Mbps	20MHz BW , FDD , 2x2 spatial multiplexing,
Peak spectral efficiency	>5bps/Hz
Average cell spectral efficiency	1.6-2.1 bps/Hz /cell	2x2 spatial multiplexing, IRC receiver,
Cell edge spectral efficiency	0.04 -0.06 bps/Hz /user	As above, Assuming 10 users/cell,
Broad cast spectral efficiency	>1bps/ Hz	Dedicated carrier for broad cast mode,

up link	Absolute requirements	comment
Peak transmission rate	>50Mbps	20MHz BW , FDD , 2x2 spatial multiplexing,
Peak spectral efficiency	>2.5bps/Hz
Average cell spectral efficiency	0.66-1 bps/Hz /cell	2x2 spatial multiplexing, IRC receiver,
Cell edge spectral efficiency	0.02 -0.03 bps/Hz /user	As above, Assuming 10 users/cell,

System	Absolute requirements	comment
User plane latency	<10ms
Connection setup latency	<100ms	Idle state is the active state
Operating band width	1.4-20 MHz	Initial requirement started at 1.25MHz with scalable band width
Cell coverage	Up to 100 km	It is found in the standard but not  achieved in release 8
VOIP capacity	NMGN preferred target >session /MHz /cell

                The requirements shown in Table are discussed and explained in more detail below.

2.4.1 peak rate and peak spectral efficiency:

For marketing purposes, the first parameter by which different radio access technologies are usually compared is the peak per-user data rate which can be achieved. This peak data rate generally scales according to amount of spectrum used. The peak rate can be defined as the maximum throughput per user assuming the whole bandwidth being allocated to a single user with the highest modulation and coding scheme and the maximum number of antennas supported .The target peak data rates for downlink and uplink in the LTE system were set at 100 Mbps .and 50 Mbps respectively within a 20 MHz bandwidth, corresponding to respective peak spectral efficiencies of 5 and 2.5 bps/Hz.

2.4.2 Cell throughput and spectral efficiency:

Performance at the cell level is an important criterion, as it relates directly to the number of cell sites that a network operator requires, and hence to the capital cost of deploying the system. For LTE, it was chosen to assess the cell level performance with full-queue traffic models (i.e. assuming that there is never a shortage of data to transmit if a user is given the opportunity) and a relatively high system load, typically 10 users per cell.

The requirements at the cell level were defined in terms of the following metrics:

• Average cell throughput [bps/cell] and spectral efficiency [bps/Hz/cell].

• Average user throughput [bps/user] and spectral efficiency [bps/Hz/user].

• Cell-edge user throughput [bps/user] and spectral efficiency [bps/Hz/user]. The metric used for this assessment is the 5-percentile user throughput, obtained from the cumulative distribution function of the user throughput.

            The original requirements for the cell level metrics were only expressed as relative gains compared to the Release 6 reference baseline. The absolute values provided in Table are based on evaluations of the reference system performance that can be found in downlink and uplink respectively.

2.4.3 Voice capacity:

It is important to set system capacity requirements for such services – a particular challenge in fully packet-based systems like LTE which rely on adaptive scheduling.

            The system capacity requirement is defined as the number of satisfied VoIP users, given a particular traffic model and delay constraints. The details of the traffic model used for evaluating LTE can be found in Here, a VoIP user is considered to be in outage (i.e. not satisfied) if more than 2% of the VoIP packets do not arrive successfully at the radio receiver

within 50 ms and are therefore discarded.

           

            This assumes an overall end-to-end delay (from mobile terminal to mobile terminal) below 200 ms. The system capacity for VoIP can then be defined as the number of users present per cell when more than 95% of the users are satisfied.

2.4.4 Mobility and cell range:

In terms of mobility, the LTE system is required to support communication with terminals moving at speeds of up to 350 km/h, or even up to 500 km/h depending on the frequency band.

These targets are to be achieved by the LTE system in typical cells of radius up to 5 km, while operation should continue to be possible for cell ranges of up to 100 km to enable wide-area deployments.

2.4.5 Broadcast mode performance:

Although not available in the first release due to higher prioritization of other service modes, LTE is required to integrate an efficient broadcast mode for high rate Multimedia Broadcast/Multicast Services (MBMS) such as Mobile TV, based on a Single Frequency Network mode of operation. This mode is able to operate either on a shared carrier frequency together with unicast transmissions, or on a dedicated broadcast carrier.

To ensure efficient broadcast performance a requirement was defined for the dedicated carrier case. In broadcast systems, the system throughput is limited to what is achievable for the users in the worst conditions.

Consequently, the broadcast performance requirement was defined in terms of an achievable system throughput (bps) and spectral efficiency (bps/Hz) assuming a coverage of 98% of the nominal coverage area of the system.

This means that only 2% of the locations in the nominal coverage area are in outage – where outage for broadcast services is defined as experiencing a packet error rate higher than 1%.This broadcast spectral efficiency requirement was set to 1 bps/Hz .

2.4.6 User plane latency:

 User plane latency is an important performance metric for real-time and interactive services.

It is defined as the average time between the first transmission of a data packet and the reception of a physical layer Acknowledgement (ACK). 

            The LTE system is also required to be able to operate with an  IP-layer one-way data-packet latency across the radio access network as low as 5 ms in optimal conditions.

            However, it is recognized that the actual delay experienced in a practical system will be dependent on system loading and radio propagation conditions.

2.4.7 Control plane latency and capacity:

In addition to the user plane latency requirement, call setup delay is required to be significantly reduced compared to existing cellular systems. This not only enables a good user experience but also affects the battery life of terminals, since a system design which allows a fast transition from an idle state to an active state enables terminals tospend more time in the low-power idle state.

Control plane latency is measured as the time required for performing the transitions between different LTE states .The LTE system is required to support transision from idle to active in less than 100 ms.

The LTE system capacity is dependent not only on the supportable throughput but also on the number of users simultaneously located within a cell which can be supported by the control signalling.

For the latter aspect, the LTE system is required to support at least 200 active-state users per cell for spectrum allocations up to 5MHz, and at least 400 users per cell for wider spectrum allocations;

Only a small subset of these users would be actively receiving or transmitting data at any given time instant, depending, for example, on the availability of data to transmit and the prevailing radio channel conditions. An even larger number of non-active users may also be present in each cell, and therefore able to be paged or to start transmitting data with low latency.

2.4.8 Deployment cost and interoperability:

Besides the system performance aspects, a number of other considerations are important for network operators. These include reduced deployment cost, spectrum flexibility and enhanced interoperability with legacy systems – essential requirements to enable deployment of LTE networks in a variety of scenarios and to facilitate migration to LTE.

2.4.9 Terminal complexity and cost:

A key consideration for competitive deployment of LTE is the availability of low-cost terminals with long battery life, both in stand-by and during activity. Therefore, low terminal complexity has been taken into account where relevant throughout the LTE system, as well as designing the system wherever possible to support low terminal power consumption.

2.4.10  Network architecture requirement:

LTE is required to allow a cost-effective deployment by an improved radio access network architecture design including:

• flat  architecture consisting of just one type of node, the base station, known in LTE as the  eNodeB.

• effective protocols for the support of packet-switched services;

• open interfaces and support of multivendor equipment interoperability;

• efficient mechanisms for operation and maintenance, including self-optimization functionalities.