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ABSTRACT: - This paper provides how Information Science and its principle work with technologies being considered for Long Term Evolution-Advanced (LTE-Advanced) in area of Air Interface. First, the Downlink Air Interface using OFDMA & Uplink Air Interface using SC-FDMA is described in term of performance requirements, operation, main characteristics with Doppler Effect & loss of Orthogonality. Then, Air Interface Technology for LTE-Advanced are explained, together with possible improvements, their associated challenges, and some approaches that have been considered to tackle those challenges.
ARTICLE INFO: - Keywords: LTE, LTE-Advanced, 4G, OFDMA, SC-FDMA, CDMA.
Consumers demand more from their technology. The latest technology purchase must have new features. With the advent of the Internet, the most-wanted feature is better, faster access, storage and retrieval of information. This paper is focused on the specifications of future generations and latest technologies to be used in future wireless mobile communication networks. The goal is: long-range, high-speed wireless, which for the purposes of this report will be called 4G, for fourth-generation wireless system.
3G systems such as high speed packet access (HSPA) provide up to round 15-20 Mbps for DL and about 5-10 Mbps for UL. The pre-4G standard is a step in the direction of LTE Advanced, a 4G of radio technologies designed to enhance the coverage as well as speed of mobile networks.
LTE offers peak data rate of 100 Mbps for DL, 50 Mbps for UL channel and also support 1.4-20 MHz scalable bandwidth. It provides low-cost and low-complexity workstation.
LTE can support up to 500 km/h of vehicular speed. LTE provides the following benefit to network operators like Low latency, High network throughput, Plug and Play architecture, Improve path from 3G networks, All-IP packet based network, low operating costs.
2. Spectrum and bandwidth management
In order to meet the requirements of IMT Advanced as well as those of 3GPP operators, LTE-Advanced considers the use of bandwidths of up to 100 MHz in the following spectrum bands .
â€¢ 450-470 MHz band (identified in WRC-07 to be used globally for IMT systems).
â€¢ 698-862 MHz band (identified in WRC-07 to be used in Region 21 and nine countries of Region 3).
Region 1: Europe, Africa, the Middle East west of the Persian Gulf including Iraq, the former Soviet Union, and Mongolia. Region 2: Americas, Greenland, and some of the eastern Pacific Islands. Region 3: most of non-former-Soviet-Union Asia, east of and including Iran, and most of Oceania.
â€¢ 790-862 MHz band (identified in WRC-07 to be used in Regions 1 and 3).
â€¢ 2.3-2.4 GHz band (identified in WRC-07 to be used globally for IMT systems).
â€¢ 3.4-4.2 GHz band (3.4-3.6 GHz identified in WRC-07 to be used in a large number of countries).
â€¢ 4.4-4.99 GHz band.
3. LTE Downlink Air Interface Management
In order to generate Downlink Air Interface orthogonal frequency-division multiplexing Access (OFDMA) Transmission technique is used. For signal generation Fast Fourier Transformation is used as basic Information Science.
Two characteristics are required to define a signal:
Time domain: It represents how long the symbol lasts on air or any other medium.
Frequency domain: It represents the spectrum needed in terms of bandwidth.
Fast Fourier Transform (FFT) and the Inverse Fast Fourier Transform (IFFT) allow moving between time and frequency domain representation and it is a fundamental block in an OFDMA system .
OFDM signals are generated using the IFFT
Fig: - Fast Fourier Transform Rectangular Pulse
Implementation of signal is simple & easier:
No need of complex filter system to detect such pulses (simplifies receiver design) and to generate them.
The pulse has a clearly defined duration:
It simplifies handling of inter-symbol interference in case of multi-path propagation environments.
Fig: - Signal in time domain
The rectangular pulse allocates a lot of spectrum in the frequency domain. Frequency fs = 1/Ts. This will be important in however exactly at multiples of the OFDM the spectral power density has null points.
Fig: - Signal in Frequency domain
OFDM Signal Generation:
Based on 3GPP Formula
Using orthogonal subcarriers it transmits hundreds or even thousands of separately modulated radio signals spread across a wideband channel .
Total transmission bandwidth
15 kHz in LTE: fixed
The peak (centre frequency) of one subcarrier â€¦
â€¦intercepts the 'nulls' of the neighbouring subcarriers
Fig: - Subcarrier Transmission
Data is sent in parallel across the set of subcarriers, each subcarrier only transports a part of the whole transmission.
The throughput is the sum of the data rates of each individual (or used) subcarrier while the power is distributed to all subcarriers.
FFT (Fast Fourier Transform) is used to create the orthogonal subcarriers. The number of subcarriers is determined by the FFT size (by the bandwidth).
Fig: - Symbol Transformation into Wave form
Doppler in OFDM and Loss of Orthogonality
Doppler Effect (shift): Change in frequency of a wave due to the relative motion of source and receiver.
Symbols are distorted in the time domain
Shifts frequency makes symbol detection inaccurate.
MSs moving at high speed are not suitable for MCS schemes with high number of bits per subcarrier (high data rates).
Doppler only impacts SINRs at the higher range i.e. > 20dB.
It reduces orthogonality.
The frequency domain subcarriers are shifted causing inter-carrier interference (ICI).
The peaks of signals and nulls of interferers will not coincide.
Fig: - Doppler Effect in OFDMA transmission
OFDMA is an extension of OFDM technique to allow multiple user transmissions and it is used in other systems like Wi-Fi, DVB and WiMAX .
OFDMA transmits data in parallel across multiple subcarriers.
E.g. OFDMA: 6 modulation symbols (01, 10,11,01,10 and 10) are transmitted per OFDMA symbol, one on each subcarrier.
Fig: - OFDMA Symbol Generation
Channel bandwidth: OFDMA bandwidths ranging from 1.4 MHz to 20 MHz
Data subcarriers: They vary with the bandwidth
72 for 1.4MHz to 1200 for 20MHz
Fig: - Data Subcarriers Bandwidth
Frame duration: 10ms created from slots and sub frames.
Sub frame duration (TTI): 1 ms (composed of 2x0.5ms slots).
Subcarrier spacing: Fixed to 15kHz (7.5 kHz defined for MBMS)
Sampling Rate: It varies with the bandwidth but always multiple or factor of 3.84 to ensure compatibility with WCDMA by using common clocking.
The parallel transmission of multiple symbols in OFDMA creates high PAR (power amplifier).
Fig: - Symbols Transmission in OFDMA
4. LTE Uplink Air Interface Management SC-FDMA
In order to generate Uplink Air Interface Single Carrier Frequency Division Multiple Access (SC-FDMA) Transmission technique is used. TS36.201 and TS36.211 provide the mathematical description of the time domain representation of an SC-FDMA symbol .
SC-FDMA transmits data in series employing multiple subcarriers.
E.g. SC-FDMA: 6 modulation symbols are transmitted per SC-FDMA symbol using all subcarriers per modulation symbol. The duration of each modulation symbol is 1/6th of the modulation symbol in OFDMA.
Fig: - SC-FDMA Symbols Generation
SC-FDMA avoids parallel transmission (where as it takes series transmission) of multiple symbols by additional processing before the IFFT: modulation symbols are presented to FFT. The output represents the frequency components of the modulation symbols.
The process have set amplitude created subcarriers that should remain nearly constant between one SC-FDMA symbol and the next for a given modulation scheme which results in little difference between the peak power and the average power radiated on a channel.
Fig: - Symbol Transmission in SC-FDMA
5. Air Interface Technology
In uplink SC-FDMA the User multiplexing is done in frequency domain & user is allocated different bandwidths (multiples of 180 kHz) where as in downlink OFDMA the user multiplexing is done in sub-carrier domain & user is allocated Resource Blocks.
One user is always continuous in frequency.
Smallest uplink bandwidth, 12 subcarriers: 180 kHz and same for OFDMA in downlink.
Fig: - Single Distribution for UL & DL
Largest uplink bandwidth: 20 MHz
Same for OFDMA in downlink.
Terminals are required to be able to receive & transmit up to 20 MHz, depending on the frequency band though .
6. Performance Differentiation [OFDMA and SC-FDMA vs. CDMA]
Larger bandwidths and frequency flexibility
OFDM provides performance benefits over CDMA based system when the bandwidth increases beyond 5 MHz.
OFDM makes it simpler to provide different bandwidths.
Frequency Domain Scheduling
OFDM can take benefit of frequency domain scheduling which increases capacity up to 50% compared to CDMA.
Reduced UE power consumption
OFDMA in DL and SC-FDMA in UL. In UMTS, TDD mode is different than FDD although some harmonization was done e.g. chip rates and coding solutions LTE uplink uses SC-FDMA which enables better power amplifier efficiency.
Simpler multi antenna operation
Multiple input multiple output (MIMO) antenna technologies, emerging over the past few years, are required to achieve the LTE bit rate targets.
MIMO is simpler to implement with OFDM than with CDMA .
Same multiple Access Techniques for FDD and TDD in LTE
OFDMA in DL and SC-FDMA in UL. In UMTS, TDD mode is different than FDD although some harmonization was done e.g. chip rates and coding solutions.
7. Functions which provides value to sustaining LTE-Advanced Technology
LTE has strong background for its research & development so that it adopts past work environment and operations LTE have wide global support among 3GPP and 3GPP2 standardization bodies .
End 2004 3GPP workshop on UTRAN Long Term Evolution.
December 2007 1st version of all radio specs approved.
December 2008 3GPP freeze of LTE as part of Release 8 (exceptions for the EPC to be completed until March2009).
2009 Start of Customer Trials.
March 2009 Ratification of 3GPP Release 8:
LTE standardization is completed and approved by 3GPP Release 8.
FDD and TDD modes supported with the same specification and hardware components.
2010 3GPP Release 9 gets ready. Self-organised networks.
2011 3GPP Release 10 gets ratified (LTE A).
Main LTE Requirements
Peak data rates of uplink/downlink 50/100 Mbps.
Enables round trip time <10 ms.
Ensure good level of mobility and security.
Optimized for low mobile speed but also support high mobile speed.
Frequency flexibility and bandwidth scalability:
with 1.25, 2.5, 5, 10, 15 and 20 MHz allocations Improved Spectrum Efficiency:
Capacity 2-4 times higher than with Release 6 HSPA.
Efficient support of the various types of services, especially from the PS domain.
Packet switched optimized.
Operation in FDD and TDD modes.
Improved terminal power efficiency.
Support for inter-working with existing 3G system and non-3GPP specified systems.
LTE-Advanced (LTE-A) in 3GPP Release 10
LTE- Advanced will be the main feature or 3GPP Release 10.
Formally submitted on the 7th October 2009 to the ITU for admission as a candidate for IMT-Advanced (IMT-A).
DL Spectral efficiency 2.4 bps/Hz/cell (1.7 bps/Hz/cell in LTE).
Downlink data rates up to 1 Gbps (low mobility) and 100 Mbps (high mobility).
Uplink data rates up to 500Mbps.
Uplink MIMO (2Tx antennas in UE) and further DL MIMO (up to 8x8) is under study.
Backwards compatibility and interworking with LTE and other 3GPP legacy systems.
First LTE-A networks expected +2014.
Support for wider bandwidth (up to 100MHz) by carrier aggregation .
Fig: - LTE-Advanced resource pool
LTE-Advanced Services and Applications
Services uniquely enabled by LTE-Advanced can be categorized into three broad classes, corresponding to the distinctive differences of 4G over 3G:
â€¢ High-Bandwidth-Based services
â€¢ Peer-To-Peer-Based Services and
8. What makes LTE-Advanced different from other technologies?
New radio transmission schemes:
OFDMA in downlink Orthogonal Frequency Division Multiple Access.
SC-FDMA in uplink Single Carrier Frequency Division Multiple Access.
MIMO Multiple Antenna Technology.
New radio protocol architecture:
Focus on shared channel operation, no dedicated channels anymore.
New network architecture:
More functionality in the base station (eNodeB).
Focus on packet switched domain.
Important for Radio Planning:
Frequency Reuse 1
No need for Frequency Planning.
Importance of interference control.
No need to define neighbour lists in LTE.
LTE requires Physical Layer Cell Identity planning (504 physical layer cell IDs organised into 168 groups of 3) .
Measurements from LTE to other systems:
UE measurements are mainly intended for handover
UTRA FDD: CPICH RSCP, CPICH Ec/No and carrier RSSI
GSM: GSM carrier RSSI
UTRA TDD: carrier RSSI, RSCP, P-CCPCH
CDMA2000: 1xRTT Pilot Strength, HRPD Pilot Strength
System is reuse 1; single frequency network operation is feasible.
No frequency planning required.
There are no dedicated physical (neither transport) channels anymore, as all resource mapping is dynamically driven by the scheduler.
LTE enables high spectral efficiency (2 to 4 x Release 6).
LTE allows spectrum refarming by bandwidth scalability.
LTE is based on flat architecture without RNC.
LTE provides smooth inter-working with GSM/HSPA/CDMA.
LTE does not required neighbour list planning.
9. Research challenges
The use of multiple spectrum bands, spectrum sharing and wider bandwidths introduces new challenges in terms of signal transmission & processing, resource management, transceiver, and error control mechanism design, among others.
Increased FFT size
LTE utilizes up to 20 MHz bandwidth, for which it requires a 2048-point FFT . In the case of LTE Advanced, a bandwidth of 100 MHz requires an FFT of increased size. If we follow the FFT size trend in LTE versus bandwidth, an FFT size of 10 240 would be needed for 100 MHz. This will directly affect memory size, and base-band processing power requirement.
The design of wideband transceivers will be affected by several factors , such as the following:
Frequency-dependent path loss: As higher frequencies are used, the path loss increases nonlinearly.
Doppler frequency and spectrum: At higher frequencies, the Doppler effects affect the signals more severely, which would require faster adaptation algorithms, increasing the overhead.
Effective noise power: As the bandwidth increases, the effective noise increases as well.
Receiver input signal: Using a wider bandwidth translates into receiving more undesired signals from other services (e.g. broadcast and radar signals). So, issues such as image rejection, reciprocal mixing have to be considered.
Reciprocal mixing: When undesired signals mix with the oscillator noise, additional noise is introduced into the receiver, resulting in an additional noise figure.
Receiver performance: The performance of the receiver will be limited by all the previous listed elements.
Sampling frequency: Sampling the entire spectrum from the lowest to highest frequency would represent an extremely high sampling frequency.
The option of using more than one spectrum band (either dedicated or shared) is immediately followed by the decision of how many bands and which bands should be used in order to satisfy the different constraints and requirements (delay, jitter, rate, interference, power consumption, mobility, reliability, subscription plan, coverage- path loss, fading, Doppler effect, etc.) .
The lower-frequency bands are better suited for longer-range, higher-mobility and lower-capacity systems, while higher-frequency bands are better suited for shorter-range, lower-mobility, and higher-capacity systems. This decision should also take into account the capabilities of the UE: multiple band support, minimum and maximum distance between component carriers, and
minimum and maximum number of frequency bands that can aggregate.
LTE uses a combination of ARQ (at the RLC layer) and hybrid ARQ (at the MAC layer) in order to achieve the low error probability required to achieve 100 Mbps. To avoid excessive overhead while achieving high throughput both methods complement each other. In LTE-Advanced, data rates of 1 Gbps are expected in scenarios of low mobility and 100 Mbps in scenarios of high mobility.
Some approaches to improve the ARQ/hybrid-ARQ interaction have been proposed.
In a shorter transmission time interval (TTI) is proposed where the hybrid-ARQ control information is transmitted each TTI, while the rest of the control information is transmitted each two or three TTIs, in order to reduce the transmission delay . In a global outer ARQ in a combination of hybrid ARQ for each component carrier is proposed to reduce the switching delay when the UE needs to switch from one component carrier to another .
Radio parameters, such as the number of carriers that are needed as guard bands between contiguous component carriers, must be optimized to achieve high utilization of the spectrum without degrading the performance. In an initial investigation of the minimum spectrum distance (carrier guard band) between component carriers in contiguous spectrum bands was done. It is reasonable to expect that more scenarios will be required and investigated .
10. Performance and Future
Mobile broadband is rapidly becoming a reality. Today, people can surf or send e-mail with HSPA-enabled handsets and notebooks; replace their DSL modems with HSPA modems; and quickly upload and download videos or music with 3G phones. LTE, which is to be introduced in 3GPP Release 8, is the next major step in mobile radio wireless communications. It will provide wide services as well as superior user experience and support even more demanding applications, such as interactive LCD/LED, user-generated videos,
advanced games, business application and professional services. LTE uses OFDM (orthogonal frequency-division multiplexing) radio access technology together with smart antenna technologies.
LTE is an advanced & versatile technology that fulfills or exceeds 3GPP requirements. Some of the most notable requirements follow below :
Downlink peak rates of more than 100Mbps and round-trip time in the radio access network (RAN) of less than 10ms. Support for flexible carrier bandwidths from less than 5MHz up to 20MHz in many new and existing spectrum bands. Support for FDD and TDD deployments.
To optimize the handling of packet-data traffic and reducing the use of transport resources operators can distribute the gateway nodes over several sites in the network. This approach also minimizes delay, which is an important prerequisite for real-time services, such as IMS multimedia telephony, Defense multimedia systems and high-peak-rate mobile broadband data access. Operators may introduce LTE flexibly [Because Ericsson's LTE hardware is the same (apart from filters) for FDD and TDD], to match current network, spectrum, and business objectives for mobile broadband and multimedia services.
This paper discussed OFDMA & SC-FDMA technique in LTE standard in 4G as well as it gives focus on sustaining & differentiating of using LTE Technology. This paper examined the benefits and challenges of using OFDMA and SC-FDMA also some existing approaches to tackle these Challenges. However, several issues in each of them are still open and require further research.
The author would like to thank Dr. Denendra Kumar Punia and Dr. M. Anil Ramesh for their valuable comments that improved the
quality of this paper and University of Petroleum & Energy Studies (UPES), for their assistance.