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Towards 4G
Technical Overview of LTE and LTE-Advanced

Zahirul Islam & Md. Maruf Ahamed
Department of Electrical Engineering
University of North Dakota
31st January, 2013

Fundamental Constraints
 Shannon’s capacity upper bound
- Achievable data rate is fundamentally limited by
bandwidth and signal-to-noise ratio (SNR)

Wider Bandwidth
 Demand for higher data rate is leading to utilization of
wider transmission bandwidth.

Challenges of Wireless Communications

Duplexing
 Two ways to duplex downlink (base station to mobile) and
uplink (mobile to base station)
 Frequency division duplexing (FDD)
 Time division duplexing (TDD)

Orthogonal Frequency Division Multiplexing
 OFDM can be viewed as a form of frequency division multiplexing
(FDM).
 Divides the transmission bandwidth into narrower equally spaced tones,
or subcarriers.
 Individual information symbols are conveyed over the subcarriers.
 Use of orthogonal subcarriers makes OFDM spectrally efficient.
 Because of the orthogonally among the subcarriers, they can overlap with
each other.

(Contd.)
 Since the bandwidth of each subcarrier is much smaller
than the coherence bandwidth of the transmission channel,
each subcarrier sees flat fading.

(Contd.)
 Design issues of OFDM
 Cyclic prefix (CP): To maintain orthogonality among subcarriers in the
presence of multi-path channel, CP longer than the channel impulse
response is needed. Also CP converts linear convolution of the channel
impulse response into a circular one.
 High peak-to-average power ratio (PAPR): Since the transmit signal is
a composition of multiple subcarriers, high peaks occur.
 Carrier frequency offset: Frequency offset breaks the orthogonality and
causes inter-carrier interference.
 Adaptive scheme or channel coding is needed to overcome the spectral
null in the channel.

Orthogonal Frequency Division Multiple Access

 OFDMA is a multi-user access scheme using OFDM.
 Each user occupies a different set of subcarriers.
 Scheduler can exploit frequency-selectivity and multi-user diversity.

Single carrier FDMA (SC-FDMA)
 SC-FDMA is a new multiple access technique.
 Utilizes single carrier modulation, DFT-spread orthogonal
frequency multiplexing, and frequency domain equalization.

 It has similar structure and performance to OFDMA.

 SC-FDMA is currently adopted as the uplink multiple
access scheme in 3GPP LTE.

SC-FDMA and OFDMA

 Similarities
 Block-based modulation and use of CP
 Divides the transmission bandwidth into smaller subcarriers
 Channel inversion/equalization is done in the frequency domain
 SC-FDMA is regarded as DFT-precoded or DFT-spread OFDMA

 Dissimilarities
 Lower transmit peak power
 Equalization performance
 Multi0carrier MIMO receiver algorithm

MIMO
 Multiple input multiple output (MIMO) technique
improves communication link quality and capacity by
using multiple transmit and receive antennas.
 Two types of gain:
 Spatial Diversity: Improves link quality (SNR) by combining multiple
independently faded signal replicas.
 Spatial Multiplexing: Increases data throughput by sending multiple streams of data
through parallel spatial channels.

LTE: Long Term Evolution
 Standardized by 3GPP (3rd generation Partnership Project)
 3GPP is a partnership of 6 regional standards
organizations.
 ARIB (Japan)
 ATIS (USA)
 CCSA (China)
 ETSI (Europe)
 TTA (South Korea)
 TTC (Japan)

Requirement of LTE
 Peak data rate
 100 Mbps DL/50 Mbps UL within 20 MHz bandwidth
 Up to 200 active users in a cell (5 MHz)
 Less than 5 ms user-plane latency
 Mobility
 Optimized for 0~15 km/hr
 15~120 km/hr supported with high performance
 Supported up to 350 km/hr or even up to 500 km/hr
 Enhanced multimedia broadcast multicast service (E-
MBMS)
 Spectrum flexibility: 1.25 ~ 20 MHz
 Enhanced support for end-to-end QoS

Key Features of LTE (R8)

 Spectrum flexibility: 1.25 ~ 20 MHz (100 MHz for LTE-A)
 Multicarrier-based radio air interface
 OFDM/OFDMA and SC-FDMA
 Support for both FDD and TDD spectrums
 Active interference avoidance and coordination
 Peak data rate
 Downlink (DL): 326.4 Mbps (20 MHz, 4x4 MIMO, 64-QAM)
 Uplink (UL): 86.4 Mbps (20 MHz, no MIMO, 64-QAM)

LTE-A Features
 Wider bandwidths, enabled by carrier aggregation
 Higher efficiency, enabled by enhanced uplink multiple access and
enhanced multiple antenna transmission (advanced MIMO techniques)
 Coordinated multipoint transmission and reception (CoMP)
 Relaying
 Support for heterogeneous networks
 LTE self-optimizing network (SON) enhancements
 Home enhanced-node-B (HeNB) mobility enhancements
 Fixed wireless customer premises equipment (CPE) RF requirements

LTE-A: Carrier Aggregation

 In order to support up to 100 MHz bandwidth, two or more
component carriers aggregated
 Component carrier (CC): Basic frequency block which comply
with R8 LTE numerology
 Each CC is limited to 20 MHz bandwidth (110 resource blocks)
 Maintains backward compatibility with R8 LTE
 Supports both contiguous and non-contiguous spectrum
 Also supports asymmetric bandwidth for FDD

LTE-A: Carrier Aggregation (Contd.)

LTE-A: Enhanced MIMO

 Downlink MIMO
 Up to 8x8 (8 layer) configuration
 Additional RS: CSI-RS and UE-specific DM RS
 Support for MU-MIMO
 Enhancements to CSI feedback

 Uplink MIMO
 Introduction of UL transmit diversity
 Introduction of up to 4x4 SU-MIMO
 Use of turbo serial interference canceller

Conclusion
 The 4G LTE technology is nothing less then ground breaking. The
advancements that have been made from 3G to 4G LTE alone are mind
blowing. With the data processing speed being increased to at least 100
Mbit/sec the possibilities are limitless in the wireless communication world.
Everything with this new technology has been brought to a new standard. The
security, with the complete IP-based solutions allows the user to use the full
capability of the phone as well as feel completely secure at the same time, this
is one of the most vital aspects that has been upgraded from the previous
wireless communication technologies. Even though the hardware and
coverage areas aren’t up to par yet though, isn’t that big of a set back. The
technology is still considered brand new and will only be improved in the
coming years. As I said previously, this technology is truly ground breaking
and makes the average person really think about what is possible with wireless
communication? If there ever is a 5G network, how powerful will it be and
what will it possibly be able to do that the 4G doesn’t already do?

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