Chirag Sureshbhai Patel

Network densification: the dominant theme for wireless evolution into 5G

This article explores network densification as the key mechanism for wireless evolution over the next decade. Network densification includes densification over space (e.g, dense deployment of small cells) and frequency (utilizing larger portions of radio spectrum in diverse bands). Large-scale cost-effective spatial densification is facilitated by self-organizing networks and intercell interference management. Full benefits of network densification can be realized only if it is complemented by backhaul densification, and advanced receivers capable of interference cancellation.

Femtocells [Industry Perspectives]

Femtocells, also known as home NodeBs (HNBs) or femto access points (FAPs), are low-transmit-power (100 mW or less) small-form-factor cellular base stations typically deployed indoors in residential, enterprise, and hotspot settings. Femtocells operate on licensed spectrum and provide voice/data service to mobile phone users by connecting them to the Internet and an operator?s core network using a broadband Internet connection (e.g. ADSL, cable) as a backhaul. Femtocell users experience excellent voice call quality and near peak data rates due to improved radio frequency (RF) coverage.

System design of CDMA2000 femtocells

Femtocells extend the cellular network coverage and provide high speed data service inside homes and enterprises for mobiles supporting existing cellular radio communication techniques. They also provide additional system capacity by offloading macro network traffic. This article reviews the characteristics of cdma2000-based femtocell systems. It discusses design and deployment aspects such as carrier allocation, access control, efficient support for femtocell discovery by idle mobiles, and active call hand-in from macrocell to femtocell. The evolution of the cdma2000 standard for optimizing performance and enriching user experience with femtocells is also discussed.

Field Results on MIMO Performance in UMB Systems

The paper presents MIMO field performance results observed using a ultra mobile broadband (UMB) testbed network. We evaluate metrics such as antenna correlations and channel condition number to characterize the MIMO channel. Results show that low condition numbers, which are beneficial to MIMO, are prevalent for a majority of the coverage area in our network. We demonstrate that the use of MIMO provides gains of the order of 20-40% over SIMO transmissions. These gains are made possible by the use of cross-polarized transmit antennas and advanced UMB features that allow dynamic MIMO vs. SIMO transmission selection based on channel conditions. These results are obtained in a truly mobile, wireless wide-area deployment, which makes them unique. Our results point to the viability and value of MIMO in future mobile wireless networks.

Downlink Transmit Power Calibration for Enterprise Femtocells

Downlink transmit (Tx) power calibration is necessary in femtocell deployments to achieve a good tradeoff between downlink femtocell coverage and interference to other macrocells and femtocells. Developing a practical Tx power calibration scheme that provides excellent performance in different deployment scenarios is challenging. In this paper, we present an enterprise deployment framework that consists of coverage planning and Tx power calibration using technician assistance. The Tx power calibration algorithms presented here use mobile feedback to learn key information such as surrounding RF environment, required coverage range, and interference impact to other cells to adapt to different deployments. Both centralized and distributed Tx power calibration algorithms are evaluated for multi-femto enterprise. It is shown that the distributed algorithm, is simple, efficient and performs very close to the centralized algorithm. Furthermore, over-the-air field tests highlight the practical efficacy of the distributed algorithm in providing excellent femtocell coverage indoors with minimal leakage to outdoors.

Performance analysis of active handoff in CDMA2000 femtocells

Cellular operators are developing femtocells to improve network coverage for indoor users. But in current CDMA systems, heavy reuse of PN codes among femtocells causes ambiguity in choosing the target for handoff. To resolve the ambiguity, a method which uses reverse link (RL) sensing at femtocells has been proposed. This method uses RL measurements of the mobiles signal to choose the femtocell with the best forward link channel, as the handoff target. The robustness of this method to fast fading and shadowing is demonstrated in this work. By making multiple RL measurements, the error in handoff due to fading can be reduced, but at the cost of increased delay. The tradeoff between probability of error in handoff and RL measurement duration is demonstrated. It is also shown that unsynchronized measurements by femtocells cause a only marginal increase in the probability of handoff error.

Downlink interference management techniques for residential femtocells

Downlink transmit (Tx) power calibration techniques are vital in self-organizing femtocell networks to achieve a good tradeoff between femtocell coverage and interference impact to nearby macrocells and femtocells. In this paper, we present two power calibration techniques that adapt to a wide variety of residential deployment scenarios: (1) Over a long term, Tx power is adapted by learning required coverage range, RF profile inside the home and interference outside using feedback from home users and macro users; (2) Tx power is adapted in real-time based on short term dynamic variations in traffic such as arrival of active macro users (guest or passer-by). The presence of nearby active macro users is detected by sensing their uplink channel. The algorithms are shown to provide superior performance compared to conventional techniques in wide variety of deployment scenarios.