Evolution Of Mobile Broadband Computer Science

'Think creative and Do creative' has become the tagline for the 'Internet Age' that has witnessed impressive and enormous development in the field of technology and attracting people of all ages. Time has change around from people talking to machines talking. Present trend in the technology is attracting the consumers to get connected everywhere, anytime on the go. Telecommunications is playing a vital role in different ways leading to change in the lifestyle of consumers, growth in economy and business sectors. Every domain in the field of telecommunications is growing and supporting each other in the advancements and striving to produce the best to make lifestyle so simple. Wireless domain has witnessed sudden and striking development in the form of 4G LTE competing with speeds of wired services. Mobile User behavior has changed noticeably from past few years and with the adoption of LTE, appetite for high speeds has been fulfilled and involved enthusiastically in exploring the benefits of the new technology. Mobile communication is all set to provide rich wireless experience and unlocking the new business models for consumers, service providers and business users.

LTE known for Long Term Evolution, defined by 3rd Generation Partnership Project (3GPP) is flexible radio interface capable of providing high speeds in the downlink of 300 Mbps, reduction in the network delay, efficient use of spectrum as compared to previous cellular systems. This new technology simplifies the operation and reduces the cost of infrastructure for the deployment. LTE offer services in wide range of system bandwidth to operate in a large number of spectrum allocations, supports time division duplex (TDD) and frequency duplex scheme (FDD) unlike other cellular systems. Earlier 3GPP systems such as WCDMA-Wideband Code Division Multiple Access, HSPA-High Speed Packet Access, TDSCDMA- Time Division Synchronous CDMA and 3GPP2 systems such as CDMA2000 has seen a significant increase in the speeds, involving the trade-offs and efforts are being made for the smooth evolution to LTE.

3GPP release 8, LTE and it's in- depth details such as basic transmission schemes in uplink and downlink schemes, spectrum flexibility, advanced multiple antenna transmission, Carrier aggregation, coordinated multipoint transmission/reception, relaying to improve coverage, reduced complexity and inter cell interference coordination are exemplified and described in this paper.

II. LTE: An Overview

Basic Transmission Scheme

OFDM know as Orthogonal Frequency Division Multiplexing in which serial stream of data is divided and put in narrow subcarriers and transmitted in parallel, is used in LTE downlink transmission. Due to use of narrow band subcarriers in conjunction with cyclic prefix, transmission is robust to time dispersion in the channel leading to no use of requirement of complex receiver side channel equalization. There is also an attractive property for the downlink that it makes the power consumption and base band processing very simple. Allowing for wide transmission bandwidth of LTE, these are used in addition with extended multiple antenna transmission, such as SDM-spatial division multiplexing.

Uplink transmission is designed significantly to enable high power efficient transmission and available power is considerably lower than for the downlink transmission. This technique enables coverage, reduces terminal cost and power consumption at the transmitter. Modulation technique such as Single carrier frequency division multiple access SC-FDMA also referred as discrete Fourier transform-pre coded OFDM is used for LTE uplink that has special property having smaller peak to average power ratio than standard OFDM. This property enables less complex and high power terminals.[1]

LTE protocol structure consists of three layers: - Radio Link Control-RLC, Medium Access Control-MAC and Physical layer-PHY. RLC and MAC layers are responsible for retransmission control, priority handling and multiplexing of data flows. Physical layer deals with turbo coding, rate matching, modulation using schemes such as Quadrate Phase Shift keying-QPSK, 16-QAMor 64-QAM (Quadrature Amplitude Modulation) followed by OFDM modulation. The subcarrier spacing is 15 kHz and there are two types of cyclic prefix are supported in uplink and downlink, normal cyclic prefix of 4.7 us and extended cyclic prefix of 16.7 us that aids in high dispersive environments. In uplink, DFT precoder is used prior to OFDM modulator that doesn't compromise orthogonality between subcarriers hence preserves the properties of single carrier. In downlink, cell specific reference signals are transmitted to support channel estimation for coherent modulation and measurement purposes like mobile management and channel quality.[2]

The transmitted signal is structured into sub-frames of 1 ms duration, each consisting of 12 or 14 OFDM symbols, depending on type of cyclic prefix used. Ten sub-frames form a radio frame. The shorter duration of sub-frame helps to reduce the delay in user data and control signaling information such as hybrid ARQ- Automatic repeat request and channel quality feedback from the terminals into the base station. LTE supports TDD, FDD but there are differences in the frame structure between the two, there is an special sub frame in TD-LTE that provide guard time for downlink to uplink conversion.

4G LTE.jpg

Figure 1. LTE basic protocol structure [1]

A fundamental property of radio communication is fading, that varies the instantaneous radio channel quality in time, frequency and space. LTE uses channel dependent scheduling in time and frequency domain that enables efficient utilization of spectrum rather than suppressing the radio channel quality variations. The scheduler that is located at the base station controls the uplink and downlink transmission and in particular determines the downlink performance in highly loaded network. For each 1ms sub-frame duration, scheduler determines which users are allowed to use or transmit, on what frequency to take place, and what date rate to use.

LTE uses robust two layered retransmission schemes: - Fast hybrid ARQ protocol in RLC layer with low overhead feedback and hold up for soft combining with incremental redundancy, followed by highly reliable selective repeat ARQ protocol in MAC layer that captures and corrects most of the errors. These error mechanisms achieve low latency and low overhead sacrificing reliability and cost. These protocols provide feedback to the transmitter for each transmitted sub-frame and are terminated in base station. Scheduling decisions, hybrid ARQ feedback, channel status, code rate and control information that support LTE features are communicated between base station and terminal. In the downlink code rate and other resources used for control signaling for each terminal can be varied dynamically to match channel variation and minimize the overhead.[3]

SPECTRUM FLEXIBILITY- Transmission Bandwidth

Radio spectrum for mobile communications is available in different frequency bands of different sizes in different geographical areas due to regulatory efforts. It is also available in paired and unpaired frequency bands. Paired frequency bands in which uplink and downlink are assigned different bands whereas in the case of unpaired uplink and downlink share the same frequency band. LTE is able not to operate in only different frequency bands; it can be deployed with transmission bandwidth that can operate in spectrum of different sizes hence enabling to operate jointly with other radio access technologies in same frequency band. This feature is called spectrum flexibility that enables efficient migration of other radio access technologies to LTE.

Bandwidth of spectrum ranges from 1.4 MHz to 20 MHz, in which later is used for higher data rates. LTE provides different bandwidths for uplink and downlink enabling asymmetric spectrum utilization. In order to access a cell, terminal should have knowledge about cell bandwidth and duplex scheme. Prior to this knowledge, system information is located in sub-frames (downlink) occupying narrow bandwidth supported by LTE. After the terminal acquiring the system information, can access the cell.[3]

SPECTRUM FLEXIBILITY- Duplex Scheme

LTE offer services in wide range of system bandwidth to operate in a large number of spectrum allocations, supports time division duplex (TDD) and frequency duplex scheme (FDD) unlike other cellular systems. Physical layer processing is same for FDD and TDD, enabling low cost terminals to hold up for these operations.

In the case of FDD, there are two frequency bands, one for uplink and other for downlink transmission. During each frame there are ten uplink sub-frames and ten downlink sub-frames. Uplink and downlink transmission operate simultaneously in the same cell. There is also one to one relation between uplink and downlink which is explored in the control signaling design.

In the case of TDD, there is only one single carrier frequency in which uplink and downlink transmission shares the same frequency but separated in time, also on the cell basis. Seven uplink-downlink configurations are supported in TDD, uplink and downlink periodicities of 5ms or 10 ms and downlink to uplink ratios of 2:3 to 9:1, to meet the requirements of uplink and downlink asymmetries. Hence there is no coherent one to one relation between uplink and downlink leading to few differences in the frame structure and control signaling design of TDD as compared to FDD.

The frame structure of TDD and FDD differs by a special sub-frame, present in the case of TDD in which guard band separates uplink and downlink enabling no overlap of signals of transmission and reception. Guard periods are created by splitting the one or two sub-frames. Each radio frame consists of three fields: -downlink part (DwPTS), a guard period (GP) and an uplink part (UpPTS).

Downlink part consists of a shorter downlink sub-frames having length varied from three to twelve OFDM symbols. Control region spans up to maximum of two symbols unlike three symbols in normal sub-frames. Third symbol is preserved for primary synchronization signal in the case of TDD operation. In the case of FDD, primary synchronization signal is located in the middle of zero and five sub-frame. The difference in the location of primary synchronization signal enables terminals to differentiate the duplex technique of the cell. Uplink part consists of shorter uplink sub-frame having one or two OFDM symbols, and can be used for transmission of sound reference signals and random access. Sound reference signals are signals transmitted from terminal and enables the base station to estimate uplink channel quality and channel estimation. Random access uses one of the sub-frames as in FDD, supporting the long random access preamble to provide capacity and coverage in large cells. The remaining symbols in special sub-frame are used for guard period for uplink to downlink and downlink to uplink switch. Guard period is sufficiently enough to handle the propagation delay in the cells. Length of guard period depends upon various factors such as proximity to the base station, base station to station interference and inter system interference. [1]

Multiple Antenna Transmission

LTE supports multiple antenna transmission, channel quality measurements for link adaptation and scheduling are important in considering this feature. Terminals support at least two antennas, it's important because that is how networks are planned estimating the presence of downlink receive diversity. More advanced multi antenna schemes are supported by LTE includes transmit diversity, spatial multiplexing (support up to four antennas), beam forming and multi layer transmission. These schemes are used based upon the scenario. Open and closed loop transmit antenna in the uplink are selected as optional features.[1]

Transmit diversity is based on space frequency block coding-SFBC and frequency switched transmit diversity-FSTD in the case of four antennas. It is primarily focused for downlink channels to provide additional diversity in the case of channel dependent scheduling is not possible. It can be applied to user data transmission such as VOIP.

In Spatial Multiplexing, multiple antennas at the transmitter and receiver are used to provide simultaneous transmission of multiple, parallel signals called as layers over a single radio link thereby increasing the data rate of peak data rates that can be provided over the radio link. For multi stream transmission, precoder is used. Precoder maps the number of transmission layers onto four antennas by means of a matrix of size NA*NL, where NA is the number of antennas, NL- number of transmission layers also known as transmission rank. Transmission rank of NL is less than or equal to number of antennas. Transmission rank and precoder matrix based on channel measurements such as quality, status reported by the terminal; know as closed loop spatial multiplexing.[1]

Considering the case of spatial multiplexing, by selecting transmission rank 1 and for antennas, precoder matrix becomes NA*1 precoder vector that performs single beam forming function. This type of beam forming is referred as codebook based beam forming. Therefore beam forming depends upon predefined precoder vectors. LTE also supports non codebook based beam forming, terminal must make an estimate of beam formed channels. LTE provides the transmission of user equipment specific reference symbols using the same beam forming as the user data so that terminal estimates the overall beam formed channel.[1]

POWER CONTROL AND INTER CELL INTERFERENCE CORDINATION

LTE provides orthogonality between the subscribers in both downlink and uplink leading to no interference within the cell, intimidating there is only interference between the cells. Hence efforts have been done to reduce the inter cell interference giving substantial benefits to the LTE performance, especially in terms of service that can be provided to users at the edge of the cell. Uplink power control mechanism is used to control the received signal strength and interference caused by neighboring cells. LTE uplink power control supports fractional path loss compensation, stating that users at the border of the cell transmit less power thus creating less interference to the neighboring cells. LTE provides advanced interference handling schemes as well.

Inter Cell Interference Coordination- ICIC is the one of the effective method to reduce inter cell interference. A simple method to improve the data rates and signal strength is done by using the static tools such as frequency reuse. This method improves cell edge data rates and signal to noise interference ratio of used frequencies. However there is reduction in bandwidth, but the loss caused by reduction of bandwidth is higher than corresponding gain due to higher signal to noise interference ratio leading to overall loss of efficiency. LTE uses advanced schemes like indicators such as high interference and overload for uplink and relative narrowband transmission power for downlink, and scheduling the users on cell edge in different cells in the complimentary parts of the spectrum. Restrictions on bandwidth are encouraged in case of traffic and radio conditions.

Conclusion

LTE thus doesn't suppress the other radio access technologies, but make use of it in different conditions providing benefits to the users. LTE enables migration of these technologies in an efficient way. LTE and some of it key components: Spectrum flexibility, multiple antenna transmission bandwidths, ICIC have huge impact in terms of service, data rates and supporting the users at any cost. Hence it offers high competitive performance and provides a good ground for further evolution.

III. REFERENCES

David Astley, Erik Dahlman, Anders Furuskar, Ylva Jading, Magnus Lindstorm, and Stefan parkvall, Ericsson Research., "LTE: Evolution of Mobile Broadband." IEEE Commun. Mag., Apr 2009

NGMN, "Next Generation mobile networks beyond HSPA and EVDO," v. 1.3, Dec.2006;www.ngmn.org

E. Dahlman., 3G Evolution: HSPA and LTE for mobile broadband, 2nd ed., Academic press. 2008

4.3GPP TR 25.913, " Requirements for Evolved UTRA and Evolved UTRAN", v.7.0.0

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