Analysis and Evaluation of Cooperative Multi-Point TransmissionReception and Soft Handover for LTE Advanced

8/13/2019 Analysis and Evaluation of Cooperative Multi-Point TransmissionReception and Soft Handover for LTE Advanced

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Analysis and Evaluation of Cooperative Multi-Point

Transmission/Reception and Soft Handover for LTE-

Advanced

Rudraksh ShrivastavaAmity University

Sector-125 Noida, U.P. (India)

Mari Carmen Aguayo-TorresDepartamento Ingeniería de Comunicaciones

Universidad de MálagaMalaga (Spain)

Abstract — Network coordination provided by Cooperative Multi-

Point transmission/reception (CoMP) in 3GPP LTE-Advanced

networks is used as a means to provide spectrally efficient and

high capacity communication with enhanced cell edge user

throughput. However, it has been observed that in LTE-

Advanced networks when CoMP scheme is employed and the

legacy handover mechanism is used to perform handover

between coordinating cells, its performance is limited. LTE-

Advanced networks using CoMP scheme are able to use a softhandover mechanism different from the hard handover

mechanism used in existing cellular networks. In this paper, we

analyze the effects on performance and efficiency for users in

LTE-Advanced networks employing CoMP Joint Processing (JP)

scheme and Adaptive OFDM with the help of simulations and

observe gains in comparison to conventional networks without

CoMP. This process can be used during soft handover, instead of

legacy hard handover mechanism.

Keywords- LTE-Advanced, CoMP, Handover, Adaptive OFDM

I. I NTRODUCTION

In mobile communication systems, it is well known that asa user changes its position, user transmission has to be

reallocated to a new cell. This handover is a complicated process involving information exchange and signalingtransmission between the user, source Base Station (BS) andthe target base station. In existing mobile networks, handover isgenerally initiated by the User Equipment (UE), in which UE

periodically measures the Reference Signal Receiving Power(RSRP) of the connected BS and the BS in the neighboringcell. When the RSRP of the connected BS drops below acertain pre-defined threshold value, while at the same time, theRSRP of the BS in the neighboring cell increases above andmaintains the threshold for a certain time, the handover isinitiated. During handover two link procedures could be used.In hard handover, the user must terminate its connection withsource BS before establishing a new connection with target BS.Soft handover is a more sophisticated technique able to

simultaneously establish a link to both source and target BS.The latter mechanism is used in 3GPP UMTS. However, byeliminating Radio Network Controller (RNC) node, 3GPP LTEstandard currently under deployment has adopted hardhandover process over soft handover [5].

The latest 3GPP standard termed as LTE-Advanced aims toachieve higher performance and efficiency as compared to

previous 3GPP specifications while maintaining backwardcompatibility. These data rates are relatively easy to reach closeto the base station. However, as the distances increase they

become more difficult to maintain. At the cell edges not onlythe signal strength is weak because of the distance from the

base station but also interference levels from neighboring basestations are likely to be higher as the UE will be closer to them.These interference levels are henceforth referred to as Inter-Cell Interference (ICI). Coordinated Multi-Point transmissionand reception (CoMP) is one of the features defined in 3GPPRelease-11 for LTE-Advanced to meet the requirements ofIMT-Advanced framework and has been a key research area inrecent years due to its ability to improve mainly cell capacityand cell-edge user throughput. CoMP transmission/receptionactually refers to a wide range of techniques that enabledynamic coordination in transmission and reception withmultiple geographically separated base stations [8]. WhenCoMP technique is employed, the handover process in LTE-Advanced can be different from the handover process used inlegacy networks. By coordinating base stations (named

eNodeB by LTE standard), a diversity scheme can be used.Figure 1 shows the operating principle for the downlink:diversity is obtained by simultaneous transmission from bothsource and target eNodeB. Coherent combination can be doneas Orthogonal Frequency Division Multiplexing (OFDM) isused as modulation/multiple access technique. In this paper, westudied and analyzed the performance and efficiency of userslocated at cell edge during handover process when acombination of Adaptive OFDM and CoMP JointTransmission scheme (JT) is used in the downlink.Performance improvement compared to that of hard handoveris also evaluated.

Figure 1 Operating principle for downlink

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II. SYSTEM MODEL

A. Channel Model

We considered a scenario with frequency selectiveRayleigh multi-path fading channel in which there were two

base stations (eNode Bs) and one receiver User Equipment(UE). The data symbols were transmitted to the UE throughtwo separate Rayleigh multi-path channels having independentchannel responses for eNode B-1 and eNode B-2 respectively.

For each of those two channels, the transmit signal reaches thereceiver through multiple paths where the path has a

complex amplitude and delay , which is kept constant

in this case. The time-varying channel impulse response

of a channel with those characteristics is given by

Where L corresponds to the number of resolvable multipath

components. When there are large numbers of paths, applyingCentral Limit Theorem, each path can be modeled ascircularly symmetric complex Gaussian random variable. Thismodel is called Rayleigh fading channel model. Under this

model, the magnitude has a probability density given by:

This is called Rayleigh random variable.

The real and imaginary part of each tap is an independentGaussian random variable with mean 0 and variance given by

the used Power Delay Profile (PDP). Average channel powerover multiple channel realizations is normalized to 1. We have

used the same PDP for each channel but as OFDM behaviour (in ideal conditions) is the same for all PDP, we

expect the same results if a different PDP had been used.

Moreover, certain pathloss was included depending on theuser equipment location. Calculations were performed usingOkumura/Hata pathloss model in real time for the urbanscenario [6]. For the height of transmit antenna, hTX [m], andthe carrier frequency of f c [MHz], the Path Loss (PL) atdistance d [m] in an urban area is given by the Hata model as

PLUrban(d)[dB] = 69.55+26.16 log f c-13.82 log hTX -C RX +(44.9-6.55 log hTX )+log d (1.3)

Where CRX is the correlation coefficient of the receivedantenna, which depends on the size of the coverage area orcell. For small to medium sized coverage CRX is given by-

C RX = 0.8+ (1.1 log f c-0.7) h RX -1.56 log f c (1.4)

Where hRX [m] is the height of transmit antenna.

B. Transmission and ReceptionWe used the OFDM multicarrier transmission model as

shown in Fig. 2, in which data symbols are generated byeach modulator at the transmitter such that ,which are then multiplexed on to N subcarriers. Please, notethat as complex symbols are transmitted from both basestations, it is assumed that a fast X2 link is established

between them.

The same time-domain samples are transmitted duringone OFDM symbol, generating them by the Inverse FastFourier Transform (IFFT) and transmitting over the channelafter adding the cyclic prefix.

Since we have two independent multi-tap Rayleigh multi-

path channels, the channel is modeled by both time-variantimpulse responses and additive whiteGaussian noise (AWGN). As transmission is simultaneouslydone from both base stations, both data symbols aresimultaneously received.

At the receiver, the cyclic extension is removed fromthe received time-domain samples, and the data samples areFast Fourier Transformed (FFT), in order to yield the receivedfrequency-domain data symbols . Maximum channel delaysare kept within the cyclic prefix range and channel responsecan be assumed to be constant for the duration of one OFDMsymbol. Combined channel frequency response can becharacterized for each OFDM symbol period by the N-pointFourier Transform of the impulse response. Thus, the received

signal at subcarrier n can be written as

Where is additive noise and for the base station. Thefrequency response of wireless channel is denoted by

Figure 2 System Model for the Simulation Scenario

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Although the addition can instantaneously beconstructive or destructive, the received power is higher inaverage. In fact, average received power at the UE can

be evaluated as the addition of the power received from botheNode Bs, which results from transmitted power and path lossdue to distance.

UE estimates the combined channel response ) of

both the channels associated with two eNode Bs and then sendit back to the eNode Bs via a feedback channel. Referencesignals included in LTE-Advanced allows this estimation [1].Channel Quality is estimated at the UE through an index called(Channel Quality Information, CQI). CQI is then fed back tothe serving eNodeB via Channel State Information (CSI)feedback mechanism that causes configurable delay and biterror probability. Reported CQI provides information aboutcurrent channel conditions and a measurement of theinstantaneous , which is used by the eNodeB to selectthe appropriate transmission parameters.

Since the noise energy in each subcarrier is independent of

the channel s frequency domain transfer function , the instantaneous Signal-to-Noise ratio (SNR) for

subcarrier can be estimated as

Where SNR is the overall Signal to Noise Ratio, whichdepends on power received from both base stations (which ingeneral are different) and on noise figure.

Constant power variable rate adaptive modulation is performed on a per subcarrier basis. Adaptive modulation iscarried out by means of predefined SNR thresholds thatdetermine the proper modulation rate to maintain the targetBER.

III. SIMULATION RESULTS

Table I and table II shows the parameters used to carry outsimulations using Matlab. Figure 3 shows BER vs. Distancecurve for Adaptive OFDM with CoMP over Rayleigh Channelemploying two Base Stations. It was observed that when theUE is close to eNode B-1, the BER is low. As the UE movesaway from the eNode B-1 and the distance between UE and

base station increases, the BER also increases but it does notexceed the target BER of and a low BER is maintainedwithin acceptable limits. The concept of CoMP is being appliedhere, as it can be seen that when the UE moves away from the

base station there is coordination in data transmission betweenthe two base-stations, eNode B-1 belonging to the serving celland eNode B-2 belonging to the non-serving cell. The UE thenreaches a point such that distance-(d1) between base station-1and distance-(d2) between base station-2 is 9 and 35 meters

respectively. Both the base stations are coordinating with eachother in order to keep the BER low, mitigate inter cellinterference (ICI) due to each other and maintain the targetQoS with the help of adaptive OFDM. This helps to achieveone of the objectives of CoMP that is, improved cell edge datarates. At this point, depending on the mobility path of the UE,the decision to initiate soft handover can be made by the eNode

B with the help of measurement reports. Soft handover in caseof CoMP provide many improvements over hard handover. It

provides macro diversity combining gain which providesenhanced link demodulation performance leading to powergains against fast fading. It also provides Multi-cell gain inwhich multiple unrelated branches in soft handover reduce therequirements on shadow fading margin. Soft handover in caseof CoMP improves load sharing, in the uplink multiple cellsreceive UE signal which reduces the transmit power of the UE

thereby increasing the battery life. In the downlink, multiplecells transmit the RF signals to the UE which reduces thetransmit power of each cell. It was also noted that due toimproved received power gain at the UE by using CoMP JPtechnique the BER vs. distance performance in this casesignificantly improved over scenario where no CoMPtechnique was applied. The BER vs. distance curve forAdaptive OFDM without CoMP over Rayleigh Channel for theUE moving away from the Base Station-1 (eNode B-1) isshown in figure 4. This proves that employing this technique ofAdaptive OFDM with Coordinated Multi-PointCommunications (CoMP) in LTE-A systems can besufficiently effective for achieving desired link performance.This technique can help in reducing OPEX (OperationalExpenses) for the operators at the same time increasing system

performance.

TABLE-I Parameters for Adaptive OFDM

Modulation Schemes SNR-Thresholds (dB) Target BER

10-2 No Transmission 0

BPSK 1 to 4

QPSK 4 to 8

16-QAM 8 to 12

64-QAM 12 onwards

TABLE-II Parameters for Adaptive OFDM & CoMP Tx/Rx

Parameters Values

Number of Sub-

Carriers

256

Length of Cyclic Prefix 40

Modulation Schemes BPSK,QPSK,16-QAM,64-QAMCut-off/Threshold SNR

values (dB)

As per table-1

Overall SNR (dB) 0:5:96

Channel Type Rayleigh (Pedestrian-As per 3GPP

TS- 25.104)

Equalizer Type MMSE

Number of Transmitted

Symbols Per Sub-

Carrier

Number of Simulation-

Iterations

Coding Gray Coding

Sub-Carrier Frequency 10 MHz

Subcarrier spacing 312.5kHz

Cyclic prefix duration,

Tcp

0.8 μs

Data symbol durationTd

3.3 μs

Total Symbol duration,

Ts

4.0 μs

Number of eNode Bs 2

Number of UEs 1

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Figure 5 Spectral Efficiency vs. Distance curve for Adaptive OFDM with

CoMP

Figure 6 Spectral Efficiency vs. Distance curve for Adaptive OFDM

without CoMP

ACKNOWLEDGMENT

For all the interesting discussions and assistance with this

work, our special thanks to Dr. Juan J. Sanchez and Mr.

Francisco Javier Martin.

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Analysis and Evaluation of Cooperative Multi-Point TransmissionReception and Soft Handover for LTE Advanced

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