Doron Ezri

Green Cellular — Optimizing the cellular network for minimal emission from mobile stations

Wireless systems, which include cellular phones, have become an essential part of the modern life. However the mounting evidence that cellular radiation might adversely affect the health of its users, leads to a growing concern among authorities and the general public. Radiating antennas in the proximity of the user, such as antennas of mobile phones are of special interest for this matter. In this paper we suggest a new architecture for wireless networks, aiming at minimal emission from mobile stations, without any additional radiation sources. The new architecture, dubbed Green Cellular, abandons the classical transceiver base station design and suggests the augmentation of transceiver base stations with receive only devices. These devices, dubbed Green Antennas, are not aiming at coverage extension but rather at minimizing the emission from mobile stations. We discuss the implications of the Green Cellular architecture on 3G and 4G cellular technologies. We conclude by showing that employing the Green Cellular approach may lead to a significant decrease in the emission from mobile stations, especially in indoor scenarios. This is achieved without exposing the user to any additional radiation source.

Location based beamforming

When channel state information is available at the transmitter, it can be exploited to increase the throughput, or to enhance the performance of a multiple input multiple output (MIMO) system. Beamforming schemes, such as closed loop MIMO and transmit beamforming constitute efficient means to achieve the aforementioned objectives. A significant shortcoming of transmit beamforming schemes is the signaling overhead required to provide the transmitter with the downlink channel knowledge. Another drawback of transmit beamforming schemes is their sensitivity to channel state information accuracy. In this paper we suggest an alternative beamforming method that uses the receiver positioning data, for example GPS positioning data, to create the transmit beamforming vector. This approach is attractive as in many modern communications systems location based services provide the transmitter (for example a cellular base station) with continuous information respective to the physical position of the receiver (the celluar phone in this case). The location based beamforming method constructs precoding vectors that are optimized for line of sight scenarios. Our simulations show that line of sight precoders are superior to regular codebooks based precoders, such as the IEEE802.16e codebook, in line of sight scenarios. Surprisingly the proposed precoders exhibit acceptable (and even superior) performance in some non line of sight scenarios.

Performance study of Green Cellular - An architecture for minimal emission from mobile stations

The mounting evidence, that cellular radiation may adversely affect the health of its users, results in growing concern among the general public. This concern only grows as cellular technologies become an essential part of modern life (mobile e-mail, social networking, etc.). Radiating antennas in the proximity of the user, such as antennas of mobile phones are of special interest for this matter. In this paper we study the performance of a recently proposed architecture for wireless networks, aiming at minimal emission from mobile stations, without any additional radiation sources. The new architecture, dubbed Green Cellular, abandons the classical transceiver base station design and suggests the augmentation of transceiver base stations with receive only devices. These devices, dubbed Green Antennas, are not aiming at coverage extension but rather at minimizing the emission from mobile stations. We employ indoor and outdoor propagation simulation tools and field experiments to study the expected impact of the Green Cellular architecture on emission from mobile stations. Our results reveal a significant, up to 50dB, decrease in emission power and respective exposure to radiation.

Near optimal multipoint receive beamforming with finite samples

Receive beamfoming techniques play an important role in MIMO communications systems as means to mitigate interference and enhance the link quality. Multipoint receive beamforming methods are expected to provide additional gains in systems employing advanced coordinated multipoint processing. Due to its importance, receive beamforming has been studied extensively in the literature. The optimal detection schemes are well known in the case of perfect channel knowledge at the receiver. The more realistic case, where the beamformer is constructed using a finite training sequence transmitted by the desired transmitter has also been addressed. The study of the latter case, known as finite sample beamforming, focused on linear detection. In this paper we propose new nonlinear finite sample detection schemes for the case of multiple spatial streams. We show that the proposed schemes significantly outperform the known linear detectors and exhibit near optimal performance. We also show that the complexity of the proposed schemes is very similar to that of standard multi stream MIMO detectors, which makes them ideal for real life systems. We apply the new schemes to uplink space division multiple access and demonstrate the implementation and respective performance gains over standard detectors. Finally, we apply our approach to the case of coordinated multipoint reception where multiple base stations jointly decode multiple spatial streams. We study the tradeoff between performance and backhaul capacity and suggest near optimal finite sample schemes.

Green handover — A new handover mechanism that minimizes radiation from mobile devices

In recent years, cellular technology has become very widespread, with over 4 billion users worldwide. The increase in usage has been accompanied by a growing concern related to the possible adverse effects of the cellular radiation on human health. The radiation from the cellular phone, which is in close proximity to the user, is of special interest for this matter. In this paper we propose a new handover mechanism for cellular networks, aiming at minimal emission from mobile phones. The criterion for handover in common cellular systems is based on the received downlink signal strength or signal quality. This criterion makes sense when the uplink and downlink are symmetric. However, the advent of MIMO technology changes this assumption since MIMO transmission and reception techniques may significantly alter the uplink and downlink characteristics. For example, the number of transmit antennas may differ from the number of receive antennas. Alternatively, the techniques may significantly differ, such as when the transmitter broadcasts the downlink signal whereas the receiver performs beamforming. Consequently a mobile phone may receive Cell A with best quality, whereas Cell B may receive the mobile at better quality than Cell A, and hence Cell B requires minimal emission from the mobile phone. The new handover mechanism uses this concept and chooses, among neighboring cells with sufficient downlink, the cell for which the emission from the mobile phone is minimized. This mechanism requires methods to estimate the expected uplink emission. We propose several such methods. These include procedures where cells broadcast their uplink reception capabilities or request mobiles to perform test transmissions. Finally, we discuss the application of the proposed handover mechanism to create a new bidding procedure aiming at minimal uplink transmission power and minimal exposure to radiation.

The impact of synchronization on receive beamforming with null steering in OFDM MIMO systems

Receive beamfoming is one of the most prominent techniques to mitigate interference in OFDM MIMO communications systems. This technique is especially important in the uplink reception of cell-edge mobiles which are naturally associated with low signal-to-interference-and-noise-ratio (SINR). In the common case where the interference is more dominant than the thermal noise, classical receive beamforming solutions, such as the minimum Variance distortionless response (MVDR), tend to place spatial nulls on the dominant interferers. It is well known that a receiver with N receive antennas can place up to N - 1 nulls while receiving a single stream desired signal. In this paper we adopt a practical MIMO channel model including delay and angular spread, and consider both synchronized and non synchronized systems. We reveal an intimate relationship between synchronization and null steering. In the case of adequate temporal and frequency synchronization, an OFDM receiver with N receive antennas may mitigate up to N - 1 interfering transmitters. However, in the absence of adequate synchronization, the same OFDM receiver may mitigate up to N - 1 interference paths. Since each transmitter is usually associated with multiple paths, this degradation may be very significant in terms of cell-edge capacity. The results in this paper may be of great practical importance, especially for the deployment of LTE systems, in which inter-cell synchronization is optional, and in comparisons between synchronized and non synchronized systems, such as next generation WiFi.

Low feedback downlink SDMA using single antenna MIMO-like detection

Space division multiple access (SDMA) is considered one of the key techniques enabling 4G spectral efficiency and capacity, and has been adopted by various advanced communications standards such as LTE and IEEE802.11ac. One of the challenges in downlink (DL) SDMA is obtaining adequate channel state information at the base-station (BS) transmitter to minimize multi user interference (MUI). MUI minimization requires the BS to acquire accurate channel state information respective to the mobiles, achieved in the form of feedback. Digital feedback is usually made of a quantized version of either the channel matrix or the singular vectors. Naturally, finer quantization leads to more accurate channel state information and less MUI, but also to higher feedback capacity. Minimizing MUI is especially important when the number of transmitted SDMA streams is larger than the number of receive antennas at the mobiles, for example when a BS transmits M SDMA streams to M mobiles, each endowed with a single receive antenna. Here the mobiles do not have enough degrees of freedom to suppress the MUI, for example through an MMSE receiver (which would be possible if each mobile had at least M receive antennas). In this paper we propose MIMO-like ML detection of all M SDMA information streams at each of the mobiles (each is endowed with a single antenna). Here each mobile discards all the irrelevant streams, targeting other mobiles, post ML detection. Although counter-intuitive, when the optimal nonlinear ML detector is employed, the standard requirement of N ≥ M receive antennas for spatial multiplexing detection is essentially eliminated. This approach is immediately extended to the case of N <; M receive antennas at the mobile, where N is not necessarily 1. The proposed method significantly outperforms standard detectors in the regime of high MUI due to insufficient feedback quantization. Thus, it paves the way to a significant decrease in feedback capacity and relaxed SDMA group selection.

New time domain equalizer for multitone vehicle to vehicle communications

Pre-FFT time domain equalization plays an important role in OFDM systems experiencing delay spread that is larger than the OFDM cyclic-prefix duration. In such systems the time domain equalizer shortens the channel, so that ordinary OFDM proceeding may follow. The equalizers usually require a long training period, which grows with the delay spread, and are to be updated to accommodate channel variations. Due to these reasons they have mostly been implemented in wireline links such as ADSL, in which long training is allowed and the channel is quasi-static. The IEEE 802.11p is an emerging OFDM based standard for vehicle to vehicle communications, which borrows much of its physical layer from the indoor centric IEEE 802.11g (WiFi). Thus, it is no surprise that the IEEE 802.11p suffers from insufficient cyclic-prefix duration and pilot density, in high mobility outdoor environments. In this paper we propose new time domain equalizers, which are tailored for the IEEE 802.11p requirements, in terms of fast convergence on the preamble and the accommodation of high mobility. We show that in benchmark scenarios, the proposed equalizers are a key to reliable vehicle to vehicle communications.