Inter-cell Interference Mitigation and Coordination in CoMP Systems Norshidah Katiran, Norsheila Fisal, Sharifah Kamilah Syed Yusof, Siti Marwangi Mohamad Maharum, Aimi Syamimi Ab Ghafar, and Faiz Asraf Saparudin UTM-MIMOS CoE Telecommunication Technology, Universiti Teknologi Malaysia, 81310 Skudai, Johor, Malaysia
[email protected], {sheila,kamilah}@fke.utm.my,
[email protected], {aimisyamimi,faiz.asraf}@gmail.com
Abstract. Coordinated Multi-Point (CoMP) transmission and reception is seen as a promising technology to achieve higher spectral efficiency in LTEAdvanced systems. Base stations cooperation can be one of the most important enabling technologies beneficial in scenarios with high inter-cell interference (ICI). In this paper, interference mitigation and coordination schemes in CoMP are being discussed. Simulation study to show the significant effect of ICI to cell-edge performance is presented. In this work, we proposed an interference cancellation scheme with minimal backhaul requirements in addition to existing inter-cell interference coordination (ICIC) schemes. Keywords: CoMP, ICI, eNB, coordination.
1 Introduction The goal of LTE-Advanced (LTE-A) standard is to further enhance system data rates and spectral efficiency while supporting backward compatibility with LTE Release 8. As part of LTE-A standard development, several enhancements including support for up to 100 MHz bandwidth and higher- order MIMO are being investigated to meet the IMT-advanced requirements [1]. An important requirement for the LTE-A system is to improve cell-edge performance and throughput. Unlike the other cellular system (e.g., CDMA) which has robust interference capability, OFDM-based cellular system (e.g., WiMAX, LTE, LTE-A) suffers from ICI at the cell boundary, especially when all frequency channels are fully reused [2]. Therefore, some means of mitigating the ICI is required to support a full frequency-reuse operation. This problem has attracted much attention and some strategies have been proposed in order to improve cell edge performance. Recently, cooperative communications (e.g., base stations, relays) have been studied extensively to exploit diversity in order to achieve better network performance [2-4]. As for LTE-A systems, Coordinated Multi-Point transmission and reception (CoMP) has been proposed as one of the key technology to enhance cell average and cell edge throughput. CoMP refers to a system where the transmission and/or A. Abd Manaf et al. (Eds.): ICIEIS 2011, Part III, CCIS 253, pp. 654–665, 2011. © Springer-Verlag Berlin Heidelberg 2011
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reception at multiple, geographically separated antenna sites is dynamically coordinated in order to improve system performance. There are two types of coordination strategy in CoMP: Joint Processing (JP) and Coordinated Scheduling/ Beamforming (CS/CB). In the first strategy, data to a single user end (UE) is simultaneously transmitted from multiple transmission points [3]. This strategy puts higher requirements on the coordination links and the backhaul since user data need to be made available at multiple coordinated transmission points. The amount of data to be exchanged over the coordination links is also large. By contrast, in coordinated scheduling (CS) scheme, a resource block (RB) is transmitted only from the serving cell [3]. An RB is assigned to the UE by scheduling of the serving cell. Scheduling/ beamforming is coordinated among multiple coordinated cells where the transmit beamforming weights are generated to reduce unnecessary interference to other UE scheduled within the coordinated cells. The rest of the paper is organized as follows: Section 2 discusses related work on ICI mitigation schemes in CoMP. Section 3 presents ICI problem formulation. Section 4 shows simulation result for ICI problem mentioned in Section 3. We proposed our ICIC strategy in Section 5. Finally, the conclusion and recommendations for future work are drawn in Section 6.
2 Related Work The goal of the ICIC scheme is to apply certain restrictions (e.g., transmit power, RB) on the resources used in different cells in a coordinated way. Such restrictions in a cell provide the possibility for improvement in SNR, and cell-edge data rates on the corresponding time-frequency resources in a neighbor cell [4]. The ICI coordination requires certain inter-eNB communication in order to configure the scheduler restrictions. From literature, we identified some ICIC schemes recently used to mitigate ICI in CoMP systems [5-17] and here we classified these schemes into three main categories. 2.1 Interference Avoiding Schemes Interference avoidance technique focuses on finding an optimal effective frequency reuse factor. It is often achieved through restrictions on frequency and power allocations to fulfill network performance goal. When frequency reuse factor, K=1 as in Fig. 1a), the entire bandwidth available for transmission is used in all sectors or cells [5]. In this case, the UEs near the cell-center will experience high signal-tointerference noise ratio (SINR) due to large path loss from adjacent cells. On the other hand, the UEs at the cell boundary will suffer from a small SINR (signal-tointerference plus noise ratio), which may increase an outage rate at the cell boundary. In order to improve the SINR throughout the cell coverage area while reducing the outage rate at the cell boundary, the whole bandwidth can be divided into three channels (Fig. 1b)). Each channel is allocated to adjacent cells in an orthogonal manner. It corresponds to K=3 and reduces the usable bandwidth for each cell. However, the UEs at the cell boundary will experience improved SINR, reducing ICI.
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A fractional frequency reuse (FFR) scheme is based on the concept of reuse partitioning. In reuse partitioning, the UEs with the highest signal quality use a lower reuse factor while UEs with low SINR use a higher reuse factor. The FFR scheme, for example uses frequency reuse of one for cell-center UEs while a reuse of three is used for the cell-edge UEs as shown in Fig. 1d) [5]. In FFR schemes, the frequency resources for cell-edge UEs in the neighboring cells are left empty in a given cell. By contrast, all the frequency resources can be used in all the cells in soft frequency reuse (SFR) schemes (Fig. 1c)). The frequency resource used for cell-center UEs in a cell is used for cell-edge UEs in neighboring cells. Although static interference schemes such as FFR and SFR achieve improved throughput [5]-[8], they may suffer seriously in terms of sector or cell throughput. An optimal partitioning approach needs to consider the distribution of the UEs, traffic arrival and channel dynamism. Therefore, any static reuse partitioning scheme always results in suboptimal solution.
Fig. 1. Some static interference avoidance schemes [5]
For LTE downlink, an interference avoidance scheme that uses dynamic ICIC facilitated through X2 interfaces among neighboring eNBs has been evaluated for LTE downlink [9]. A UE restricts I inter-eNB most dominant interferers and then determines the achievable rates on physical resource block (PRB), n. A pair of sectors will have restrictions for the same PRB in some cases (e.g., when ICI received at particular sector/cell is unacceptable). In this case, the corresponding eNB communicates with the neighboring eNBs using X2 interfaces about restricting the PRB. Then, the corresponding eNB will decide either the restricted PRB are unused or used with lower power. [10] considered the objective of network-wide proportional fairness (PF) through load balancing where under-loaded cells avoid using the same RBs as used by cell-edge UEs in neighboring over-loaded cells. Thus, ICI imposed on them can be reduced and further increase the capacity of over-loaded cells. In another approach, [11] presented a decentralized ICIC scheme in order to mitigate ICI and enhance the capacity of the users found near the cell-edge area of a wireless multi-cell OFDMA system. The scheme aims at maximizing the throughput of the cell-edge users which experience the most severe form of ICI with a minimal coordination between eNBs. The algorithm tries to find the subchannel that is less reused by neighboring eNBs and then finds the pair of cell-edge user and subchannel that yields the higher channel gain. Then, it allocates equal amount of power to all
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users of the cell-edge group. Each cell solves its own problem with a minimal coordination via information exchange through X2 interface. 2.2 Coordinated Scheduling/Beamforming (CS/CB) In CB/CS coordination strategy, it is not necessary to share the UE’s data across multiple eNBs. To achieve the coordination, the UE needs to feed back information about the CSIs of the serving cell and the other cells in the CoMP set. [12] presented coordinated scheduling method based on FFR and multi-cell precoding matrix index PMI coordination in CoMP system (Fig. 2). To support PMI coordination, the celledge UE is required to feedback information that includes the preferred PMI indices for each frequency sub-band to its serving cell. Also, it recommends or restricts precoding index for neighboring cells. Through multi-cell coordination, the neighboring cell is requested to either use the recommended precoder or not to use the restricted precoder. PMI recommendation is more effective than PMI restriction in suppressing the interference. In order to avoid excessive feedback overhead, the PMI information can be limited to one or two strongly interfering cells.
Fig. 2. Inter-eNB signaling for PMI coordination [12]
By comparison, [13] proposed a CB scheme with explicit feedback which denotes full CSI and designed the precoding vector through exploitation of the signal leakage information to other cells to reduce ICI. To mitigate ICI in CoMP based on CS/CB, [14] used a downlink transmit beamforming with nulling under partial channel state information (CSI) and no data sharing condition for cell-edge users with low mobility. By considering all possible interfering channel direction to UEs in adjacent cell, both inter-user interference (IUI) and ICI can be canceled perfectly. However, this strategy induces a trade-off between mitigating interference for cell-edge users and maximizing the total throughput.
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2.3 High Interference Indicator (HII) and Overload Indicator (OI) Mechanisms The LTE system defines a load indication procedure for uplink ICIC. This mechanism is used to send an interference overload indication when an eNB experiences too high interference on some resource blocks. The eNB experiencing excessive interference initiates the procedure by sending an uplink interference overload indication (OI) message to intra-frequency neighboring eNBs. The information exchanged consists of high interference, medium interference or low interference indication for each PRB. On the other hand, HII is in general a proactive operation, aiming at preventing the occurrence of ICI. The serving cell informs the neighboring cells of which resources will be used by its edge users. Then neighboring cells control the allocation of resources, such as lowering the power or resources re-allocation. While work in [15] and [16] employed OI and HII separately which will incur inevitable shortcomings, [17] used an OI and HII hybrid scheme that can enhance system performance and reduce backhaul signaling.
Fig. 3. ICIC based on HII and OI X2 signaling [18]
3 System Model 3.1 ICI Problem As a UE moves away from the cell-center, SINR degrades due to two factors. Firstly, the received signal strength goes down as the path-loss increases with distance from the serving eNB. Secondly, the ICI goes up because when a UE moves away from one eNB, it is generally getting closer to another eNB as shown in Fig. 4. We assume that the UE is connected to eNB1 and moving away from eNB1 towards eNB2. Furthermore, we assume frequency reuse of one, which means that both eNB1 and eNB2 transmit on the same frequency resources. Therefore, the signal transmitted from eNB2 appears as interference to the UE.
Fig. 4. Inter-cell Interference (ICI)
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The SINR experienced by the UE at a distance r from eNB2 can be expressed as [4]:
P1r −α ρ= . N 0W + P2 (2 R − r ) −α
(1)
where α is the path-loss exponent and Pk is the transmit power for the kth eNB. Also, R is the cell-radius with 2R the distance between eNB1 and eNB2. In general, all the eNBs in a system use the same transmit power and therefore we will assume P1 = P2. In a severely interference-limited scenario, the background noise, N0 can be ignored and Equation (1) can be simplified as: α
2R ρ = − 1 . r
(2)
Let us assume the path loss model for 2 GHz frequency:
PLs = 128.1 + 37.8 log10 (r ) dB.
(3)
where r is the distance between the UE and eNB in kilometers. In addition, we assume in-building penetration loss of 20 dB. The same path-loss model is assumed for the interferer eNB2.
PLi = 128.1 + 37.8 log10 (2 R − r ) dB.
(4)
The SINR experienced by the UE can be written as:
ρ ICI
PLs P10 10 . = PLi N 0 + P10 10
(5)
When the ICI is not present, the SINR experienced by the UE can be written as:
ρ No − ICI
s PL P10 10 = N0
.
(6)
Shannon showed that the system capacity C of a channel perturbed by additive white Gaussian noise (AWGN) is a function of the average received signal power S, the average noise power, N and the bandwidth, W [19]. The capacity relationship according to Shannon theorem can be stated as:
C = W log 2 (1 +
S ). N
(7)
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3.2 Conventional Non-cooperative Scheme The conventional system is characterized by single cell signal reception and suffers from ICI from its neighboring cell (shown as shaded hexagonal cell area).
Fig. 5. Non-cooperative system
3.2 2-Cell CoMP (JP) In the class of joint processing (JP), multiple eNBs jointly transmit signals to a single UE terminal improve the received signal quality [20]. In this case, data intended for a particular UE terminal is shared by the neighboring cell (Cell 2) and is jointly processed at this cell.
Fig. 6. CoMP system with 2 cells
We assume that UE1 is receiving signals from the two cells: Cell 1 and Cell 2 (denoted as C1 and C2). Assume Hi1is the channel gain from Ci to UE1, the received signal Y1 at UE1 can be expressed as
Y1 = H11W1 X 1 + H 21W2 X 2 + Z1 .
(8)
where Xi is the signal transmitted at Ci, Wi is the precoding matrix at Ci, and Z1 is the adaptive white Gaussian noise (AWGN) at the receiver.
4 Results and Discussion 4.1 ICI Analysis In Fig. 7, we presented SINR with and without assuming ICI as a function of distance from the cell-center, r for a UE receiving transmission over a 20 MHz bandwidth.
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The total background noise is No = -104 dBm. We also assume eNB transmit power of P = 43 dBm. We note that SINR degrades with increasing r, which is the case for cell-edge UEs. Also, for a given r < R, the SINR is higher for a larger path-loss exponent α. This is because the interference travels a longer distance for r < R and is attenuated more for larger α.
Signal to Interference Plus Noise Ratio,SINR(dB)
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Interference is the major limiting factor in the performance of cellular radio systems. The relative gains in throughput by eliminating ICI are expected to be even larger for low SINR UEs as the capacity scales almost linearly at lower SINR. For high SINR UEs, small gains in SINR by ICI elimination do not translate into any meaningful gains in throughput as shown in Fig. 8. From this discussion, we can 5 4.5 4
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Fig. 8. Capacity gain result
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conclude that ICI is more important for cell-edge UEs than for the cell-center UEs. Therefore, ICI mitigation schemes can be used to improve system cell-edge performance. 4.2 CoMP Performance We further evaluated the performance of the CoMP (JP) scheme and compared to the conventional non-cooperative system. At this stage, we assumed that both systems are single-input single-output (SISO) based. In Fig. 9 we compare the error performance of the CoMP scheme and the conventional scheme over Rayleigh fading channel for downlink. -3
10
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-4
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Fig. 9. Error performance comparison
The CoMP scheme gives better error performance over the conventional noncooperative. Cooperation among eNBs can effectively turn harmful ICI into useful signals, allowing significant power gain to be exploited. Note that the above analysis is based on the fact only one UE is served by the CoMP cluster which is called CoMP single user (SU) MIMO mode. It is expected that CoMP (JP) will bring more significant system improvement at a higher implementation cost [19]. In CoMP (JP), the data together with channel related information for different UEs need to be exchanged among the cells within the CoMP cluster. This data exchange can be done in wired backhaul. However, this will cause additional latency and impose stringent requirements for backhaul technologies.
5 Proposed ICIC Strategy We consider a CoMP-JP system with N cooperative eNBs assigned with the same carrier frequency. Each eNB tries to detect the UEs transmitting in their own sector
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without any support from other eNBs. In case that the cell-edge UEs have been detected, the data transmitted by them is signaled to all cooperating eNBs. When all cooperating eNBs have accurate multi-cell CSI, they can accurately reconstruct the interference caused by these UEs and subtract it from their own received signals. As a result, the probability that the UE transmitting in the respective cells can be decoded can be improved. Note that this interference cancellation scheme is applied to celledge users only so that we do not burden the backhaul link with excessive data and information exchange. This is one of the main contributions of our proposed scheme. As shown in Fig. 11, assume there are 3 cooperating eNBs in the system and signal transmitted by UEI to its own eNB interfers the signal transmitted by UE2 and UE3 to their respective eNBs as both of them are located at cell boundary region. All cells decode their own user’s data and then transmit their cell-edge user’s data to the neighboring cells. For example, Cell 1 decodes transmitting data by UE1 and signals it to Cell 2 and Cell 3 on the backhaul links. Then, Cell 2 and Cell 3 subtract the interfering signal (from UE1) before detecting their own received signal. In general, the proposed scheme can reduce the requirements on backhaul link since only data of users located at cell boundary region is shared by the cooperating eNBs. Besides, its capability to mitigate ICI is feasible and undoubted.
Fig. 11. System model of the proposed scheme
6 Conclusion The LTE system targets better cell-edge performance in order to provide service consistency in terms of geographical coverage and throughput. In interference-limited scenarios, the cell-edge performance can be improved via ICIC. In this paper, intercell interference coordination (ICIC) schemes in CoMP system have been discussed. However, there are some limitation and drawback that we have addressed throughout our review on these schemes. In FFR and SFR schemes for example, interference is mitigated by sacrificing cell throughput and are suboptimal since the nature of channel dynamism in wireless environment has not been considered. As for CoMP (CS/CB) schemes, the challenge lies in the precoder design complexity that can optimally suppress interference. In addition, excessive feedback overhead (e.g. CSI, PMI) from UE to eNB should be avoided. Utilizing HII and OI mechanism to mitigate
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ICI should be performed with minimum inter-eNB signaling to minimize signaling delay and overhead. In general there is tradeoff between cooperation and information exchange which needs be further explored. We have shown simulation results and discussed the significant effect of ICI to the received SNR and capacity. We also performed basic system performance of CoMP scheme over conventional noncooperative scheme. Besides, we also proposed an interference cancellation scheme in CoMP system with minimal signaling as to reduce the X2 bandwidth requirements and processing delay. Currently, the proposed work is still in progress and in future we will be sharing the resulting outcomes. Acknowledgement. The authors would like to thank to Ministry of Higher Education, Malaysia (MOHE), Research Management Centre (UTM-RMC), and Universiti Teknologi Malaysia (UTM) for their support. This work was funded by Vote No. Q.J130000.7123.01F.01H35 grant.
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