Zhang et al EURASIP Journal on Wireless Communications and Networking 2011, 2011:187 http://jwcn.eurasipjournals.com/content/2011/1/187 RESEARCH Open Access Analysis and modeling of spatial characteristics in urban microscenario of heterogeneous network Jianhua Zhang1*, Nan Sheng1, Fenghua Zhang1, Lei Tian1, Guangyi Liu2, Weihui Dong2, Ping Zhang1 and Chia-Chin Chong3 Abstract Heterogeneous network (HetNet) is a typical deployment scenario for the IMT-Advanced system whereby the macro enhanced node B (eNB) provides the wide coverage while the lower power nodes such as micro, pico, femto, and relay nodes extend the coverage/capacity for coverage hole or hotspot This literature addresses the spatial propagation modeling for urban micro (UMi) scenario of HetNet Due to users distributed in canyon streets, the multipath with high power is not always coming from the line-of-sight (LoS) direction in UMi scenario Moreover, considering the impact of the directional antenna pattern, the current IMT-Advanced UMi channel model may lead to inaccurate interference modeling To verify this, multiple-input multiple-output (MIMO) field channel measurement is conducted in downtown Beijing for typical UMi Based on the measurement data analysis, the multipath’s angular offset from the LoS direction is clearly observed In order to capture such spatial characteristic into the existing IMT-Advanced UMi channel model, the angular offset models are proposed for both LoS and non-LoS (NLoS) cases Finally, the interference and capacity simulation prove that it is necessary to capture the angle offset model into the MIMO channel model in UMi scenario Keywords: IMT-Advanced, multiple-input multiple-output (MIMO), channel model, interference, HetNet Introduction With the expansion of the mobile data market, the mobile operators have more and more pressure to expand the cellular capacity by cell splitting or carrier aggregation [1] In order to make full use of the expensive spectrum, the cellular technology is required to improve the spectrum efficiency as much as possible As reported in [2], the mobile data market will increase more than 50 times from 2010 to 2015 In order to meet the requirements of the future data market, 3rd Generation Partnership Project (3GPP) has started the research and standardization of the next generation cellular network technology, which is called as LTEAdvanced [3] The cellular system is usually planned as hierarchical coverage The macro enhanced node B (eNB) with high transmit power and high antenna height is deployed to * Correspondence: jhzhang@bupt.edu.cn Key Laboratory of Universal Wireless Communications, Ministry of Education, Beijing University of Posts and Telecommunications, P.O Box 92, Beijing 100876, China Full list of author information is available at the end of the article provide wide coverage as the basic layer, whereas some low power nodes such as micro, pico, femto, and relay [4] nodes are deployed for the coverage/capacity expansion as the secondary layer In order to alleviate the complexity of the network planning and optimization in hierarchical cellular deployment like Global System for Mobile (GSM) and Universal Mobile Telecommunication System, the macro eNB and micro/pico nodes are allocated with different carrier frequencies, and thus the interference between different coverage layers can be ignored According to the prediction from International Telecommunication Union-Radio communication sector (ITU-R) [5], the required spectrum for IMT-Advanced is above GHz, while the spectrum allocated for the IMT-Advanced by ITU-R is less than 500 MHz now In order to fill the spectrum gap between the required and the available, more aggressive spectrum usage strategies have to be considered for IMT-Advanced In 3GPP, the hierarchical network with the same spectrum allocated for both basic and secondary layers is defined as heterogeneous network (HetNet) [6] Compared to the © 2011 Zhang et al; licensee Springer This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited Zhang et al EURASIP Journal on Wireless Communications and Networking 2011, 2011:187 http://jwcn.eurasipjournals.com/content/2011/1/187 homogeneous interference among the macro eNBs, the heterogeneous interference between macro eNB and lower power nodes becomes more serious In order to deal with the serious inter-cell interference, the enhanced inter-cell interference coordination (eICIC) [6] and heterogeneous coordinated multiple point transmission and reception (CoMP) [7,8] are proposed in 3GPP To facilitate the corresponding performance evaluation for HetNet, 3GPP has defined the evaluation methodology for HetNet and eICIC [6] However, only the path loss and shadow fading are explicitly defined based on the existing models, such as the IMT-Advanced model [9] and ITU-R M.1225 [10], whereas the fast fading is not defined explicitly For the performance evaluation of eICIC in time domain, the path loss and shadowing may be sufficient; however, for the eICIC and CoMP in the spatial and frequency domains, the fast fading is necessary to show a reliable performance Therefore, both the fast fading and slow fading of the MIMO channel should be captured in performance evaluation methodology of HetNet As shown in Figure 1, the eNB sites in typical urban scenarios are usually surrounded by high-rise buildings For the outdoor users distributed in the streets outside of the building, the multipaths with high power are not always coming from the line-of-sight (LoS) direction due to the special street environment in urban micro (UMi) scenario However, in the current geometry-based spatial channel model (GBSM), e g., IMT-Advanced UMi channel model [9], the high power multipath always focuses around the LoS direction for user equipment (UE) Therefore, the conflict between the characteristics of the typical UMi scenario and its corresponding GBSM model may happen, which lead to inconsistency between the real Page of 12 propagation characteristics and the corresponding spatial channel modeling In this literature, the spatial models of the typical UMi scenario of HetNet are addressed Regarding the existing IMT-Advanced UMi channel model, due to the impact of the directional antenna pattern of eNB transmitter and the phenomenon as described above, the intra-site interference from the neighboring sectors of the same micro eNB may be underestimated and thus, leads to overestimation of the single point MIMO system performance Dedicated MIMO field channel measurements were conducted in downtown Beijing for typical UMi scenario The angular offset of the multipath is observed from the measurement results, and a modified MIMO channel model is proposed to capture such spatial characteristics into the UMi channel model, where a random angular offset is captured into the fast fading To verify the influence of the proposed model, the theoretical channel capacity based on the measured data is analyzed, and system level simulations of Time Division LTE-Advanced (TD-LTE-Advanced) system [11] are performed The numerical results show that the intra-site interference has been underestimated by the original IMT-Advanced UMi model, while the proposed model provide better CoMP gain due to taking into account the impact of the directional antenna pattern and the angular offset on the multipath The rest of the article is organized as follows The limitations of the existing channel models are discussed in Section The field channel measurement is described in Section The proposed channel model for UMi is presented in Section The theoretical analysis and system level simulation results are given in Sections and 6, respectively Finally, the conclusions are drawn in Section Figure HetNet deployment of macro/micro in typical urban environment Zhang et al EURASIP Journal on Wireless Communications and Networking 2011, 2011:187 http://jwcn.eurasipjournals.com/content/2011/1/187 Limitations of existing channel model Wireless channel consists of many propagation paths, which diffuse in the spatial domain at both the transmitter and the receiver The performance of MIMO system is greatly affected by the extent of the angular dispersion of angle of departure (AoD) and angle of arrival (AoA), which is described by the angular spread (AS) in the existing channel models Moreover, due to the application of sectorized antennas, the spatial characteristics of the channel between the eNB and a certain UE may be influenced by the antenna pattern of the eNB As illustrated in Figure 2, the eNB has three sectors, i e., sectors A, B, and C UE is served by sector A Without loss of generality, we take the downlink interference from sector B to the UE for example The intra-site interference could arrive at the UE either straightly from the backside of the antenna (the green line in Figure 2) or from the reflection by remote scatterers (the red line in Figure 2) In existing GBSM, e.g., IMT-Advanced channel model [9], almost all the intra-site interference from sector B is supposed to come from the backside of sector B antenna and has experienced extra 25 dB attenuation because of the front to back ratio of the antenna [9] However, it might not be the case in UMi scenario, especially in the downtown of dense urban like Beijing The canyon-like streets environment in such scenario may lead to peculiar spatial characteristics of the Page of 12 propagation channel, e.g., the center of the PAS has some offset from the LoS direction Considering the impact of the sectored antenna pattern, it may influence the interference modeling much and thus influence the network capacity To facilitate the analysis, the intra-site interference from sector B is defined as follows I=X· PL · AeNB (θ ) · P(θ ) · AUE · dθ , ◦ −180