Sara Linder

Urban peer-to-peer MIMO channel measurements and analysis at 300 MHz

Multiple-input multiple-output (MIMO) systems operating at frequencies in the upper VHF and lower UHF region is attractive for peer-to-peer communication applications where robustness is of high importance, e.g., in tactical networks and emergency response systems. When designing and evaluating such systems, knowledge of realistic propagation conditions is required. This paper presents results from an urban MIMO measurement campaign at 300 MHz. Measurements are performed along 25 receiver routes and for three fixed transmitter locations, using antenna arrays mounted on two cars. Channel characteristics and ergodic capacity for the 7 times 7 MIMO channels are extracted from the measured data. A path-loss model is derived for the measured scenario, and the distributions of the large-scale fading, the Ricean K-factor, the delay spread, and the ergodic capacity are studied in detail. The correlation distance for the different channel parameters is also examined. Furthermore, the analysis reveals that several of the channel parameters are correlated, and also have a strong correlation with the capacity.

Combat radio system range reduction due to radiated electromagnetic interference from COTS electronics

Co-location between digital radio systems and civilian electronic equipment can lead to severe interference problems. Unintentional radiated electromagnetic emission can increase the bit error probability (BEP) in the digital radio receiver, and thus reduce the range of secure operation. Furthermore the intersystem interference causes a reduction of the jamming resistance. In this paper, the corresponding decrease of the operating range to compensate for this increase in BEP is determined Furthermore, the increase of the operating range for a hostile jammer is determined for a chosen example. The results are visualized as simulated coverage diagrams for chosen terrain environments. The analyses show that the operating range is decreased with approximately 25% for a co-location distance of 20 meters between the radio receiver and disturbing COTS (commercial of the shelf) equipment, if typical measured emission levels are used. In this scenario a hostile jammer gets nearly 50% larger operating range compared to a scenario without the COTS-computers. If a part of the emission equals the EN 55022 Class B emission limit, required for commercial equipment sold in the European Union, a range reduction of 50% is obtained. Thus, the overall conclusion is that interference from COTS can cause severe reductions of operating range for digital communication links. This interference can also have significant impact on the jamming resistance. These are important drawbacks that must be considered before co-locating COTS equipment close to tactical communication systems.

Electromagnetic interference on tactical radio systems from collocated medical equipment on military camps

On military camps for joint international operations, the intersystem interference can be highly unpredictable, and situation changes can occur very fast. For such situations it is highly important to perform intersystem interference analyses not only for intentional transmitters but also for electromagnetic interference from other electric equipment. One example of such an interference source to consider is medical equipment in field hospitals since the hospitals can contain a large amount of interfering equipment. In this article, we show examples of necessary safety distances between medical equipment and tactical radio systems at military camps for international missions. We give an example of how wideband electromagnetic interference can degrade the performance even for a wideband frequency hopping army combat radio.

A new approach for estimating the impact of pulsed interference on digital communication systems

Determining the impact on digital communication systems from electromagnetic interference often requires complex analyses or computations. Therefore, there is a need for low-complexity approximate methods suitable in computer-based tools for intersystem interference analyses. A commonly used approximation is to treat the interference as additive white Gaussian noise (AWGN), when estimating the resulting bit error probability (BEP). However, the validity of this approximation varies for different types of interference. Previously, it has been shown that higher-order statistics (HOS) can be used to estimate the quality of the AWGN-approximation for periodic pulsed interference with fixed amplitude. In this paper, we show that it is possible to use HOS to estimate the quality of the AWGN-approximation for a more general pulsed interference with random amplitude and arrival time. The results open the possibility to use HOS as a quality measure of the resulting BEP when using low-complexity approximate methods in computer-based tools.

A novel method to determine the performance degradation of co-located frequency-hopping systems

In practical applications, frequency-hopping (FH) systems using different independent hopping patterns can be co-located close together. Even if the different FH systems use the same frequency band, the use of independent hopping patterns reduces the impact of intersystem interference. However, it is important to determine the performance degradation due to intersystem interference in the early design stage of such applications. If this is not done, any interference problems in the final application can be very difficult and expensive to handle. A method for such analyses has been developed where both the desired signals and the undesired spurious emissions are considered. With this method the bit error probability (BEP) for the co-located radio systems is estimated by use of the complete transmitted frequency spectrum of the radio systems. The method has been validated through measurements on real co-location situations for the Swedish army combat radio, the Ra 180. The analysis method is justified by the tight agreement between theoretical and experimental results. The results from this validation are presented, as well as a comparison with a simpler method where only the desired signal is considered. It is shown that for small co-location distances, the BEP for the co-located radio receiver will be seriously underestimated if other emissions than the desired output are neglected. Furthermore, three examples of applications of the method in radio design issues are shown.

Capacity benefits of airborne nodes in ad hoc networks with broadcast traffic

Tactical military networks are often ad hoc networks since it is a highly robust solution. The data rate in ground-based networks is often limited due to low antenna heights and a challenging wave propagation environment. In such networks it can be hard to achieve high enough data rate to support the requested services. A large portion of the traffic in tactical networks is broadcast or multicast traffic. Often, the used data rate is assumed to be the same for all links and tends to be low in order to reach all nodes in a network. A possible solution to improve the capacity is to add an airborne node to the network. In this paper, the performance of ground networks at 300 MHz is analyzed with and without an airborne node. The airborne node is positioned directly above and at a distance from the ground network. Also, both a detailed channel model including terrain data and a distance dependent channel model are considered. Results show the capacity improvement when adding an airborne node to an ad hoc network. Moreover, the use of a channel model with terrain considerations has a great impact on the results. For example, the required height of the airborne node, to achieve a certain capacity improvement, is severely underestimated when using the distance dependent model.

Measurement-Based Analysis of Two-Hop Cooperative Relaying

For tactical military communications, ad hoc networks are an attractive choice because they can provide good area coverage without the vulnerabilities of any central node or base station. Their performance is, however, strongly dependent on the ability to maintain link quality information, and to distribute the changing routing tables, in a timely and efficient manner in highly mobile scenarios. A relatively new concept of cooperative relaying, in which several relay nodes simultaneously retransmits identical copies of the message on the same channel, is promising as it eliminates the need of routing. In this paper, we analyze the performance of an OFDM based cooperative simultaneous relaying scheme for two-hop networks. The networks are simulated based on measured channels for the individual links. The results are compared with the performance for the traditional single relay technique with optimal and suboptimal route information. The results show that the performance of the cooperative relaying scheme in the investigated scenario is equal to, or better than, the performance for the best route for the single relay scheme in a large majority of the simulated networks. However, some fraction of the networks shows a worse result for the cooperative scheme. This fraction decreases as the bandwidth increases. Furthermore, the performance for the cooperative relay scheme show significant increase in performance when compared to the single relay scheme for the next-best route. Moreover, we observe that the performance of the cooperative relay scheme, in general, is non-reciprocal with respect to the direction of transmission for a given pair of terminal nodes.

A new method for estimating the impact of interference on digital communication systems in intersystem interference analysis tools

In this paper we present a low-complexity method for estimating the impact of intersystem interference on digital radio systems. For a digital radio receiver, the bit error probability (BEP) is a widely used measure of the receivers performance. However, the computations necessary to determine the BEP, caused by electromagnetic interference, are often quite complicated. Hence, simplified methods are needed in computer-based tools for intersystem interference analysis. One such simplified method is to approximate the interfering signal as additive white Gaussian noise (AWGN). However, this approximation is not always valid. In this work we use a higher-order statistics (HOS) measure, the kurtosis, to estimate the quality of the AWGN-approximation and also to create a method where the BEP for different kinds of interference is estimated. The method is designed to handle both modulated interfering signals as well as pulsed interference. This low-complexity method combines the kurtosis measure and the AWGN-approximation in order to obtain a better estimation of the BEP in a digital radio receiver.

Effects of interference between co-located frequency-hopping ad hoc networks

Traffic in military ad hoc networks are often broadcast or multicast traffic. A robust and efficient way to handle this type of traffic is the multi-point-relay (MPR) method. Furthermore, frequency spectrum is a scarce resource and must often be shared by several networks resulting in a risk for internetwork interference. The objective of this paper is to analyze the effects from internetwork interference in frequency-hopping (FH) ad hoc networks. Earlier work show that performance on a communication link can be degraded by interference between FH systems sharing the same spectrum resource. Results in this paper show that ad hoc networks are robust against FH interference in terms of a maintained high delivery ratio, but it is achieved at the cost of reduced capacity in the network. This reduction can in some scenarios be severe resulting in two networks with interference being less efficient than time-sharing between the networks.

Analysis of cooperative relaying in an urban scenario

For tactical military communication independence of a fixed infrastructure, base stations or central node is an advantage. Multihop routing in ad hoc networks have these benefits, it is, however, highly dependent on accurate link status in the network to find routes which can be problematic in mobile scenarios. A relatively new concept of cooperative relaying, in which several relay nodes simultaneously retransmits identical copies of the message on the same channel, is promising as it eliminates the need of routing. In this paper, we analyze the performance of an OFDM based cooperative simultaneous relaying scheme in a city scenario with relay nodes located at the same street. The performance is evaluated on measured channels for the individual links and compared with the performance of a traditional single relay technique. The results show that in the investigated scenario, cooperative relaying can increase the median path gain several dB compared to a single relay. However, even if the median path gain is higher there are occurrences of lower path gain with cooperation than with a single relay. The number of occurrences are decreased with more cooperating nodes and can be further decreased by specific cooperative relay techniques, such as phase dithering or delay diversity.