Performance Gain of Smart Antennas with Diversity Combining at Handsets for the 3Gpp Wcdma System
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13th International Conference on Wireless Communications (Wireless 2001), July 2001. Calgary, Alberta, Canada PERFORMANCE GAIN OF SMART ANTENNAS WITH DIVERSITY COMBINING AT HANDSETS FOR THE 3GPP WCDMA SYSTEM Suk Won Kim1, Dong Sam Ha1, and Jeong Ho Kim2 1VTVT (Virginia Tech VLSI for Telecommunications) Laboratory Department of Electrical and Computer Engineering, Virginia Tech, Blacksburg, Virginia 24061 USA 2Next Generation Wireless Communication Research Laboratory LG Electronics, Inc, Hogye-dong 533, Dongan-gu, Anyang-shi, Kyongki-do, Korea Phone: +1-540-231-4942 Fax: +1-540-231-3362 E-mail: [email protected], [email protected], and [email protected] ABSTRACT Smart antenna techniques have been considered mostly for base stations so far [2],[3] because of In this paper, we propose a dual smart antenna high system complexity and high power system incorporated into handsets for the 3GPP consumption. In addition, two (or multiple) WCDMA system, in which a diversity combiner antennas at a handset are in proximity, which may combines the two rake receiver outputs using a reduce the effectiveness of the antenna system. One diversity combining scheme. We considered three of the third generation wireless personal diversity combining schemes, selection diversity, communication systems, 3GPP (third generation square law combining, and equal gain combining partnership project) WCDMA (wideband code (EGC), and two types of the channel model for the division multiple access) system [4], requires dual antenna signals, loosely correlated fading antenna diversity at base stations and optionally at channel model (LCFCM) and spatially correlated mobile stations [5]. The feasibility of implementing fading channel model (SCFCM). To obtain the dual antennas at a mobile handset was investigated channel profile, we adopted a geometrically based in [6]. Due to the compact size and stringent cost of channel model known as GBSB (geometrically handsets and the limited battery capacity, smart based single bounce) elliptical model in the antennas at handsets should have low circuit simulation. The simulation results indicate that i) complexity and low power dissipation. To justify the EGC scheme performs the best among the three employing smart antenna techniques at handsets, diversity combining schemes and ii) as expected, the performance gain should be large enough to the higher performance gain is achieved under the offset the additional cost and power consumption. LCFCM than the SCFCM. The simulation results Recently, the smart antenna technique has been under the two channel models are likely to be the applied to mobile stations [7]-[9]. For example, the lower and the upper bounds for the performance HDR (high data rate) system of Qualcomm gain of the dual smart antenna system. Our employed dual antennas at a mobile station [7]. simulation results indicate that a dual smart antenna Each antenna signal is applied to its own rake system with diversity combining at handsets is receiver followed by the maximal ratio diversity beneficial for the 3GPP WCDMA system. combining, which combines the two rake receiver signals. A dual antenna system for handsets was I. INTRODUCTION also investigated for the digital European cordless Smart antenna not only combats multipath telephone (DECT) system for the indoor radio fading, but also suppresses interference signals. channel [8]. The dual antenna handset receiver When spatial signal processing achieved through selects one of two signals of the receivers based on smart antenna is combined with temporal signal the signal-to-interference plus noise ratio (SINR). processing, the space-time processing can repair Each receiver processes a signal that is an equal signal impairments to result in a higher network gain combining of the signal from one antenna and capacity, coverage, and quality [1]. When the phase-shifted signal from the other antenna. compared with the conventional single antenna Wong and Cox proposed a dual antenna system system, a smart antenna system requires additional which could be applied to handheld devices as well antennas and circuitry to process multiple antenna as base stations [9]. Summing the signals from two signals. The additional antennas and circuitry result antennas with proper weights in complex numbers in higher cost and more power consumption. cancels the dominant interference and hence increases the signal-to-interference ratio (SIR). To 235 13th International Conference on Wireless Communications (Wireless 2001), July 2001. Calgary, Alberta, Canada compute the antenna weights, a technique to for the dual antenna signals, i) loosely correlated optimize the SIR was proposed. Unlike the above fading channel model (LCFCM) and ii) spatially two methods, the signal weighting and summing correlated fading channel model (SCFCM). In the was implemented at the radio frequency (RF) level LCFCM, each antenna signal is assumed to have instead of at the baseband signal level. Thus, it independent Rayleigh fading. However, each reduces the complexity of the receiver since it antenna signal in the SCFCM is subject to the same requires only one baseband processor. Rayleigh fading, and the two signals are different Smart antennas may employ two different only in phase due to a nonzero angle of arrival. A combining schemes, diversity combining and channel model with less correlated dual antenna adaptive combining. The diversity combining signals, which is the LCFCM in our models, is scheme exploits the spatial diversity among expected to yield higher diversity gain [12]. We multiple antenna signals. Thus, the diversity believe that the actual channel of dual antenna combining achieves higher performance when signals lies in between these two channel models. multiple antenna signals are less correlated. For To obtain the channel profile (such as delay, instance, if each antenna signal undergoes average power, and angle of arrival of each independent fading, the diversity combing scheme multipath signal), we adopted a geometrically based would perform well. The adaptive combining channel model known as GBSB (geometrically scheme adjusts the antenna weights dynamically to based single bounce) elliptical model in our enhance the desired signal while suppressing simulation [13][14]. The GBSB elliptical model is interference signals. Since the adaptive combining applicable for microcell environments found in scheme aims to add multiple antenna signals urban areas. constructively, the scheme performs better for The paper is organized as follows. The channel correlated antenna signals. Thus, if multiple models employed for our simulation are presented antenna signals are exactly the same except the in Section 2. The 3GPP WCDMA system is briefly phase difference due to a nonzero angle of arrival, described in Section 3. The system setup for the adaptive combining achieves the highest simulation and the simulation results are presented performance. in Section 4. Finally, Section 5 concludes the paper. In [10], we showed that smart antennas with a diversity combining scheme at handsets for the II. CHANNEL MODEL cdma2000 system improve the performance Because the channel model influences the (through reduction of the frame error rate) by from design of receivers and their performance, channel 1.7 dB to as high as 12.7 dB depending on the modeling is important when evaluating a smart correlation of the dual antenna signals. We also antenna system. In the uplink of the 3GPP showed that smart antennas with an adaptive WCDMA system, each user signal is transmitted combining scheme at handsets for the cdma2000 asynchronously and traverses different paths from a system improve the performance (through reduction mobile station to the base station. Thus, the main of the frame error rate) by from 0.8 dB to 2.2 dB source of interference is coming from other users’ depending on the mobile velocity [11]. signals within the same cell (intra-cell interference). In this paper, we propose a dual smart antenna However, in the downlink of the 3GPP WCDMA system incorporated into handsets for the 3GPP system, the signal transmitted from the base station WCDMA system and present simulation results on is the superposition of all active users’ signals and the performance gain for a diversity combining. A common control signals. The desired user signal diversity combiner combines the two rake receiver and multiple access interferences traverse the same outputs using a diversity combining scheme. We paths, but they are inherently orthogonal to each consider three diversity combining schemes, i) other. So it dose not pose a serious problem at selection diversity (SD), ii) square law combining handsets. A multipath signal is effectively an (SLC), and iii) equal gain combining (EGC). The interference signal to another multipath signal. SD scheme selects the signal with higher power. However, a rake receiver can manage multipath The SLC scheme weights each signal by its signal signals to its advantage to improve the level and adds them according to the formula, performance. Another source of interference in the | a | | b | a + b ,wherea and b are downlink is coming from adjacent cells (inter-cell a2 + b2 a 2 + b2 interference), which can have a substantial impact on the performance. Note that the latter case the two rake receiver outputs. The EGC scheme becomes manifest when the soft handover is simply adds two signals with an equal weight of occurred. Since the number of adjacent base 0.5. We consider two types of the channel model 236 13th International