Generation Y, web design, and eye tracking Generation Y (age 18-31) is a very large and economically powerful generation, containing eighty-two million people and spending $200 billion annually. It is not surprising that companies are interested in gaining the patronage of this group, particularly via the web. Surprisingly, very little research into making web pages appealing to this important demographic has been done. This paper addresses this need through two separate studies. The first, an online survey, provides evidence that our proposed score for predicting the visual appeal of web pages reflects the self-report measure of what pages Generation Y likes. To refine these findings, an eye tracking study is conducted using the pages that were most and least liked in Study I. Participants' eye-movement is tracked while browsing these pages, providing evidence of what attracts their attention. The results of these two studies suggest that Generation Y may prefer pages that include a main large image, images of celebrities, little text, and a search feature. This research has both important theoretical and practical implications. Many companies use the World Wide Web as a major channel to broaden their reach and market share (see Loiacono and McCoy 2004) . This is evident by the total sales of goods and services over the Internet, which increased from $172.4 billion in 2005 to $201.7 billion in 2006. This number is projected to further increase to $328.6 billion in 2010 (USCensusBureau 2007) . Given this, it is not surprising that improving users' web experiences has become a major theme in industry research labs (e.g., Tedesco and Tullis 2006; Tullis and Stetson 2004) . Research indicates that whether a user finds a website visually appealing (Lavie and Tractinsky 2004; Lindgaard et al. 2006) often has a powerful impact on forming his or her perception of website usability (Schenkman and Jönsson 2000; Tractinsky et al. 2000) . Thus, making websites visually appealing has become of great importance to organizations (Djamasbi et al. 2007; Djamasbi et al. 2008) . In particular, companies are interested in capturing the patronage of Generation Y (age 18-31). At eighty-two million people, this generation could account for as much as "half the spending in the economy," or $200 billion in annual spending (Waters 2006) . This demographic has the added distinction of having been exposed to technology since childhood, making it a unique target market. As an extremely technologically savvy demographic, Generation Y demands that corporations understand them and their needs and adapt accordingly in order to maintain relevance in a competitive marketplace (Neuborne and Kerwin 1999) . Despite the compelling relevance of Generation Y to both business (Waters 2006 ) and academics, very little research has been done on their web preferences. Because Generation Y is such an important segment of the market, making websites visually appealing to this group of users has become an important challenge for many companies (Djamasbi et al. 2008 ).. The following section provides background on the subject of this paper. Next, the two studies conducted are presented: Study I, an online survey, and Study II, an eye tracking study. Finally, the implications of the findings are discussed, as well as the ways this research can be extended by future studies. Grounded in prior research, it was argued that particular web page characteristics (a large main picture, search feature, images of celebrities, and little text) were likely to be visually appealing to Generation Y on retail homepages. Four expert judges categorized the web pages based on the absence or presence of these characteristics and, based on the number of present characteristics, a score was assigned to each. Based on the score, the web pages were grouped into high and low characteristic score groups. Analysis showed that the pages in the high characteristic score group were most visually appealing to Generation Y, and the pages in the low characteristic score group were less visually appealing. The results suggested the characteristics used in calculating the score may have contributed to formation of users' visual appeal ratings. To investigate this possibility more closely, participants' eyes were tracked in Study II. This data enabled an examination of the areas of the page that attracted participants' attention and, therefore, their potential to contribute to the first impression and visual appeal. The results of Study II showed that the identified characteristics did indeed attract the attention of Generation Y. This research has important implications both theoretically and practically. Theoretically, this research contributes to human-computer interaction literature, particularly visual appeal research. It also provides a methodology and characteristics to support future research. Moreover, the findings of this study suggest that capturing users' gaze may be a productive way of evaluating a stimulus as whole while simultaneously examining the contributing parts. Practically, we provide retail companies targeting Generation Y with specific characteristics that improve the visual appeal of their homepages, potentially leading to a higher rate of return and the attraction of more Generation Y users. With the advance of technology, targeting specific audiences is becoming more practical and, as it is easier to understand and appeal to a specific market segment, should be a preferred method. This research sought to take a step towards understanding the web preferences of Generation Y. An index was defined based on several characteristics that, according to prior research, were likely to be appealing to Generation Y. This index can help to estimate how visually appealing a page is to Generation Y users. To test this index, an online study was conducted, the results of which showed that the characteristic score index used in this research was closely related to visual appeal. These results suggested that the four characteristics used in the index (large main picture, little text, search bar, and images of celebrities), may have contributed to Generation Y finding the homepage of a retail website visually appealing. As a first step towards understanding whether these characteristics played a role in the formation of users' impressions of the page, eye tracking was conducted in Study II to determine whether the characteristics under investigation attracted users' attention. Users' fixations were recorded as they viewed the web pages, providing objective evidence to determine which aspects of web pages attracted their attention. The eye tracking data showed that users fixated on main large pictures, faces of celebrities, and the search feature. Because fixation is a reliable measure of attention (Chapman 2005) , this data indicated that these characteristics attracted users' attention. It also showed relatively few fixations on the text which, along with exploratory analysis showing lower ratings for pages with more textual information, suggested that text amount may play a role in the impression formation of Generation Y.. Research showing that first impressions are formed in the first few seconds suggests that what captures users' attention on a page is likely to have a great impact on informing users' opinion about the visual appeal of the page (Lindgaard et al. 2006) . To this point, the areas of the page that were viewed within the first five seconds of viewing, as well as the order in which these areas were viewed, were examined in this research. To do so, an ordered plot of average fixation information was created for the first five seconds of viewing. The fixation order plots showed that a main large image was likely to be attended to within the first two fixations. When a main image was absent from a page, the first three fixations included navigation presented as list of links with a relatively large amount of textual information (see CVS and Bidz). The pages that included a main large image were rated as the most appealing, and those that didn't were rated as least appealing. The self report measure of appeal, along with the fixation order plots for the most and least appealing pages, suggests that the presence of a large main image and/or the absence of textual information may also play an important role in the formation of an impression of a page. Similarly, the data showed that faces of celebrities received early attention, e.g., within the first two fixations (Figure 3 and 4) and received a high amount of fixation (Figure 1 and 2) . This is consistent with previous research showing that users tend to fixate on faces when they are present and prefer images of people that include faces (Cyr et al. 2009; Tullis et al. 2009 ). The self report data showing more favorable ratings for pages with faces of celebrities combined with the objective measure of early and intense fixations suggest that the images of celebrities may play a role in the formation of favorable ratings. The top right corner of the page, where search is typically located (Shaikh et al. 2006 ) received fixation during the viewing process, as shown in the heat maps. This is particularly interesting because participants were asked to browse the pages; they were not oriented to complete specific tasks. In the first five seconds of viewing, however, the search feature was fixated upon on the least appealing pages only. It may be that, because of the higher number of components on the least appealing pages, users were compelled to look at the search feature. This is a topic that bears further research. These plots showed that the first few fixations rested on the center and top of the pages, suggesting that these areas may play an important role in forming the first impression of a page. The data also showed that logos were viewed on almost all pages within the first few fixations. It is possible that the users anticipated seeing logos in top left corner of web pages and their expectation was met when this was the case. It is also possible that individuals look to the top left because they expect to find important general information -for example, two common locations for the navigation bar on a page is across the top or along the left side. By looking at the top left, in addition to orienting themselves by locating the logo, participants may have been orienting themselves relative to the navigation and layout of the web page. In fact, participants fixated on the navigation within the first four fixations on average for all pages, indicating that this may be an important feature on a web page for users. These possibilities can be clarified by future research in this area. No matter the reason, these results suggest that top left corner may be ideal for providing company information such as logos. These findings are consistent with and support the prior literature that was used to select the characteristics identified in this study. There was a positive reaction to the pages containing a large main image and pages with lower word counts, along with relatively high fixations on large main images and low fixation on textual information. These findings are aligned with research that has shown that Generation Y prefers picture-based communication (Oh et al. 2008) , as well as literature that states that Generation Y dislikes reading "long boring text" (Perez 2008) . It also strengthens the argument that the age of the participants in Tullis & Tullis may have skewed the results; because the majority of participants were under thirty years of age, the finding of a correlation between the size of the largest image on the page and visual appeal may be applicable only to younger users. The high fixation levels on celebrities, as well as the high visual appeal ratings given to web pages that included celebrities, corroborate literature that states that images of celebrities may be particularly important to creating an emotional relationship with Generation Y (Tsui and Hughes 2001) . This is further supported by the advertising literature that suggests that Generation Y finds well-respected messengers, including "TV and movie stars, athletes, and musicians," appealing. Finally, these findings support literature that states that, when people are asked to build their ideal web page, people under forty are more likely to include a search feature (Chadwick-Dias et al. 2004) . The eye tracking results of Study II show that certain locations consistently received fixation. The top left corner of the page, where the logo is typically located (Shaikh and Lenz 2006) , consistently received fixation, even within the first few seconds of viewing (see Figures 3 & 4) . The navigation (or "internal links") was another area that received early fixations (within four fixations on average), suggesting that it also may contribute to visual appeal. Users may look to navigation for basic orientation on what products or information the website offers. In contrast to the areas that received consistent fixation, other areas consistently did not receive fixation (see Figure 2 and 4). For example, the top part of the page received high levels of fixation while the area below the fold received no fixation within the first five seconds of viewing, and very little fixation over the entire viewing time. This is consistent with the theory of visual hierarchy (Faraday 2000) that asserts that location is an important factor in attracting users' attention (Table 1) . Similarly, large text and images consistently received high levels of fixation, again supporting literature on visual hierarchy suggesting that, by manipulating the size of objects on a page and including images on a page, one can cue users to focus on elements in a particular order. Because a hierarchy defines a viewing order, the features at the top of the hierarchy are more likely to draw attention in the first few seconds of viewing, and thus contribute to the formation of the first impression (Lavie and Tractinsky 2004) . These findings suggest that the principles of visual hierarchy may be particularly helpful in guiding future visual aesthetic research. The results of this study have important theoretical implications for human-computer interaction studies, particularly those that investigate visual appeal. As mentioned previously, visual appeal studies generally fall into two categories, each focusing on a different aspect of user behavior. One focuses on the users' reaction to a holistic stimulus or web page, while the other studies the effect of individual components. Because of concern about ecological validity and wholeness, the former method calls for the inspection of the actual whole stimuli by people. Because of its reliance on scientific methods of inquiry, the latter approach examines reactions to isolated parts of the stimuli. In case of web pages, the former method advocates an examination of reactions by exposing subjects to the actual unmanipulated webpage, while the latter recommends examining reactions to isolated web components on a page. While these approaches are complementary in nature, they are difficult, though particularly useful, to employ simultaneously. The research described in this paper, particularly Study II, shows that the use of eye tracking may be a valuable method through which studies can benefit from both approaches simultaneously. For example, eye tracking can reveal the components that attract participants' attention without affecting the wholeness of a page (e.g., examining reactions to one component such as isolated images) or manipulating the webpage (e.g., creating different prototypes for each component). This, in turn, can help to identify components that are likely to affect user impressions. In addition to the combination of the above mentioned complementary methods of visual appeal investigation, the results have also other implications for human-computer interaction studies. Self report measures, while valuable in capturing a user's subjective reaction, measure only a snapshot of user behavior at a specific point in time (e.g., after viewing a web page). To supplement this, eye tracking measures can provide an objective and continuous measure of the user's reactions through eye movement and gaze. Using this, a more complete picture of users' reaction to a web page can be formed. Thus, this work provides further support for including eye tracking methods in humancomputer interaction studies. This research not only shows that eye tracking can be particularly useful method for investigating visual appeal, but also provides a basic step towards understanding Generation Y's web preferences. Our results suggest that certain web components can affect how visually appealing a web page is perceived to be by Generation Y. The characteristics and methodology used in this study can be utilized to investigate the visual appeal preferences of other generation and demographics. Using the four characteristics identified in this study, future works can refine our understanding of the preferences of Generation Y. For example, our results show that the characteristics score used in this study was a reliable predictor of the visual appeal preferences of Generation Y in regard to retail homepages. This suggests that refining this binary (presence/absence) score may lead to a productive research avenue in visual appeal. These results also have practical implications for companies targeting Generation Y. These findings indicate that including images of celebrities, a search feature, a large image, and little text may increase the visual appeal of a retail homepage for this demographic. Because images of celebrities can set the tone for a web page, it is likely that choosing the proper representative for the product will have a more positive impact on the first impression of Generation Y. Future research is needed to test this possibility. The inclusion of search feature may seem obvious, but many sites do not have one. The fact that participants fixated on the search bar on the most cluttered pages suggests that this is an important feature to include, particularly since the task was merely browsing. Eye tracking indicates that a large image is likely to draw the focus of Generation Y and may be particularly attractive to them. For example, the Mac pages contained a single large main image; the fixations on this page were focused primarily on this element. In contrast, the Bidz web page contained many smaller pictures; attention was more fractured, and only the first few images received fixation. Finally, since Generation Y does not like to read large blocks of text, the inclusion of little text is likely to increase visual appeal. Through eye tracking, it was found that Generation Y did not scroll, suggesting that important information should be placed above the fold of a web page or that scrolling should be avoided when possible. The top left corner of the page appears to be an ideal place for placing company information, such as a logo, as it attracted participants' eyes. In general, and particularly when there is no main focal point, the gaze plots showed that the left side of a page tended to get more attention. This suggests that companies may benefit from placing important information on the left side of the page. Of course, these observations are specific to Generation Y, and practitioners should be cautioned that they may not apply to all audiences. For example, Generation Y does not like to read large blocks of text (Nielson 2005) , but Baby Boomers, another large demographic, might (Neuborne and Kerwin 1999). Additionally, prior research provides evidence that education influences aesthetic preferences (Csikszentmihalyi and Robinson 1990; Devlin and Nasar 1989; Getzels and Csikszentmihalyi 1969) . Because Generation Y has been educated and grown with technology, it is likely that their visual appeal preferences may vary from those of other generations. Such evidence for possible differences suggest that future studies should look into finding similarities and differences between users of different generations and investigate how companies could use personalization to avoid possible conflicts between the visual appeal preferences of their target market. As mentioned previously, a characteristic score was calculated for each web page. Based on these scores, the web pages were placed into either a high characteristic score group or a low characteristic score group. Pages in the high characteristic score group had a greater number of the characteristics that were likely to be appealing to Generation Y. Hence, this group of pages were likely to be more visually appealing to Generation Y than those in the low characteristic score group. To test this possibility, a paired t-test was conducted on the mean visual appeal ratings given by participants for the pages in the high characteristic score group and the low characteristic score group. The results of the t-test, shown in Table 2 below, showed that the pages in the high characteristic score group received significantly higher visual appeal ratings from the participants than the pages in the low preference score group. To better understand users' reactions, several additional analyses were conducted. For example, we tested whether the participants rated pages that contained images of celebrities more favorably than those that included images of people who were not celebrities. The results of the t-test showed that the mean of users' visual appeal rating was significantly higher for the former group of pages (see Table 3 ). Next, we examined whether web pages with a large main picture were more appealing to Generation Y. The results of the t-test showed that the mean of users' visual appeal rating was significantly higher for pages that had a main large image. These results are displayed in Table 4 . The next comparison was between web pages that included little text and those that contained a great deal of text. Once again, a t-test was used to compare the mean of visual appeal for pages with little text with the mean of visual appeal for pages that included more text. The results of the t-test (see Table 5 ) showed that pages with little text received significantly higher ratings than pages that contained more text. Finally, a t-test was used to compare the mean of visual appeal between pages that contained a search feature and those that did not include a search feature. This analysis, as displayed in Table 6 , showed that the mean of visual appeal ratings was significantly higher for pages with the search feature. The results showing that users rated pages with high characteristic scores as significantly more appealing than those with low characteristic scores (Table 2) warrant further examination. While the exploratory results comparing pages with certain characteristics to those that lacked those particular characteristics (displayed in Tables 2-6) are aligned with the finding that pages with high characteristics scores were rated more favorably (reported in Table 2), they do not directly test whether users noticed those characteristics on a page. This is because visual appeal is a holistic assessment of overall reaction to a page, not individual characteristics. If those characteristics were not noticed by users, they were less likely to play a part in users' ratings. To test whether these characteristics attracted users' attention, a second study was conducted. In Study II, users not only rated the visual appeal of web pages (a self report and overall measure), but also had their eye movement tracked (an objective measure revealing users' focused attention). This facilitated a better understanding of users' reaction to a web page because it ascertained which characteristics on the page captured users' attention. The details and results of this second study are reported in the next section.. The results of Study II are split into three sections. First, the analysis of the visual appeal ratings, which were collected via a self report survey, is reported. Next, the results of the fixation analysis and heat maps are explained. Finally, an analysis of the order in which participants viewed areas on the pages is provided. While the visual appeal self-report survey was a reliable way to measure a users' reaction to a web page, it provided only a snapshot of users' behavior. Eye tracking provided a continuous measure of the users' reactions because it recorded users' eye movement throughout the experiment. Information on a web page is typically communicated through its perceptual elements, such as text and image. Creating a 'visual hierarchy' through the proper arrangement of these perceptual elements can naturally guide users in viewing the page (Faraday 2000) . For example, the size of an object is an important factor in its perceived visual importance -the larger the item, the greater its importance and, consequently, the higher its level in the visual hierarchy (Faraday 2000) . Similarly, images and graphics affect visual hierarchy because viewers tend to process these first. As an example, a number of studies show that people tend to gather information on a newspaper page by scanning images on the page first, then reading the text (Brandt 1954 ) . The location of perceptual elements on a web page also influences the visual hierarchy -components that are placed at the top of a page tend to be perceived as more important (Faraday 2000) . It is not surprising, then, that people tend to expect the title to be located at the top of a page (Bernard 2001) . Creating a visual hierarchy on a web page can make it easier to understand (Faraday 2000) , consequently making it more usable. Table 1 displays the summary of factors that affect visual hierarchy by influencing a user's entry point to a page (Faraday 2000) . Table 1 : Factors affecting visual hierarchy by influencing point of entry to a page 1. Motion (animated elements draw user attention before any other elements) 2. Size (larger objects attract more attention ) 3. Images (images attract more attention than text) 4. Color (elements with brighter colors attract more attention than those with darker colors) 5. Text style (typographical variations serve as effective non verbal cueing systems for attracting attention) 6. Position (top elements attract more attention than those located on the bottom). Making web pages more usable is important to companies because a good web design can keep users from moving away from the website. While users may move away from a website for technical reasons (slow download) or content reasons (the information on the page), studies suggest that form (the impression of a page) can also be a reason for moving away from a website (Schenkman and Jönsson 2000) . A good web design can also attract new users such as those surfing the web without any particular intention (e.g., to look up a certain piece of information). Such new users may be attracted to a page through various methods, but the visual appeal of the page may be particularly important (Schenkman and Jönsson 2000) In addition to keeping users from moving away from the website or attracting new users, the visual appeal of a web page can have a strong effect on the evaluation and enjoyment of a website (Lindgaard et al. 2006) In social psychology literature, this is often called the halo effect or confirmation bias (Lindgaard et al. 2006; Nisbett and Ross 1980) . No matter its name, this effect holds true even when a site is little more than marginally usable; one study found that although they could, on average, complete less than half the assigned tasks successfully, participants still valued websites that they had previously found highly visually appealing (Lindgaard and Dudek 2002) . Even major usability flaws on a website can be overlooked based on an extremely positive first impression. The lasting effect of a first impression is not limited to positive first impressions (Everard and Galletta 2003) . In fact, if users' first impression of a website is negative, confirmation bias will keep them from changing their opinion in spite of positive evidence to the contrary (Lindgaard et al. 2006) . Consequently, visual appeal has been recognized as an important factor in determining a website's quality and a good predictor of a user's intention to purchase an item or revisit the website (Loiacono et al. 2002 ) . Furthermore, an appealing website may be important in developing the trust of users (Karvonen 2000) . Users' statements, such as "if it looks pleasant, I just trust it," in studies provide evidence that visual appeal has a significant effect on important issues such as "trusting a service to be reliable and secure" (Karvonen 2000; Karvonen et al. 2000) .

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O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O 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Blended interaction spaces for distributed team collaboration In recent years there has been an introduction of sophisticated new video conferencing technologies (e.g., HP Halo, Cisco Telepresence) that have led to enhancements in the collaborative user experience over traditional video conferencing technologies. Traditional video conferencing set-ups often distort the shared spatial properties of action and communication due to screen and camera orientation disparities and other asymmetries. These distortions affect access to the common resources used to mutually organize action and communication. By contrast, new systems, such as Halo, are physically configured to reduce these asymmetries and orientation disparities, thereby minimizing these spatial distortions. By creating appropriate shared spatial geometries, the distributed spaces become "blended" where the spatial geometries of the local space continue coherently across the distributed boundary into the remote site, providing the illusion of a single unified space. Drawing on theories of embodied action and workplace design we discuss the importance of this geometric "blending" of space for distributed collaboration and how this is achieved in systems such as Halo. We then extend these arguments to explore the concept of Blended Interaction Spaces: blended spaces in which interactive groupware is incorporated in ways spatially consistent with the physical geometries of the video-mediated set-up. We illustrate this discussion through a system called BISi that introduces interactive horizontal and vertical multipoint surfaces into a blended video-mediated collaboration space. In presenting this system, we highlight some of the particular challenges of creating these systems arising from the spatial consequences of different interaction mechanisms (e.g., direct touch or remote control) and how they affect movement and spatial configuration of people in these spaces. Over the last couple of decades, we have seen continual shifts in the way organizations are structured and the way that work gets done. As organizations operate more within a global environment this has created a greater imperative for work practices to operate more and more over distance. Within this climate, teams of workers are no longer assembled to just work in collocated settings. The adoption of a myriad of computermediated communication technologies has enabled teams to be assembled according Authors' addresses: K. O'Hara (corresponding author), Microsoft Research, Cambridge, UK; email: v-keohar@microsoft.com; J. Kjeldskov, Department of Computer Science, Aalborg University, Denmark; email: jesper@cs.aau.dk; J. Paay, Department of Computer Science, Aalborg University, Denmark; email: jeni@cs.aau.dk. Permission to make digital or hard copies of part or all of this work for personal or classroom use is granted without fee provided that copies are not made or distributed for profit or commercial advantage and that copies show this notice on the first page or initial screen of a display along with the full citation. Copyrights for components of this work owned by others than ACM must be honored. Abstracting with credit is permitted. To copy otherwise, to republish, to post on servers, to redistribute to lists, or to use any component of this work in other works requires prior specific permission and/or a fee. Permissions may be requested from3:2 K. O'Hara et al. to the appropriate expertise required wherever it is located. The issue here is not simply about enabling a distributed set of individuals to work together which arguably are supported by a range of conventional desktop collaboration products such as video chat, NetMeeting, Google Docs, and so forth. Rather the concern is also about how to enable multiple collocated teams situated at different locations to work more effectively together when collaborating across a distance.. In spite of the progress made in collaborative technologies, the experience of distributed teamwork remains a difficult and frustrating one. Travel remains an important component in any effective operation of such teams in order to enable face-to-face interaction. Aside from significant environmental impacts, this need to travel is also costly and time consuming for the organization, creating intermittent collaboration rather than the more fluid, regular, and serendipitous interaction that characterizes collocated teamwork.. Traditional video conferencing technologies sought to overcome some of these collaborative difficulties but it is well documented that the rhetoric behind such systems never quite matched the reality of collaborative experience with such systems. There has been much research over the last 20 years that sought to understand and explain why this was the case (e.g., Buxton [2009] , Dourish et al. [1996] , Finn et al. [1997] , Gaver et al. [1993] , Harrison [2009] , Luff [1991, 1992] , Hirsh etal. [2005] , Mantei et al. [1991] , Nguyen and Canny [2005] , Noll [1992] , O'Connaill et al. [1993] , Olson et al. [1997] , Olson and Olson [2000] , Sellen, [1992 Sellen, [ , 1995 , Sellen et al. [1992] , Sellen and Harper [1997] , Short et al. [1976] , and Tang and Isaacs [1993] ). As such, while these systems support some limited communication activities within distributed teams, they often remain underutilized for collaborative activities of any realistic complexity. A recent study conducted in one particular organization has indicated that traditional video conferencing systems are used on average for only 12 hours per month [Weinstein 2005 ]. This level of usage matches well with the research findings in the literature relating to user experience and organizational factors that often hinder use.. In recent years though, we have seen the introduction of more sophisticated video conferencing technologies that have led to a stepwise increase in the collaborative experience between remote team members. Systems such as HP Halo (see Figure 1 ), Cisco Telepresence, Tandberg T3, and Polycom TPX provide an enhanced user experience, with research showing usage rates of such systems as much as ten times higher than traditional video conference systems [Weinstein 2005] . Our intention here is not to overplay the significance of these comparative statistics, as there are a number of factors underpinning them. For example, the fact there are more of the traditional video conferencing units relative to the higher-end systems is a contributing factor. But in part they are also attributable to the enhanced user experience associated with these higher-end systems with users saying the technology "disappears" enabling them to focus on the collaboration [Gorzynski et al. 2009; Weinstein 2005] . The experience and usage rates with these high-end systems, then, appear to challenge some of the well-established difficulties with traditional video conferencing systems.. Articulating the reasons for why these systems challenge some of our assumptions about the value of video-mediated communication is one of the concerns of this article. While there are a number of possibilities for how we may approach this, the way we aim to do so in this article is through a closer look at the design characteristics of these systems using the Halo environment as an example, drawing on and extending the work of Gorzynski et al. [2009] . Our concerns here are more than simply a characterization and explanation of the Halo system design. Rather, we want to use this characterization to exemplify the broader design philosophy behind such systems. In particular, we want to articulate further what Gorzynski et al. have come to call "Blended Spaces, " 1 that is, distributed set-ups in which the design of the whole environment produces a geometrically coherent representation of the remote site, faithfully representing its spatial geometries with respect to the local site. This provides the perception of unified spatial frame of reference for all parties.. Taking this as a starting point, we want to argue how the basic blended space philosophy can be extended to think about an ecology of other distributed workplace environments that support a broader variety of distributed collaborative work practices. In particular, one of the key aims of the article is to extend the Blended Spaces work to see how collaborative practices can further be supported by the introduction of interactive information workspace elements into such environments. That is, we will move towards what we have come to call "Blended Interaction Spaces." Our concerns here are to illustrate some of the design challenges of making such a move and the ways that different interaction mechanisms and approaches can affect the way that we achieve geometrically coherent representation of the remote site with respect to the local site. As with the characterization of the Halo system we will adopt a similar approach of articulating the characteristics of Blended Interaction Spaces through a system we have developed for data-intensive collaboration for distributed small groups.. Before we delve into a deeper articulation of Blended Spaces and Blended Interaction Spaces, we want to first take a step back and set some context within which to ground some of the subsequent discussion. We discuss first of all notions of embodied action and the importance of physical space as the basis for coordinated action, meaning making, and intersubjective understanding. We see how the notion of embodied action is used to make sense of the communication and collaboration behaviors arising in media spaces and traditional video-mediated communication and through this set the scene for understanding some of the design directions of Blended Spaces. Building on the ideas of how action is enacted embodied in space, we move on to a discussion of the ways that our social action and behavior are influenced by our spatial configuration with respect to others and objects. This discussion draws in particular on Hall's [1966] notions of proxemics and Kendon's theory of F-formations of interactional space. We go on then to discuss workplace design where again, notions of physical space are fundamental. Like CSCW, workplace design has as one of its central concerns the way effective workplace collaboration is achieved. In contrast to much of CSCW, workplace design approaches this from the point of view of designing physical space, from architectural elements through to specifics of furniture design. We then articulate where CSCW and media space research have drawn on notions of spatiality in the design of distributed environments. This leads on to our discussion of Blended Spaces and Blended Interaction Spaces. In this article, we have presented an analysis of the concept of Blended Interaction Spaces. Our chosen approach here has been to combine the theoretical underpinnings of the concept with important design issues illustrated through analysis of concrete system examples. In adopting this approach, our aims go beyond the specifics of the systems explored in the article. Rather, our intention has been to provide some of the important analytic foundations through which such systems can be conceived and studied in a more principled way.. Out of this analytic foundation there arises a broad set of interesting empirical issues and questions that can form part of a larger research agenda. These relate to a number of different areas. The first concerns the impact of these environments and proxemic concerns on the mechanical aspects of communication and collaboration. As with other media space research, these concerns apply not simply to performance outcome measures but also to characterization of process of communication and collaboration. That is, these systems can be evaluated in terms of particular spatial configurations that result in more effective decision-making or more effective collaborative interpretations. They can also be measured in terms of the ways that they change communication and collaboration style, for example, do they result in different utterance or turn taking patterns, or different patterns of gesture? As well as these quantitative analyses, it is important to complement this with more detailed qualitative interaction analysis of these kinds of systems such as those applied by Heath and Luff [1991 , 2000 in earlier media space research. In the same way that they highlighted some of the problems of media space asymmetries, these kinds of analyses can help relate particular interaction behaviors and utterances to the geometrically faithful representations of Blended Interaction Spaces. This kind of research into the mechanical aspects of communication and collaboration applies not just to the specific Halo and BISi systems but also to a broader ecology of Blended Interaction Spaces with different proxemic configurations.. A second area of empirical interest arising from the discussion here relates to the specific question of interaction proxemics. The systems discussed here highlighted issues related to the particular interaction mechanisms and configurations used in their design. This points to a larger agenda of understanding the spatial consequences of existing input technologies and interaction techniques. Experimental studies around this issue can, of course, utilize spatial tracking technologies of bodies and body parts, as well as input logging technologies to create important data components in the visualization and systematic analyses of these spatial consequences. However, the research agenda here is more complicated than simply taking a particular interaction technique and mapping out the particular spatial behaviors around it. As we have argued earlier, these spatial consequences will also be shaped by the presence of other competing influences on spatial behavior within blended space environments. For example, the need to be positioned on camera, or the presence of other interaction mechanisms are likely to interact with the basic interaction proxemics of a particular input device. As such, in conducting experimental analyses of interaction proxemics, it will be important not just to examine interaction mechanisms in isolation but within the context of other artefacts and issues with distributed collaboration and Blended Interaction Spaces.. A final area of empirical work, drawing on arguments made by Schmidt [2009] , concerns the less mechanical aspects of communication and collaboration. The current presentation of Blended Interaction Spaces put forward in this article has focused on the mechanical aspects of collaboration and conversation and how this is facilitated by particular material properties and design characteristics of these spaces. This focus has enabled a level of clarity in the presentation of the arguments but it is important to recognize this as just the initial part of the understanding of these kinds of Blended Interaction Spaces. Missing from this more mechanical emphasis are any details of actual work practice and behaviors within particular organizational contexts. There is a need then for an empirical assessment of Blended Space systems, such as Halo, BISi, and others, within real-world organizational contexts. Rather than focusing on the mechanical properties of communication, such an empirical agenda will aim to understand how and why particular details and characteristics of work practice within particular organizational settings relate to the geometrical properties of these distributed spaces. Through more ethnographic enquiry into these work practices, it should be possible to articulate further reasons why, in particular organizational contexts, people do and don't orient to common spatial references in the shaping of their work practices and the ways embodied interaction within a range of organizationally situated Blended Interaction Spaces creates meaning and the production of "place." Our concerns in this article have been to articulate the key features and theoretical foundations of what we call Blended Interaction Spaces. These are distributed collaboration environments that attempt to faithfully incorporate geometrical properties and configuration of space, which have been shown significant in the organization of communication behavior in physical space. In developing these arguments, we have drawn extensively on a long tradition of literature within media space research and work examining the relationship between spatiality and human interaction. Of particular significance here have been the theoretical framework of embodied interaction and theories of the spatial organization of behavior such as Hall's [1966] proxemics and Kendon's [1990] interactional spaces. These understandings of the spatial organization of behavior provide important analytical lenses through which developments in media space research and design have and can continue to be understood. More specifically, they provide the resources with which we can think about new kinds of media space environments, of concern in this article, that explicitly address these spatial concerns in their design. In order to more concretely articulate the design ideas, principles, and issues involved, the article takes as a launch point a discussion of Gorzynski's [2009] original concept of Blended Spaces. Our approach here has been to further elucidate the ideas concerned through an examination of a canonical example of a Blended Space, namely HP's Halo collaboration environment. This has allowed us to explore some of the key design features that contribute to the perception of a shared spatial geometry between users working across distributed environments. Our concerns here are not simply with the specifics of the Halo environment, but more broadly how they help communicate some of the more general design considerations and ways of thinking that need to be applied in developing different types of Blended Spaces.. It is our thinking about other types of Blended Spaces that requires us to extend the original notion to encompass Blended Interaction Spaces: distributed environments that encompass possibilities for richer collaboration over distributed interactive workspaces in which the communication is an integral part of a collaborative analysis and interpretation of presented information; where the group needs to gesture, point, interact with and manipulate presented information in the context of their ongoing discussion. While such environments in some sense form part of the larger set of Blended Spaces, there are important reasons why such a conceptual extension is necessary and valuable. By introducing interactive workspace capabilities within such environments, we fundamentally change some of the behavioral and spatial assumptions that can be relied upon in the creation of a perception of shared spatial geometries. Through making the extension to Blended Interaction Spaces, we specifically draw attention to the spatial consequences of the interactive workspaces. Again, as with the Halo discussion, our approach in articulating these concerns is through an examination of a canonical example of a Blended Interaction Space called BISi. And again, our aims in articulating these design details lie in more than just the specifics of the BISi system itself. Our intent through this illustration is to highlight some of the key design considerations, issues, and a more general way of thinking that is pertinent to the design of other systems of this genre. With other systems, some of the specifics will change, but the overarching design philosophy will remain the same.. In presenting this discussion, there are a number of key issues that the work has highlighted. First is the important notion of interaction proxemics. Drawing on Hall's [1966] original concept of proxemics, this notion makes particular reference to the spatial consequences of different interaction mechanisms in terms of the way they position people with respect to information resources and in terms of how they dynamically configure people with respect to each other during the ongoing course of collaboration. What we are arguing for, then, is for greater understanding and reflection on the spatial consequences of particular interaction mechanisms so that they can be combined appropriately with architectural and furniture elements with particular effect. Understanding the interaction proxemics of particular interaction mechanisms forms an essential part of the spatial assumptions that need to be worked with in the creation of a blended environment. So in the same way that table design allows embodiment of assumptions about spatial positioning of people in the environment, we need to do something similar with the interaction mechanisms. We are beginning to see some informative work to this end, for example, in the domain of tabletop interaction mechanisms and territoriality (e.g., Nacenta et al. [2007] ). While this work has focused its attention on collocated collaboration, it is nevertheless informative to our more specific spatial concerns with distributed collaboration environments. What we would argue is that this kind of work needs to be extended to a broader range of interaction mechanisms and configurations that might usefully be incorporated within other Blended Interaction Spaces (e.g., touchless gesture-based interaction).. A second set of issues arising out of the move from Blended Spaces to Blended Interaction Spaces comes from the spatial qualities of the data space and in particular how to map this onto the geometric properties of the space envisioned. In our discussion of BISi we have seen ways this can be informed from other work in multidisplay environments but also the additional complexities of trying to do this across distributed environments. What we have also seen is that the attempts to pursue spatial continuity across the distributed boundary can potentially lead to design conflicts with well-established design philosophies in CSCW such as WYSIWIS. In developing the Blended Interaction Space paradigm, such potential conflicts need to be the subject of a systematic research effort to more fully understand their implications for distributed collaborative practices.. What we also hope to have highlighted in our arguments about Blended Interaction Space systems is an attention not simply to the details of technology design but also to the details of architectural and furniture elements. In presenting our discussion of Halo and BISi, we have shown how these components are tightly coupled together in mutually dependent relationships and that adjustment in one component intimately affects our experience with other components. By paying attention to all these different components together, it is possible to exploit these interdependencies to good effect. For example, we have seen the use of furniture design to embody assumptions in the environment about the positioning of people. This information is then something that can be used in configuration and placement of cameras and in configuration and placement of displays. We have also seen examples where architectural design has been used to compensate for certain aspects of the technology such as image distortions. An example here is the lack of features on the walls, avoiding horizontal and vertical lines that make salient any distortions. As well as thinking about this issue in relation to Blended Interaction Spaces, we believe that these considerations of architectural, technological, and furniture relationships can be applied more generally to CSCW design in collocated situations, that is, applying attention not just to the technological but also giving consideration to the technological within the context of architectural and furniture elements. By attending to these other elements, it is possible to change people's spatial relationships with the technological. Hall's [1966] proxemics are again informative here in his articulation of how architecture and furniture can affect the nature of interpersonal interactions and social action.. Following on from this in blended interaction space philosophy is the argument, drawn from traditional workplace design literature, for a varied set of spaces, each designed specifically to afford different aspects of individual and collaborative work practice. That is, in everyday work practices, people work in an ecology of interlinked spaces moving between them and exploiting their respective affordances for the unfolding demands of work. While we have illustrated the concept of Blended Interaction Spaces through the discussion of the BISi system, this system is really a starting point in thinking about the development of a larger ecology of these spaces. This is an important reason why our concerns go beyond the specifics of the Halo and BISi systems to encourage thinking about this broader potential ecology of Blended Interaction Spaces. Each of these points in the design space can draw inspiration from the varied physical workspaces that make up the spatial ecology of the modern office place. This approach attempts to deal with the varied nature of work practices through a combined system of purpose-designed spaces as opposed to designing single general-purpose spaces or technologies that accommodate varied practices and circumstances through configurability and tailorability. In taking this system-of-spaces approach, Blended Interaction Spaces trades off the values of purpose-built design with the values of flexibility and extendibility (e.g., extending to more and more nodes than one gets, for example, with Access Grid-type technology). In this respect, we can draw some parallels between the appliance (purpose built) and convergence (general purpose) debate discussed by authors such as Norman [1998] with Blended Interaction Spaces being at the appliance end of the design approach : well-designed spaces with very particular purpose. As with the appliance/convergence model these devices can coexist (refer to Dourish [2001] ). In advocating the importance of Blended Spaces, our intent is not that these should be at the expense of other more general-purpose video-mediated communication tools. Rather, it is envisioned that these different kinds of systems will coexist according to the particular values they have acquired.. In presenting the ideas here, and in particular the emphasis on providing shared spatial geometries across distributed environments, there is some concern that the Blended Interaction Space approach be viewed simply as an uncritical and blind pursuit of the "being there" experience. However, just because we are highlighting spatial characteristics and the importance of physical space design does not mean that we are advocating what Fitzpatrick [2003] and others such as Harrison and Dourish [1996] might characterize as a "space-based" approach to the design of these systems. Rather our concerns are much more in line with the "place-based" approach [Fitzpatrick 2003; Harrison and Dourish 1996 ] to the design of collaboration systems; that is, the design and use of space for place-making where meaning making takes place through the actions in the space [Fitzpatrick 2003 ]. With this in mind, our reasons for trying to maintain spatial geometries in the design of distributed environments can be grounded in the ideas of embodied action. From these it is argued that the representation of geometrically accurate spatial continuity across distributed spaces can facilitate visible access to and understanding of the gestures, actions, and orientations of others in relation to the spatial environment and understanding of how our own actions, gestures, and orientations appear to others in relation to the environment. Having a shared spatial geometry can help overcome negative consequences arising from orientation and perspective disparities that can affect in-the-moment coordination of conversational mechanics. Through this, it is possible to overcome some of the difficulties associated with the fractured and disembodied experiences with some early video-mediated communication and have a space that affords "place-making" through the more embodied experience.. When making these arguments, it is important to treat them with an appropriate perspective. As we discussed earlier, the claims being made are not about the necessity or sufficiency of shared spatial geometries as a basis for successful collaboration. The numerous examples of distributed collaboration technologies that are usefully incorporated into people's lives, making no attempt to represent spatial geometries, are testament to this (telephone, instant messaging, etc.). So while people can and do communicate without coherently represented spatial geometries, our arguments are that the provision of shared spatial geometries can facilitate and make easier aspects of communication, coordination, and collaboration. As Healey and Battersby [2009] argue, people make use of them when they are available, offloading the mental demands of conversational and gestural reference to the spatial geometries of physical and interactional space. The facilitation effects of these cues are particularly relevant where visual communication channels are used to provide spatial reference points and which are "assumed" to be congruous but where orientation disparities lead to a confusing incongruity, such as in video-mediated communication.

O O O O O O B O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O 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Performance characterization of a rover navigation algorithm using large-scale simulation Autonomous rover navigation is a critical technology for robotic exploration of Mars. Simulation allows more extensive testing of such technologies than would be possible with hardware test beds alone. A large number of simulations, running in parallel, can test an algorithm under many different operating conditions to quickly identify the operational envelope of the technology and identify failure modes that were not discovered in more limited testing. GESTALT is the autonomous navigation algorithm developed for NASA's Mars rovers. ROAMS is a rover simulator developed to support the Mars program. We have integrated GESTALT into ROAMS to test closed-loop, autonomous navigation in simulation. We have developed a prototype capability to run many copies of ROAMS in parallel on a supercomputer, varying input parameters to rapidly explore GESTALT's performance across a parameter space. Using these tools, we have demonstrated that large scale simulation can identify performance limits and unexpected behaviors in an algorithm. Such parallel simulation was able to test approximately 500 parameter combinations in the time required for a single test on a hardware test bed. Autonomous rover navigation algorithms choose commands to direct a robotic vehicle toward a goal while avoiding obstacles. Efficient control of Mars rovers requires such algorithms, because the time delay in communicating with Mars excludes earth-bound operators from all but the slowest control loops. NASA's Mars Rovers, MER (current) and MSL (planned), use the GESTALT algorithm [4, 9] to provide autonomous navigation. Using autonomous algorithms in uncertain environments is inherently risky, but the risk can be mitigated by testing the algorithms extensively on the entire range of conditions that the algorithm may face. This establishes the algorithm's operational envelope, allowing a rover operator to maximize efficiency on Mars by using autonomy without jeopardizing the rover or being overly cautious.. Such large-scale testing is best done in simulation. Hardware testing is severely limited by the availability and speed of hardware testbeds and the expense of constructing terrains. By contrast, multiple copies of a simulator can be available early in a program, and the parameters of virtual terrains are easily modified. Many copies of a simulation, running simultaneously on many terrains, can rapidly and systematically explore a navigation algorithm's performance over a large parameter space. In addition, simulation can test conditions not available to hardware test beds because of expense, risk to the hardware test bed, or simply physical impossibility.. This paper describes a prototype capability we developed to rapidly explore the operational envelope of rover technologies within a large parameter space, using as a specific example the GESTALT autonomous navigation algorithm operating on terrains with various slopes, obstacle height and distribution, and wheel slip-page. We integrated GESTALT with the ROAMS rover simulator to test the algorithm's performance over a broad range of terrain environments. We developed a progressive sequence of tests to verify the integration of GESTALT with ROAMS and the performance of component algorithms within GESTALT. We configured the ROAMS Monte Carlo (RMC) batch manager to perform these tests, running ROAMS with GESTALT many times, over a range of terrain parameters. We developed a tool to evaluate the performance on each parameter set. We also adapted RMC to operate on a supercomputer, running many tests in parallel to radically decrease the time required for a thorough walk through the parameter space. Finally, we conducted three tests to evaluate the utility of this ensemble of programs in identifying the performance envelopes of rover technologies, through the specific example of GESTALT on various terrains. Testing GESTALT or other algorithms in simulation on a supercomputer allows a much more thorough investigation of the limits of our navigation software than is possible using hardware testbeds alone. To demonstrate the viability of such software testing, we integrated the GESTALT algorithm into the ROAMS simulator, allowing closed-loop testing in simulation. We adapted the Roams Monte Carlo batch manager to run the simulation many times using varying input parameters, exploring the parameter space. We ran this on the JPL Supercomputer cluster, testing GESTALT's performance at about 400 points in the parameter space per hour. We developed a visualization tool to help rapidly identify trends in the results. We ran three experiments to verify that the ensemble of programs functioned together. These tests revealed some surprising behaviors of GESTALT that might have been overlooked if limited hardware-test bed testing had prevented exploration of the regions of the parameter space where the behaviors occurred. This capability should be beneficial to MSL, which is slated to use GESTALT and unlikely to have schedule for extensive hardware test bed testing of GESTALT.. The impact of this initial capability to do large-scale simulation extends beyond the example application to GESTALT. The task developed the capability to evaluate the performance of a rover technology over a range of environmental (or other) parameters by embedding the technology in the ROAMS simulator and running many simulations in parallel on a supercomputer, each evaluating one point in the parameter space. It demonstrated exploring a space of terrain parameters such as wheel slippage and slope that may not even be available to hardware testbeds. It demonstrated the ability to run hundreds of tests in the time required to run a single test on a hardware test bed.. The framework is now available for testing GESTALT against additional parameters such as pose estimation noise. It is also useful for testing the impact of environmental or other parameters on other rover technologies. It also can be used to test rover algorithms in general and, more broadly, to test new rover hardware designs.. Two future extensions to enhance the current capabilities are presented here. The first is to conduct the remaining experiments in the test plan. The second is to identify when the rover drives into a region that GESTALT considers unsafe. One way to do this is to add a tool that applies GESTALT's traversability evaluator to the entire terrain map. A user would use this tool before running ROAMS to create a map of areas that GESTALT considers dangerous. Then as ROAMS generates and reports the actual path of the rover, it could monitor the safety map and report, along with the simulation end criterion, the traversability of the worst point on the path.

O O O O O O O O O B O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O B I O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O


Reduced-dimension multiuser detection We present a new framework for reduceddimension multiuser detection (RD-MUD) that trades off complexity for bit-error-rate (BER) performance. This approach can significantly reduce the number of matched filter branches required by classic multiuser detection designs. We show that the RD-MUD can perform similarly to the linear MUD detector when M is sufficiently large relative to N and K, where N and K are the number of total and active users, respectively. We also study the inherent RD-MUD tradeoff between complexity (the number of correlating signals) and BER performance. This leads to a new notion of approximate sufficient statistics, whereby sufficient statistics are approximated to reduce complexity at the expense of some BER performance loss. 1 978-1-4244-8216-0/10/$26.00 Multiuser detection (MUD) is a classical problem in communications and signal processing: given a noisy received signal, we would like to detect all signals (out of a set of possible choices) that comprise the received signal. For instance, in a CDMA uplink system, a number of mobile users communicate simultaneously with a given base station (BS). The BS must demodulate all of these simultaneously received signals in the presence of noise and other channel impairments.. One of the conventional methods for MUD is a matched filter (MF) bank followed by a sampler and demodulator [1] . The bank of MFs correlates the received signal with all possible waveforms, and the sampler and demodulator demodulate all users' information bits simultaneously. When the waveforms are orthogonal this MF bank with a sampler and demodulator maximizes the output signal-to-noise ratio (SNR) of each individual user.. When the waveforms are not orthogonal this MF bank structure does not optimize output signal-to-interference-plusnoise-ratio (SINR) and it suffers from the near-far problem [2] , namely, that any user that has strong power at the receiver can degrade the bit-error-rate (BER) performance of other users. In general, the MUD that minimizes BER is the maximum likelihood sequence estimator (MLSE) [1] , but it is exponentially complex in the number of users even with orthogonal signals. A lower complexity sub-optimal linear MUD detector is the decorrelator, which consists of an MF bank followed by an appropriate linear transform which eliminates multiuser interference, thereby solving the near-far problem. An issue with all of these conventional detectors using an MF bank front end is that when the number of users is large, building correlators for all possible waveforms can be costly in terms of hardware.. In practical systems the number of active users, K, is much smaller than the total number of users N . Motivated by the idea of compressed sensing [3] , we would like to exploit this sparsity in the context of MUD to reduce the number of correlators. The reduced-dimension multiuser detector (RD-MUD) we propose can reduce the number of correlators needed at the front end of MUDs. The RD-MUD correlates the received signal with a set of M correlating signals, where M is typically much smaller than N , followed by appropriate processing on the outputs. As we show, the BER performance of RD-MUD can be similar to that of standard linear MUD receivers.. Previous work exploited compressed sensing ideas in MUD by equating user detection with support recovery [4] [5] [6] . These works establish conditions for the number of correlating signals M required to achieve a zero probability-of-falsedetection (PFD) when the number of signals N tends to infinity. However, this asymptotic analysis sheds little insight into system design questions such as, e.g., how many correlating signals we should use to achieve a given probability of error target, and how to choose the correlating signals.. A key aspect of our work is that we process analog signals; most existing work on exploiting compressed sensing ideas for MUD assumes discrete signals. In particular, these detectors start with discrete samples obtained by sampling the received analog signal at the Nyquist sampling rate. Some of these detectors apply compressed sensing techniques by compressing these samples via matrix multiplication [4] [10] . The RD-MUD takes a different approach that is closely related to analog compressive sensing [11] [12] [13] . However, those works focus on sparse signal estimation rather than the multiuser detection problem considered herein.. Finally, there is another branch of compressive detection, which focuses on detecting the presence of a particular signal with the signal itself being sparse [7] [8][9] [10] . The differ-ences between these works and our own are as follows. First, the sparsity we employ is in the number of active signals; however, each user's signal may not be sparse. Second, their formulations primarily focus on binary detection, which corresponds to a single-user case. The PFD for a binarydetection problem was given in these previous works using the restricted-isometry-property (RIP) of correlation matrices. Although [7] showed that their research can be extended to a signal set with multiple waveforms, there was no further PFD analysis.. In this paper, we present a framework for reduceddimension multiuser detection (RD-MUD) that exploits the sparsity in the number of active users. Our contributions include the following: (1) We present an RD-MUD which can reduce the number of correlators compared to that of the MF bank. The RD-MUD correlates the received signal with M ≪ N correlating signals, followed by a linear transform of the correlation coefficients (essentially correlating the received signals with all possible discrete signal vectors). The correlating signals are linear combinations of the bi-orthogonal signals of the original signal set. (2) We characterize the tradeoff between complexity (the number of correlating signals) and BER performance, which offers a continuum of design choices for practical implementations. (3) We introduce the notion of approximate sufficient statistics: correlating with our correlating signals does not provide a set of sufficient statistics (which may be obtained by correlating with N appropriate signals); however, the RD-MUD yields approximate sufficient statistics in the sense that it attains a BER performance similar to that which can be obtained by linearly processing the sufficient statistics.. The rest of the paper is organized as follows. Section II presents the system model and the RD-MUD structure. In Section III we discuss the RD-MUD noise performance and introduce the notion of approximate sufficient statistics. Section IV demonstrates the performance of the RD-MUD via several numerical examples. Throughout, we use standard notation: x(t), y(t) = 1 T T 0 x(t)y(t)dt denotes the inner product between two real analog signals in l 2 ; x(t) = x(t), x(t) 1/2 is the norm of x(t); [X] ij indicates the ijth entry of a matrix X; diag{x 1 , · · · , x n } denotes a diagonal matrix with the specified entries on the diagonal; I represents the identity matrix; X T , X −1 , and X † denote the transpose, inverse, and Moore-Penrose pseudo-inverse of a matrix (or vector) X, respectively; x = (x T x) 1/2 is the norm of the vector x. The function δ ij is defined such that δ ij = 1 only when i = j and equals 0 otherwise. The sign function is defined as

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