a novel approach for cell selection and synchronization in lte-advanced

International Journal of IT, Engineering and Applied Sciences Research (IJIEASR) ISSN: 2319-4413 Volume 3, No. 5, May 2014

i-Explore International Research Journal Consortium www.irjcjournals.org

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A Novel Approach for Cell Selection and Synchronization in LTE-Advanced

Sunita Kumari, M.E. (ECE) Fellowship, Chitkara University, Chandigarh T. L. Singal, Professor & Guide, ECE Department, Chitkara University, Chandigarh ABSTRACT Long Term Evolution (LTE) is the result of the standardization work done by the 3rd Generation Partnership Project (3GPP) to achieve a new high speed radio access in the mobile communications frame. Cell selection by a mobile UE is another issue in LTE. In particularly, an interesting challenge in the physical layer of LTE is how the mobile unit immediately after powering on, select a radio cell and locks on to it. More specifically, to understand how the mobile unit establishes the connection with the strongest cell station in surrounding region. To do this, the mobile unit has to overcome the challenges of estimating the channel to communicate with the cell site and frequency synchronization. To appropriately synchronize the mobile unit with the base station when multiple mobile unit are communicating with same receiver from various distances. Keywords LTE, LTE-A, Cell Selection, Synchronization 1. INTRODUCTION Initially, Long Term Evolution and abbreviated as LTE was introduced in the Release 8 in 2008. 3GPP published and introduced the various standards for IP based system LTE. In 2010, the Release 9 was introduced to provide enhancements to LTE and in 2011 Release 10 was brought as LTE-Advanced. The objective was to expand the limits and features of Release 8 and to meet the requirements of the International Mobile Telecommunications-Advanced (IMT-Advanced) of ITU-R for the fourth generation mobile technologies (4G), and the future operator and end user’s requirements. The key reason of the evident of the LTE-A is the growing demand for network services, such as web browsing, VoIP, video telephony, and video streaming, with constraints on delays and bandwidth requirements, poses new challenges in the design of the future generation cellular networks. [1]. The overall goal of LTE technology is to significantly increasing peak data rates scaled linearly according to spectrum allocation, improving spectral efficiency, lowering costs, improving services making use of new spectrum opportunities, improved quality of service, and better integration with other open standards.

2. NEW TECHNIQUES USED IN LTE i) Multicarrier Technology: The access scheme is different for the uplink and downlink in LTE. OFDMA (Orthogonal Frequency Division Multiple Access) is used in the downlink; whereas, SC-FDMA (Single Carrier - Frequency Division Multiple Access) is used in the uplink. Refer Figure 1. OFDM technology has been incorporated into LTE because it enables high data bandwidths to be transmitted efficiently while still providing a high degree of resilience to reflections and interference.

Fig. 1: OFDMA and SC-FDMA in LTE

The Peak-to-Average Power Ratio (PAPR) of an OFDM signal is relatively high, causes difficult to tolerate for the transmitter of the mobile terminal. SC-FDMA has a significantly lower PAPR, the more constant power enables high RF power amplifier efficiency in the mobile handsets. [2]. ii) Multiple Input Multiple Output (MIMO): MIMO is used to improve the data bit rates and spectral efficiency. It consists of using multiple antennas in both the receiver and transmitter in order to use the multipath effects, which decreases the interference and leads to high transmission rates. See Figure 2. One of the main problems that previous telecommunications systems have encountered is that of multiple signals arising from the many reflections that they encountered. By using MIMO these additional signal paths can be used to advantage and to increase the throughput. [3].



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Fig. 2: MIMO in LTE iii) System Architecture Evolution (SAE): As there is need for high data rate and low latency for 3G LTE, it is necessary to evolve the system architecture so that improved performance can be achieved. The new SAE network is based upon the GSM/WCDMA core networks to enable simplified operations and easy deployment, as shown in Figure 3.

Fig. 3: LTE Architecture System 3. SALIENT FEATURES OF LTE-A The features of LTE are extended in LTE- Advanced in order to exceed or at least meet the IMT-Advanced requirements. It also must fulfil operator’s demands like a reduced cost per Mbit transmitted, better service providing in terms of homogeneity, constant quality of the connection, smaller latency and compatibility with all 3GPP previous systems. a) Support of asymmetrical bandwidths and larger bandwidth: In LTE (release 8), the bandwidth could have different sizes but had to be the same in the downlink and in the uplink. In LTE-Advanced (Release 10) bandwidths

can be different because due to actual demand in mobile networks, the load from the station to the user is more than from the user to the station. b) Enhanced multi-antenna transmission techniques: LTE introduced MIMO in the data transmission. However in LTE-Advanced, the MIMO scheme has to be extended to gain spectrum efficiency, cell edge performance and average data rates. LTE-Advanced uses a configuration 8x8 in the downlink and 4x4 in the uplink. LTE-Advanced tries to get the network closer to the user to provide a uniform user experience and to increase the capacity of the network by using advanced topology networks. Advanced topology networks provide the benefits and the performance increase. Some of the characteristics of this type of networks are: • Self-organizing networks • Intelligent Node Association • Support for relays • Adaptive Resource Allocation • Multicarrier (spectrum aggregation)

c) Coordinated multipoint transmission and reception (CoMP): It is used to improve the received signal of the user terminal. Both the serving and neighbour cells are used in a way that the co-channel interference from neighbouring cells is reduced. It implies dynamic coordination between geographically separated transmission points in the downlink and reception at separated points in the uplink. This mechanism is used to improve the coverage of high data rates and to increase the system bit rate. d) Relaying: Relaying increases the coverage area and capacity of the network. User’s mobile devices communicate with the relay node, which communicates with a donor eNB (enhanced Node B). See Figure 4.

Fig. 4: Relay types in LTE systems Relay nodes can also support higher layer functionality like decoding user data from the donor eNB and re-encoding the data before transmitting it to the user terminal. Type 1 relay nodes control their cells with their own cell identity, and are used for the purpose of the transmission of synchronization channels and reference symbols. Type 2 relay nodes don’t own an identity, so the



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mobile user won’t be able to distinguish if a transmission comes from the donor eNB or from the relay. 4. A COMPARISON: LTE vs. LTE-A LTE and LTE Advanced are high speed 4G wireless technologies. These 4G technologies give the real experience of the triple play services such as voice, video and high speed data. Both LTE and LTE Advanced offer high speed access to internet equivalent to FE connection. The key differences between LTE and LTE-Advanced are mentioned below and in Table 1.

Table 1: Specifications of LTE and LTE-Advanced

Parameter LTE LTE-A Peak Data Rate Down Link

150 Mbps 1 Gbps

Peak Data Rate Up Link

75 Mbps 500 Mbps

Transmission Band (DL)

20 MHz 100 MHz

Transmission Band (UL)

20 MHz 40 MHz

Scalable Bandwidths

1.3,3,5,10, and 20MHz

Up to 20-100 MHz

Capacity 200 active users per cell in 5MHz

3 times higher than LTE

5. CHALLLENGES IN IMPLEMENTING LTE-A i) Uplink scheduler limitations: OFDMA is used in downlink. Without ordering constraints, the scheduler can fill out the RB allocation. Due to the use of SC-FDMA, the uplink allows the UEs to transmit only in a single carrier mode. Thus, the scheduler for the uplink has limited degrees of freedom. It has to allocate contiguous RBs to each user without any choice among the best one available. [4]. ii) Spectral efficiency: In LTE, one of the main goal is to achieve effective utilization of radio resource. Hence, several types of performance indicators need to be calculated. For example, a specific policy could be planned to maximize the number of users served in a given time interval, or the spectral efficiency, by always serving users that are experiencing the best channel conditions. User throughput can be used as efficiency indicator which is defined as the actual transmission data rate without including layer two overheads and packet retransmissions due to physical errors. iii) Fairness: If overall cell throughput increases to greatest extent, positively it enables effective utilization in

terms of spectral efficiency, but also leads to very unfair resource sharing among users. Therefore fairness is a key requirement that should be taken into account to guarantee minimum performance to the cell-edge users and the users who experiencing bad channel conditions. iv) QoS Provisioning: It is a major feature in all-IP architectures and very important in next generation mobile. LTE maps QoS constrained flows to dedicated radio bearers that depending on their QCIs enable special RRM procedures QoS constraints may vary depending on the application and they are usually mapped into some parameters: maximum delivering delay, minimum guaranteed bitrate, and packet loss rate. Hence, it is important to define QoS-aware schedulers. 6. THE CELL SEARCH PROCEDURE In LTE, Similar to all mobile communication systems, terminal must perform certain steps before it can receive or transmit data. [5]. These steps can be categorized in cell search and cell selection, derivation of system information, and random access. Refer Figure 5.

Fig. 5: LTE Initial Access

A se A user equipment willing to access an LTE cell must first undertake a cell search procedure. Cell search procedure is a group of procedures which consists of a series of synchronization stages through which the UE determines time and frequency synchronization parameters that are necessary to demodulate the downlink data and to transmit in uplink slot with the correct timing so as the signal maintains orthogonality with other users. 7. CELL SYNCHRONIZATION PROCESS Matching up with time and frequency parameters of the reference or source is called synchronization. In case of LTE networks, eNodeB is the source which controls access to the UE. Thus, UE should adjust its frequency and time according to the eNodeB. This is done with the help of special Zadoff Chu (ZC) sequences. [6]. eNodeB transmits these sequences periodically so that all UEs can



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Beginning of the cell search

PSS Detection

Physical Layer Cell ID

Frequency Synchronization

Slot Boundaries

SSS Detection

Frame Timing

Group Cell

End of Cell Search

synchronize to the reference accordingly. There are three synchronization requirements in LTE:

- symbol timing acquisition by which the correct symbol start is determined;

- carrier frequency synchronization which mitigates the effect of frequency errors resulting from Doppler shift, and errors from electronics circuitry; and

- sampling clock synchronization. This is achieved by two types of sequences called as primary synchronization sequences (PSS) and secondary synchronization sequences (SSS). Refer Figure 6.

Fig. 6: Cell search and Synchronization Algorithm

PSS detection is the first step in cell search. To start synchronization, UE should understand the time clock and frequency on which eNodeB is working. [7]. For this UE after powering up performs the downlink synchronization which is detection of PSS and SSS and acquiring time, frequency and system configuration information from broadcast channel. a) Uplink Synchronization: After downlink synchronization, the UE has synchronization with eNodeB clock and frequency. In addition, to obtain the uplink timing advance information from eNodeB, UE performs transmits one of the ranging code supported in the system and waits for the ranging reply in the downlink control channel. Once the ranging is successful, UE receives the message and new dedicated ranging code with temporary ID in the downlink control channel. If the ranging successful message is not received, then UE tries again till it succeeds. b) New cell identification or initial synchronization: If the UE is already registered with one eNodeB and moving out of coverage area then new cell identification procedure is carried out where the connected eNodeB assists the UE in ranging and registration to the new cell. UE is assigned with a dedicated ranging sequence and then ranging is performed with the new cell. Whereas in case of initial synchronization, the UE performs ranging procedure on the contention basis. PSS and SSS synchronization signals are used in cell search process where slot start time, frequency offset and physical layer ID is achieved after detecting PSS. The detection of SSS gives radio frame timing, cell ID, cyclic prefix length and TDD/FDD frame system configuration. PSS sequences and SSS sequences are transmitted twice per 10 ms radio frame. The PSS is located in the last OFDM symbol of the first and 11th slot of each radio frame which allows the UE to acquire the slot boundary timing which is independent of the type of cyclic prefix length. The PSS signal is the same for any given cell in every sub frame in which it is transmitted. The location of the SSS immediately precedes the PSS – in the before to last symbol of the first and 11th slot of each radio frame. The UE would be able to determine the position of the 10 ms frame boundary as the SSS signal alternates in a specific manner between two transmissions. 8. CONCLUSION To understand how the mobile unit establishes the connection with the strongest cell station in surrounding region. To do this, the mobile unit has to overcome the challenges of estimating the channel to communicate with the cell site and frequency synchronization to



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appropriately synchronize the mobile unit with the base station when multiple mobile unit are communicating with same receiver from various distances. To identify the available cell site so that the mobile unit will connect successfully. For this, it uses two signals, the Primary Synchronization Signal and the Secondary Synchronization Signal sequentially. The primary signal is based on Zadoff-Chu sequence which is a Constant Amplitude Zero Auto Correlation (CAZAC) sequence. This Research simulates the mobile cell search procedure and the challenges associated with it can be analyzed using MATLAB tool. REFERENCES

[1] Brian Katumba, Johannes Lindgren and Kateryna Mariushkina “The LTE Access Procedure”, IEEE wireless communications and networks, vol. 7, no. 3, Jan 2011.

[2] D. Supratim, P. Monogioudis, J. Miernik And P. Seymour “Algorithms for enhanced inter cell interference coordination in LTE Hetnets”, IEEE wireless communications and networks, vol. 16, no. 3, Jan 2013.

[3] S. Fulani and Q. Liang, “Physical layer test trials and analysis of call drops and real time throughput versus channel capacity of the long term evolution (4G) technology”, International Journal of electronics Engineering, vol. 9, no. 3, pp.120-130, 2011.

[4] G. Berardinelli, “Air interface for next generation mobile communication networks: physical layer design: a LTE-A uplink”, Department of Electronic Systems, pp. 56-60, 2010.

[5] K. Davaslioglu and E. Ayanoglu, “Interference-based cell selection in heterogenous networks”, Department of Electrical Engineering and Computer Science, IEEE wireless communications, vol. 11, no. 4, pp.13-25, Feb. 2013.

[6] K. Son, S. Chong and G. Veciana, “Dynamic association for load balancing and Interference avoidance in multi-cell networks”, IEEE Transactions on wireless communications, vol. 8, no. 7, pp. 3566- 3576, July 2009.

[7] A.Zakrzewska, Sarah Ruepp and S. Michael, “Cell selection using recursive bipartite matching”, IEEE International Conference on Computer Communications (INFOCOM 2013), vol. 13, no. 6, pp.656-666, 2013.