Secondary Use of Radio Spectrum: A Feasibility Analysis

Arnon Tonmukayakul and Martin B.H. Weiss

School of Information Sciences University of Pittsburgh

Pittsburgh, PA 15260

25 August 2004

Abstract Enhancing spectrum efficiency and use is a significant task of regulatory authorities worldwide. A number of measurement studies of spectrum utilization have indicated that spectrum is sporadically used in many geographical areas and times. Low utilization and increased demand for the radio spectrum resource suggests the notion of secondary use, which allows unused parts of spectrum to become available temporarily for commercial purposes. The secondary use of spectrum is one of the promising ideas that can mitigate unsatisfied spectrum demand, potentially without major changes to incumbents. In this paper, we intend to outline the issues and discuss further study with the goal to determine what the necessary conditions are for spectrum sharing to be feasible. We consider the basic elements of wireless communications that utilize radio spectrum space (signals and the channels) in our analysis. The signals and channels of potential interest are expected to exist in at least three dimensions: geographical space, time, and frequency. Different signals/channels occupy different subspaces, therefore allowing us to locate and distinguish one from another. The model provides useful graphical information to clarify the concerned topics at hands. This feasibility study of the secondary use takes several factors into consideration, including the availability of spectrum, interference, mobility, practicality of communications, and service applications. We perform preliminary studies of three basic scenarios relating to three basic methods of separating channels – Frequency Division (FDM), Time Division (TDM), and Code Division Multiplexing (CDM). Exploring the problems of providing secondary use of spectrum gives us the ability to consider basic obstacles to secondary use, including why primary users would allow secondary use and, equally important, under what circumstances secondary users might emerge. The reality of identifying the pre-conditions for secondary use is a crucial step towards higher level of efficient spectrum utilization.

1. Introduction Wireless communications offer a number of benefits. With advances in wireless technologies and its declining cost, more industries are relying on modern wireless communications to provide services and assist their operations. Regardless of types of wireless services and technologies, a critical component common to all wireless deployments is access to radio spectrum. It is generally agreed that radio spectrum is a limited and scarce resource when managed by the traditional “command and control” spectrum management policy that does not recognize the explosive demand for spectrum access and the ability of new technologies to efficiently utilize the radio spectrum. As a part of the U.S. spectrum policy reform, the Federal Communications Commission (FCC), in its Spectrum Policy Task Force (SPTF) report, has supported the idea of using the market mechanism to use spectrum dynamically and efficiently [1]. One of the market-oriented approaches to manage spectrum is the creation of secondary spectrum market. Due to the uncertainties of and changes in market conditions, licensed incumbents may not be able to put their spectrum to the highest and best use. The incumbents who own excess spectrum in the short run are not allowed to lease to others who might have a compelling short-term need. They must either sell their spectrum outright or hold it through their full license terms. Therefore, the desirability of promoting the development of a secondary market for spectrum use is emerging. The secondary spectrum market, in broad terms, allows a permanent transfer of the rights of using licensees’ spectrum to users who could put it into a better use. If the trade can be conducted with transparency and accountability, the spectrum trading may impose a clear, market-based opportunity cost upon incumbents, thereby providing them with correct incentives to conserve spectrum. The process of spectrum commoditization enables spectrum use to become more efficient, resulting in increasing innovation and more competition. In 2003, the FCC authorized spectrum licensees to exercise spectrum-leasing options for many wireless services [2]. The policy is aimed at encouraging the development of secondary spectrum markets by facilitating the spectrum leasing arrangements among radio spectrum users. Under this order, the commission has clarified and provided options regarding to the scope of rights and responsibilities of parties entering the spectrum leasing arrangement. The licensee must comply with the original technical and service rules applicable to the particular frequency bands or type of wireless services. Although the commission has taken this first step to simplify the regulatory process, there are several issues regarding technical, economic, and strategic that existing licensee might not have an incentive to make unused spectrum available. With the ultimate goal of putting spectrum resource to its highest valued use, we need to identify methods to encourage existing spectrum licensees to make an efficient use of their assigned spectrum either by lease their unused or under-utilized parts to other users or by deploy a more efficient technology to reduce the amount of spectrum use. The objective of this paper is to outline issues that needed to be taking into considerations for the spectrum sharing idea to become reality. We define the term “secondary use of radio spectrum” as a temporal use (by secondary user) of existing licensed spectrum currently occupied by incumbent (primary user). This includes various scenarios of secondary use -- from a conservative method that requires regulatory approval to a fully dynamic scenario where spectrum can be obtained on-demand through the spot markets and the flexibility of software-defined radio (SDR) technology.

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The next section of this paper identifies research questions that are critical components to the successful deployment of secondary radio spectrum markets. The third section gives an overview of a representation of spectrum use through the signal space model, which is used as a graphical tool used to clarify technical issues. In the fourth section, we take a further step reviewing major technical factors that would play a critical role in defining technical parameters for parties participating in spectrum trading. Finally, we discuss our future work on this feasibility analysis. 2. Survey of the Issues The creation of the spectrum markets for secondary use brings about many research questions necessary to be addressed in order to develop appropriate policies that lead to a successful market [3]. The feasibility of the market depends not only upon technical feasibility of various technologies that would contribute various scenarios of how spectrum can be shared, it is also depend upon economics, regulatory, and political issues. Some of the issues that have been raised regarding secondary use or secondary markets of spectrum include:

- Capabilities of advanced radio technologies - Spectrum hoarding, speculation, and monopoly - Secondary use in government frequency bands - Government control - Concerns about public safety services - Regulatory issues relating spectrum leasing - Rights, responsibilities, and enforcement problems - Efficiency of the market - Pricing and billing

And the list goes on. However, we attempt to outline what we believe as fundamental elements that would constitute the starting point of the secondary use of radio spectrum. These would allow us to incorporate miscellaneous issues as listed above and systematically perform a feasibility study. The following are fundamental questions that needed to be unfolded for the concept of secondary use to become reality. 2.1 What are conditions for primary users to share spectrum? For spectrum sharing to take place, current spectrum incumbents need to have some incentives to sell or lease their own spectrum. From economic and financial aspects, the interest of trading the spectrum for secondary use largely depends on the classification of primary users. For instance, governmental agency users who do not pay for the rights to use radio spectrum tend to have less financial incentives to share their spectrum than those who have paid for the spectrum -- private spectrum users. Strategic factors also play an important role among the private users. Existing commercial users of spectrum could have small incentives to sell or lease excess or unused spectrum if potential buyers will use their acquired spectrum to provide a service competing with the sellers. In terms of technical issues, spectrum has multiple access dimensions, as we will elaborate in the next section. Regardless of the dimensions of sharing, a lessee’s bid and a licensee’s offer must match in all dimensions for trading to occur. This could imply that the number of participants in the spectrum market may be very low. The lack of liquidity decreases the likelihood that the

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trade takes place, leading to the need of developing policies, incentives, and market mechanisms that increase market liquidity and enhance the willingness of spectrum users to conduct a trade. The transaction costs of this market mechanism are still unclear. It is likely to be related to the negotiation process, which in turns depends on types of spectrum use and dimensions of sharing. Peha and Panichpapiboon [4] assessed the costs and benefits of secondary use where the incumbent is a GSM-based cellular carrier. Although the paper demonstrates the existence of a financial incentive of a primary user to share the spectrum, much work needed to be done to clarify the transaction costs, practical implementation and other essential elements for the market deployment. 2.2 What are conditions for secondary users to exist? While it is clearly a necessary to examine the incentives for spectrum sharing, it is equally important to consider under what circumstances potential secondary users would emerge. This answer, it turns out, depends in large measure on the application that the secondary users have in mind. In general, new spectrum users have several options to obtain access to spectrum as follows:

- Obtain a license for exclusive use - Lease spectrum from existing users - Opportunistic use of the idle spectrum through agile radio (secondary use) - Use dedicated unlicensed spectrum - Use underlay transmissions (Ultra-wideband)

The ultimate advantages of secondary use might be easy to perceive, but its obstacles, hidden costs, and efficiencies are still unclear. The availability of spectrum and the level of participation of primary spectrum users have to be taken into account. A limited pool of usable spectrum can result in insufficient liquidity of the market. Technological factors may place a limit on spectrum sharing capability. Different technologies of primary and secondary users may cause barriers in developing spectrum markets. Equipment capabilities and costs also influence the feasibility of the market. One important assumption we have to make for this framework is the availability of Software Defined Radios (SDR). The capabilities of SDR can be used to provide real-time spectrum management functions that are essential parts of the pre-conditions of the markets [5]. The degree of flexibility a device should have in order to function in the spectrum market is also needed to be determined. Secondary users hold a risky position in their service operations since they may not have direct control over the availability of radio spectrum, quality of service, and coverage expectation. This may make it difficult for the secondary users to control the quality of service they provide to their clients. To approach the problem, we need to analyze the business viability of secondary users with respect to the types of services, transmission technologies, transaction costs, and costs of software defined radios needed in providing spectrum sharing for some levels of quality of service. 2.3 How do primary and secondary users find each other? This question gives a rise to technical and economic issues. The market is framed in significant measure by the technical features of the spectrum in question. For example, trading in higher

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frequency bands implies a local spectrum markets because the higher frequencies suffer greater attenuation than lower ones. Depending on the types of sharing, demand and supply could become very specific in each of the sharing dimensions such as in time, geographical location, frequency, or modulation scheme, etc., resulting in a more complicated matching and negotiation mechanisms especially when the markets progress towards real-time markets. The trading of secondary use may occur through intermediaries such as bandwidth brokers or distributed market makers or through the process of online-automated spectrum sharing and trading in a real-time fashion. In general, the mechanisms of searching for a match between the primary and the secondary users largely rely on types of services, access characteristics, and service levels requested by secondary users. The access types could consist of a long-term lease, a scheduled lease, and a short-term lease or spot markets. Each type requires different discovery mechanisms and applies with different levels of service agreements. 2.4 What has to be negotiated and how? The essence of the problem is to identify technical parameters that primary and secondary users must negotiate for spectrum usage right trading. Hence, the necessity of developing practical negotiation mechanisms becomes apparent. We need to investigate whether the negotiation mechanisms can be generalized. All systems could apply with a uniformity pattern by means of constructing an agreement on standard-based parameters or the negotiation mechanisms depend on specific types of technologies or market mechanisms. In both cases, the development of the negotiation mechanisms needs to be more specifically defined. Negotiation parameters generally include technical (frequency, location, time, transmit power, modulation, etc.), financial (price, payment options, etc.), service quality (interference protection, signal-to-noise ratio, etc.). Actual parameters in negotiation may be more or less specific depending on characteristics of services offered by the primary user and the secondary user. Similar to the discovery mechanism, negotiation mechanism depends on types of spectrum use and access models. Performance of each possible mechanism could be measured from its transaction costs (costs associated with providing information, matching mechanism, negotiation, payment, enforcement, etc.), ability to support different types of services, and ability to support real-time markets. It is also important to take into account the situation where a secondary user intends to access radio spectrum on an opportunistic use basis. This type of users utilizes adaptive radio system that is capable of detecting spectrum environment for opportunity to access spectrum and adapting its transmission to avoid harmful interference to the primary user. In this case, the set of negotiation parameters may be different. The negotiation process may be a one-time process provided that opportunistic users has equipped with proven adaptive techniques or has complied with a set of initial agreements.

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3. Radio Spectrum Space & Spectrum Management In this section and the rest of the paper we focus on the technical perspective of providing for the secondary use of radio spectrum. 3.1 Radio Spectrum Space It is helpful to have a framework to systematically define dimensions of radio spectrum space. Theoretically, every dimension can be divided to provide multiple access to the spectrum space. Spectrum dimensioning can clarify how traditional spectrum management techniques have been employed, how new technological developments potentially allow access to different dimensions, and what dimensions secondary use can occur effectively. Characterization of spectrum dimensions is also important for policy makers to define efficient policy for secondary use. Several researchers have proposed models that capture multiple dimensions of radio spectrum space [6-8]. In general, the dimensions can be differentiated from one another by the their orthogonality property or by using statistical methods. Table 1 summarizes various dimensions of radio spectrum and their importance. With these parameters, one can identify each wireless transmission as a unique constellation defined by an n-dimensional vector V = <V1, V2, …, Vn> in the n-dimensional spectrum space. This concept, however, is theoretical and is based on several assumptions such as ideal transmitters, receivers, and environmental conditions. In the real world, each dimension has its practical limitations as noted in the table. In this paper, we choose geographical space, time, and frequency dimensions to illustrate the ongoing wireless traffic. These three particular dimensions are primarily used to define a portion of spectrum resource in the licensing process. Accordingly, each licensed portion of spectrum is a subspace sketched as a box inside this three-dimensional spectrum space model. Wireless transmissions of each licensed system would appear as a pattern or a constellation resides in the licensed subspace in the multidimensional spectrum space. Such a constellation can be either static or dynamic. Static constellations would imply stationary systems such as fixed point-to-point microwave, in which location, frequency, and perhaps time of usage are predictable. Dynamic constellations would refer to non-stationary systems such as mobile cellular networks, in which spectrum usage changes in multiple dimensions according to location of the mobile station, frequency handoff, and time. Various technological options can be represented in this graphical spectrum space. Figure 1 illustrates three well-known methods of separating channels – CDM, FDM, and TDM. Different signals/channels occupy different subspaces; therefore, allow us to locate and distinguish one from another. It is important to keep in mind that the selection of dimensions to describe wireless transmission signals can reveal different aspects of the problems of interest.

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Table 1: Dimensions of radio spectrum

Dimensions Parameters Comments

Frequency Frequency

- Subdivision of too narrow frequency bandwidth could result in an unreasonable filter requirement. - In some cases, systems that utilize underlay transmissions can coexist with spectrum users in the same frequency range.

Latitude

Longtitude Geographical

space Elevation

- Spectrum users cannot practically terminate the propagation of signal right at the physical boundaries. Thus, subdivision of physical space needs to consider the signal propagation of the systems and environmental conditions in the area as they directly affect the geographical coverage.

Time Time

- Subdivision of time influences the degree of coordination needed. The smaller time scale (ms or ns) implies a closer and more complicate coordination (synchronization) between users.

Horizontal angle (Azimuth)

Signal direction Vertical angle

- Although it is similar to the geographical division, the former was assumed that signal arrives at the receiver’s antenna from every dimension. - Signal cannot be perfectly confined to a particular angle due to the effects of multipath propagation. - Transmission between a pair of transmitter and receiver is limited to only one direct path. However, advanced technology (e.g. space-time coding) can exploit multipath to create multiple independent channels between a transmitter-receiver pair.

Modulation/Coding scheme Transmission characteristic Polarization

- These can be viewed as secondary dimensions of radio spectrum, used as information bearing parameters and logical channelization. They can be distinguished by their characteristics or by probability.

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Figure 1: Multidimensional spectrum space

3.2 Spectrum management From the technical aspect, one of the key objectives of spectrum management is obviously to subdivide and allocate spectrum space for specific uses or services so that the demand is met and harmful interference created by one service to another is eliminated or minimized. The uncertainty caused by interference is also the main barrier to the success of secondary spectrum use. The primary users may not willing to allow secondary access because of the potential interference created by the secondary systems that can lead to performance degradation. On the other hand, new spectrum users may not be willing to access the spectrum on secondary basis because of the interference uncertainty created by the primary systems. The effect of interference can be illustrated as spillover from the subspace of the intended signal in the three-dimensional signal space in Figure 2. 3.2.1 Taxonomy of interference types Spillover or interference comes from all dimensions of the intended transmission subspace. In wireless communications, we can characterize types of interference in each of the corresponding dimensions in this three-dimensional radio spectrum space as follows: 1. Geographical space dimension

Spillover of energy from one geographical area to the adjacent areas while keeping the frequency and time parameters constant is known as the problem of co-channel interference.

2. Time dimension Overlapping of signals in time dimension also causes another form of interference. This kind of interference comes in several forms depending on the scale of the time dimension under

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consideration. They include spillover between timeslots in time division multiple access (TDMA) systems due to the effects of propagation delay, intersymbol interference caused by multipath propagation of signals, etc.

3. Frequency dimension Spillover from one frequency to the adjacent frequency bands is referred to as the problem of adjacent channel interference.

Figure 2: Spectrum space with interferences

3.2.2 Interference management Interference management has long been a continuing challenge of regulators for decades. Interference creates pollution around the subspace that prohibits multiple systems to locate closely to each other. The traditional approach to avoid interference is to grant a license for exclusive use of radio spectrum with gaps in all dimensions in order to keep the acceptable level of interference. The interference management approach can be visualized in the model as reserving disjoint subspaces in the spectrum space model for exclusive use as shown in Figure 3. These licensed subspaces have to be well separated in all dimensions from other subspaces in proximity to avoid interference.

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Figure 3: Traditional interference management

First, separation in the frequency axis represents systems in the same area with different frequency channels. Second, separation in the space axis represents systems in different geographical areas. Finally, separation in the time axis corresponds to systems utilizing spectrum at different points of time. Examples of the licensed systems in frequency and space are evident, since these two dimensions are primarily, yet not solely, used to define physical boundaries of most of the licenses. Dividing radio spectrum use in time is less common; it exists, for example, in AM broadcasting where some AM radio stations are required to reduce power or cease their operations at night time to avoid interference to other AM stations [9]. Sharing in the time dimension represents opportunities for additional spectrum access, as mentioned in the SPTF report [1]. These separations are referred to as guard band, guard time, and limited transmitted power (related to path loss distance that gives enough attenuation) corresponding to the frequency, time, and space dimensions respectively. In the current spectrum management scheme, the sizes of the gaps in frequency axis and space axis are determined based on the transmitter’s operations. Generally, these interference protections through limits on in-band transmitted power and out-of-band emissions are established case-by-case basis varying by different radio services that interfere with each other. The receiver’s ability to pick up the intended transmission often becomes an afterthought. Therefore, these amounts of separations are based on general characteristics of devices or a set of worst-case parameters. This current scheme is reasonable for an environment where demand for spectrum access is low and with the fixed radio services. However, today’s spectrum environment has changed. The FCC has recognized that the worst-case predictive interference model would significantly reduce the potential of additional spectrum access. With the number of users, devices, and services proliferates, a new approach to interference management and a regulatory reform are needed to create opportunities to access unused part of radio spectrum.

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4. Technical considerations in secondary use of radio spectrum We use the spectrum model described in the previous section to outline the effects of technical factors to the development of secondary use of radio spectrum. 4.1 Opportunity of secondary use of spectrum The opportunity of secondary access in spectrum resource is essentially that of identifying unoccupied subspaces, determining and arranging signal constellations of the secondary systems so as to avoid or minimize interferences with the licensee’s signal. With this multidimensional signal space, we try to identify types of secondary access that would best exploit the remaining spectrum resource and factors that would render the idea of sharing feasible/infeasible.

Figure 4: Secondary use opportunity

Figure 4 illustrates the unoccupied parts of the spectrum space in the green areas. The figure shows three licensed transmissions in CDM, FDM, and TDM techniques. Consider that the overall radio spectrum is divided into multiple licensed subspaces. Each has buffer spaces for interference protection in every dimension used to define subspace (see Figure 3). We can initially classify types of secondary access into two types -- unused parts of spectrum outside the licensed subspaces and unused parts of spectrum inside the licensed subspaces. The former case corresponds to access into guard-bands in the frequency domain and access into coverage boundaries in the geographical space domain. These additional spectrum and geographical areas are set aside for the purpose of interference avoidance between neighboring licensed systems. Therefore, only spurious emissions are expected in this area. From a

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secondary user’s perspective, secondary access in this case only needs to handle the out-of-band emissions from systems operating in adjacent frequencies in the same area (or adjacent channel interferences). In the space domain, it would confront with the in-band emissions from the licensed systems operating in the nearby geographical areas (or co-channel interferences). The major technical challenges of using spectrum in this type of subspaces are the problems of interferences from secondary users to primary users and vice versa. Since primary user does not expect any transmission in this type of spectrum spaces, the secondary user may have be an opportunistic user who is able to detect the opportunity and adapt its transmission to avoid causing interference to the primary user. Although secondary access into guard spaces appears to have less negotiation and lower transaction cost, the availability of this type of spectrum is very limited in terms of the amount of spectrum allocated for this purpose and the opportunity to access without interferences. The second case, in which secondary users operate in the same part of spectrum as primary users, introduces greater challenges. First, secondary transmitters may need to conform to a set of technical or operational rules defined in the particular license of the primary user. In essence, the parameters include designated frequency range and bandwidth, geographical space, and maximum in-band and out-of-band emissions. Second, secondary users may have to negotiate with primary users regarding the operational characteristics and the amount of interference they are willing to accept and that they are going to create. This task becomes very complicated, as not every part within the licensed subspace may available for secondary use. For spectrum leasing, secondary users have to negotiate with primary users for the secondary use such as operational frequency, geographical area, channel, etc. For opportunistic use, secondary users have to recognize the ongoing transmission of primary users and adapt their transmissions so as to avoid collision and harmful interference to the primary users. Negotiation for opportunistic use, if any, may be aimed to provide interference protection such as minimum performance of spectrum sensing technique, maximum interference level in each dimension, maximum delay for opportunistic devices to vacate the channel upon detecting spectrum activity, etc. In both cases, technical parameters associated with the transmission characteristics of the primary users need to be taken into account. 4.2 Technical factors For new spectrum users, there are several technical factors to be reviewed in determining the suitable spectrum access method (i.e. obtain license, leasing, etc.) and in seeking appropriate parts of spectrum space to operate a service. Technical considerations generally include:

• Dimensions of radio spectrum access

Not every part of spectrum space has the same properties. Some parts may be suitable for some types of services but not others. For example, lower frequencies can propagate further. In contrast, the propagation of high frequency signals is more sensitive to environmental conditions such as rain and obstacles and may require a line-of-sight (LOS) between a transmitter and a receiver. These propagation characteristics and LOS requirement directly influence the selection of frequency band of operation. In any particular geographical location, the frequency band choice is defined by the physical location requirements of the new service. For example, a broadcast service needs to obtain spectrum access in all coverage areas. On the other hand, a fixed point-

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to-point communication service may only need to secure access in the geographical area of the transmission path. The time dimension is largely dependant on characteristics of the service or spectrum use. If the services requires real-time or continuous transmissions such as television broadcast or multimedia streaming, they may need to have access to spectrum at all times. If the services are delay insensitive or bursty such daily data backup, they have more flexibility and are able to access spectrum whenever it is available for a period of time. These selections of spectrum parameters play an important role in system design, equipment costs, and costs relating to spectrum access.

• Amount of spectrum needed

The previous issue allows us to evaluate and seek the most suitable part of the spectrum space for each secondary service based on characteristics of the secondary user. This issue deals with the amount of each dimension required for a successful operation of the service. Secondary users have to evaluate the amount of spectrum needed in every dimension based on every aspect of their services including service types, choices of technologies, QoS requirements, system costs, spectrum access costs, etc. For example, a narrowband broadcast channel may transmit at high power to cover a large service area but it may need a small amount of bandwidth. An UWB system connecting household devices can utilize much larger frequency bandwidth but it has very low transmission power for a very small coverage area. In terms of QoS requirements, a voice transmission may require a constant amount of bandwidth with continuous access to the time dimension of spectrum. A bulk data transfer application can trade off between transmission delay and the amount of bandwidth needed depending on the QoS requirements.

• Cost and time to obtain access to radio spectrum

This factor is building upon system characteristics of secondary users and the parts of spectrum space in which they wish to operate. This issue, together with choices of technologies and service requirements, will likely to influence the selection of the optimal spectrum space and the appropriate access option for operation of the service. Depending on secondary users, parameters of spectrum access can be engineered, to some extent, to satisfy the service requirements. The use of less crowded higher frequency bands is likely to yield a lower spectrum access cost, but with the expense of more advanced and expensive equipment. Flexible secondary users with the ability to use a variety of frequency ranges or non-contiguous frequency bands would likely be able to take advantage of a larger pool of available spectrum. In addition to the access costs, secondary users need to consider how much delay they are willing to accept before gaining access. In every spectrum access option, the negotiation process is likely to contribute to a significant portion of the time delay. Again, depending on the flexibility of secondary services and types of sharing, time and cost are varied by what technical issues they need to negotiate and how much of technical issues they can agree to be flexible upon.

• Existing use of services

Evaluation of secondary use should consider the current use of the primary user in the spectrum space where secondary users wish to operate. The degree of sharing and

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negotiation may be bounded by technology employed by the primary user. The operating characteristics of the primary user, such as types of service and transmission technology, are likely to suggest an array of possible types of sharing along with a set of technical parameters that need to be negotiated. Accordingly, they may have an important effect on the negotiation process, which in turn, influences the cost and time to obtain spectrum access. When analyzed with characteristics of the secondary service, this factor may shape the amount of spectrum needed in each dimension. In extreme cases, it may render some parts of spectrum incompatible or cost-prohibitive for some particular types of secondary use. Depending on the type of service, secondary users may have to conform to the existing allocation and operation rules associated with the band in which they intend to operate. International standards and equipment standards could create strong restrictions on the choices of spectrum access and types of spectrum sharing, as they may be difficult to modify.

• Service and technical limitations

Service rules and technical limits on a frequency band or in a location can impede secondary use. These limits were often established to specify types of services that are permitted, amount of in-band transmitted power, and amount of out-of-band emissions. Thus, transmissions of secondary users may have to be compatible with the service rules in the band. It is also important to ensure that additional traffic created by secondary use will not cause harmful interference to the neighboring services in every dimension and vice versa.

4.3 Some examples of secondary use The general criteria for the evaluation of secondary use as described above demonstrate that the study of secondary use is a complicated task. We identified several technical factors that have important effects on the feasibility analysis. As mentioned earlier, it is important to include characteristics of primary users into the study. Thus, we provide some illustrative examples of secondary use given various scenarios of primary users. We do not intend to provide a complete solution for each scenario; rather we use them to show the detail of complexity of this study. Intuitively, we anticipate that secondary use of radio spectrum would occur in the simplest forms possible. In many cases, spectrum may not be available for secondary use in its entirely. Primary users may share spectrum by the way they define a channel in the primary systems. With this method, primary users can share their unused capacity and still able to operate in the same part of spectrum. We consider three basic multiplexing methods: FDM, TDM, and CDM.

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4.3.1 Frequency Division Multiplexing (FDM) FDM system is broadly used in satellite and terrestrial communications. Figure 5 shows typical frequency allocation in a FDM system. Channels are divided in frequency domain with a guard band between channels to reduce adjacent channel interference. Frequency reuse in the FDM system requires a sufficient guard space between adjacent co-channel areas to avoid co-channel interference.

Figure 5: Dedicated frequency channel spectrum sharing in FDM system

Frequency-Shared FDM The very basic form of sharing in FDM is perhaps to lease one or more of the frequency channels to secondary users. Depending on the frequency planning, a list of frequency channels is varied by geographical areas of the primary user’s spectrum space. In general, technical parameters that define a FDM channel are center frequency, bandwidth, and geographical area. Since the secondary user operates in the well-defined dedicated frequency channels, it is confined to the subspace defined by the channel parameters. In other words, these parameters impose sufficient limits on co-channel interferences and adjacent channel interferences to the nearby channels in the primary system. In some applications (e.g. broadcast spectrum) this scenario is scarcely different from a traditional frequency allocation, although the channel may be “leased” from the licensees whose channels might now suffer increased interference from the new user (the green channel in Figure 5). In others (e.g. cellular systems) it is quite clearly new. Defining geographical boundary for a secondary use channel, however, can be problematic especially in the frequency reuse systems. For instance, a cellular system may not have a perfectly defined boundary of its cell, because it is not practical to perfectly predict signal propagation under real-world conditions. By considering transmit power and the location of the

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secondary transmitter, the primary user needs to ensure that emission of the secondary user will not create co-channel interference to other cells. Since it is not necessary for the secondary transmitter to have the same location as the primary transmitter, the secondary system must have a smaller coverage area relative to coverage area of the primary system in the shared channel. Accordingly, dedicated frequency channel sharing in a larger area such as a rural macro cell or a television broadcast area may be more feasible than sharing in a smaller area. Space-Shared FDM In a more complex scenario, the primary user may not want to give up an entire frequency channel in the entire area for the entire time. S/he can further subdivide the use in time or geographical dimensions. Since the channel is not dedicated to the secondary user in every access dimension, we refer to the sharing of a frequency channel in space domain as Space-Shared FDM. Subdividing the use of the frequency channel in the time dimension (Time-Shared FDM) is considered in the TDM example as it has features similar to secondary use in TDM systems. Subdividing in geographical space is illustrated in Figure 6.

Figure 6: Space-Shared in FDM system

Not only does this scenario fully inherit the problems associated with dedicated channel spectrum sharing, it also presents more complicated issues that require attention from both the primary and secondary spectrum users. We elaborate the issues that could arise with regard to two technical factors: mobility and interference protection.

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Mobility: Mobility reflects the movement of secondary user’s signal in the geographical dimension of the spectrum space model. For a fixed service, the location and technical parameters of the system are chosen and remain fixed for the entire session. Future spectrum access attempted by either primary or secondary users can consider the parameters of the existing use and determine appropriate location and operational parameters for the new access. For a service that requires mobility, interference considerations become more challenging as devices can move from one place to another. Mobile users require some mechanisms to acquire the current state of the spectrum in the location they are moving to. Recall that in the case of dedicated channel spectrum sharing, spectrum users do not have difficulty with the mobility factor, since the entire physical location of the frequency channel is dedicated to the secondary system. Thus, the movement of secondary user will not affect the primary user as long as the secondary user’s emission is confined to the assigned spectrum space. Interference protection of secondary user: Depending on service requirements, secondary user may be required to have its spectrum use be protected from interference in order to guarantee a reliable service. For secondary users with interference protection, primary users may need to add extra mechanisms such as coordination, negotiation, or smart radios to their normal operations in order to avoid interference to secondary users. Without the protection, primary users are likely to require less involvement after the initial negotiation. For primary systems, we assume that primary users always entitle to interference protection. Using these two technical factors, we create different spectrum sharing scenarios and attempt to describe the coexistence mechanisms in Table 2. Many methods have been suggested to evaluate the availability of frequency channels [4, 10, 11]; Summing them up, interference caused by a fixed service of either primary or secondary systems, can be determined by the following techniques: • Use technical parameters and geographical location of a fixed service to evaluate the

interference. The other user could be informed of this or it can be made available through the development of a universal database.

• A fixed system, typically through a base station, could convey the availability of the channel in the area by broadcasting a signal on a well-known channel.

• The other user could employ spectrum-sensing techniques and make use of adaptive radio to adjust or cease its transmission to avoid interferences.

In the case of a mobile service, techniques similar to the last two options could be used to evaluate interference created by a mobile service.

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Table 2: Some scenarios in shared frequency channel within the same area

Mobility

Primary user

Secondary user

Secondary user interference protection Coexistence mechanisms

Fixed Fixed No - Secondary user avoids/negotiates interference effects upon installation

Fixed Fixed Yes - Both users avoid/negotiate interference effects upon installation

Fixed Mobile No - Secondary user constantly determines the interference effects from static spectrum use of the primary user

Fixed Mobile Yes

- Secondary user constantly determines the interference effects from static spectrum use of the primary user - Primary user avoids/negotiates interference effects upon installation

Mobile Fixed No

- Secondary user avoids/negotiates interference effects upon installation - Secondary may be required to vacate the channel upon detecting primary user nearby - Primary user constantly determines the interference effects from static spectrum use of the secondary user

Mobile Fixed Yes

- Secondary user avoids/negotiates interference effects upon installation - Primary user constantly determines the interference effects from static spectrum use of the secondary user

Mobile Mobile No

- Secondary user constantly determines the interference effects from dynamic spectrum use of the primary user - Secondary may be required to vacate the channel upon detecting primary user nearby

Mobile Mobile Yes - Both users negotiate technical parameters to avoid collision

4.3.2 Time Division Multiplexing (TDM)

In a TDM system, all users exploit the same frequency channel and are separated by distinct time slots. It is typical to have more than one frequency channel in the TDM system in a given geographical area. Figure 5 depicts spectrum sharing in TDM system. If spectrum is shared by frequency channels without recognizing the time slot, it will be the same case as sharing in FDM system. Sharing by time slots may be attractive since it offers high multiplexing gain especially with bursty traffic. However, this has the main disadvantage of synchronization complexity.

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Figure 7: Spectrum sharing in TDM system

For a secondary system, the first requirement is that it has to support discontinuous mode of transmission and reception. In order to avoid overlapping of time slots, the time burst transmission requires sufficient amount of guard time between adjacent time slots to counter the effects of different propagation delays and ramp-up and ramp-down delays of a transmitter. Limited buffer space makes synchronization of the transmitter and receiver a crucial task in the TDM system. Systems with shorter guard time (e.g. milliseconds) require a more precise synchronization mechanism than those with longer guard time (e.g. minutes, hours). Without accurate timing, secondary users may create interference with the primary system. This condition may force secondary devices to acquire synchronization signal directly from the primary system. Consequently, this implies that the devices in secondary system must be compatible with operation of the primary system to acquire timing information. The problem is greatly simplified when the primary system has long guard time, as the secondary user could employ other methods to protect interference to and from the primary user. Since the length of guard time reduces the usable channel time, hence the capacity of the system, we would normally observe that the guard time is proportional to the time slot duration and is depending on the capacity of the system. 4.3.3 Code Division Multiplexing (CDM) The basis of CDM system is to allow signal of multiple users to occupy the same spectrum space in both frequency and time dimensions. Secondary access in the CDM system is a very complicated issue and is dependent on spread spectrum techniques.

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In direct-sequence spread spectrum, each channel is assigned with a unique spreading code. The interference from all other users is eliminated if the spreading codes are orthogonal. In practice, self-interference and multiple access interference (MAI) still remain in the system due to imperfect codes and the presence of multipath propagation effects. The system also requires extremely accurate power control to prevent near/far problems. Thus, sharing spectrum through orthogonal code is merely impractical due to very complex signal processing frameworks that are not likely to be accomplished in the secondary system without the same standard and very close coordination. In CDM, the system capacity is limited by self-interference and MAI, which results from channel characteristics and the number of active users (Figure 8). When capacity is not reached, secondary transmission could theoretically be permitted as long as it appears as wideband noise to the primary system. In real-world with changing channel conditions and interferences from adjacent cells, it would be difficult for secondary user to guarantee Signal-to-Interference Ratio (SIR) to primary user and vice versa.

Figure 8: Theoretical CDM system capacity

For frequency-hopping spread spectrum system, multiple users are hopped over a set of frequency channels and are distinguish by different hopping sequences. In order to gain secondary access, secondary user in this case needs to obtain a unique hopping pattern to avoid collision with primary system. More importantly, it has to synchronize the timing with the primary system. Although this establishes the similar synchronization problem as in the TDM system, it could become more problematic, as the hopping rate is generally much faster than the time slot in the TDM systems (for example, the Bluetooth hop rate is 1600 hops/s). Multi-carrier transmissions such as OFDM and MC-CDMA have recently been proposed for the spectrum sharing platform [12, 13]. The basic principle relies on the transmission of data by subdividing a given large bandwidth into individual frequency channels and transmitting a low data rate stream in each channel. Strictly speaking, this scheme is a FDM, because the data is multiplexed over the frequency dimension. The difference, however, is that all frequency channels are assigned to one user. The user can divide its total data stream and simultaneously

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transmit sub-streams over a large number of frequency channels (carriers). The main advantage of using multi-carrier transmission for spectrum sharing is put in the context of secondary user. Given that secondary user employs multi-carrier scheme, it can dynamically rearrange its transmission to appear in any specific frequency bands (both contiguous and non-contiguous) of the primary user. To avoid interferences, this type of sharing, however, may require the primary user to be a multi-carrier system and have close technical coordination such as synchronization, sub-carrier organization. In other cases, it may demonstrate the need for adaptive guard band or highly selective filters to isolate transmission of secondary user. 4.3.4 Distant Communication

One of the important issues in achieving viable communications using secondary spectrum access is the problem of finding the right-of-way. Accessing spectrum on a secondary basis does not guarantee that the unoccupied spectrum at the secondary transmitter will also be unoccupied at the secondary receiver and at the geographical space in between. Thus, the secondary transmitter and receiver need to secure the right-of-way between their locations in every spectrum dimension before their communications can take place, shown in Figure 9. In the signal model, the horizontal lines on the frequency-space pane represent the viable communications passages between location A and location B.

Figure 9: Right-of-way of secondary spectrum access

The right-of-way issue plays an important role in the feasibility study of secondary spectrum access. The availability of paths and the capability of devices to detect them could become an important factor in determining the feasible types of secondary access for a given primary system.

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For example, the figure shows that, without an effective spectrum sensing mechanism, the short-distance communications appears to be more viable than the long-distance. One of the reasons is that the availability of the passages tends to decrease as the distance between transmitter and receiver increases. This issue is especially important in designing the adaptive radio devices. Although, advanced technologies can provide both adaptive transmitter and receiver an ability to sense the unoccupied parts of spectrum, the information may not be the same to both devices due to the differences in locations. As a result, they need a method to establish an agreement on the part of spectrum and the point of time that they are going to communicate. By relying solely on adaptive devices, some certain long-distance communications may be eliminated from the secondary use due to their limited capabilities to sense the spectrum conditions on the entire path between transmitter and receiver. A similar problem could also occur when we consider the mobility of the users. Hence, these issues may demonstrate the need for an interference monitoring network or a broadcast of spectrum conditions from the primary user. 5. Conclusion and Future Work Determining the feasibility of secondary use of radio spectrum from a technical perspective is complicated. One of the major technical problems involves determining the potential interference impact between primary and secondary users. Many key technical factors must be examined to assist in determination of the possibility of spectrum sharing for each service. As demonstrated in the paper, some types of secondary use of spectrum are potentially suitable in some environments and for some wireless services and not in others. It is thus necessary to evaluate the characteristics of both the primary and secondary users in each of the different spectrum environments. Thus, the next step in our study is to classify types of applications and services of spectrum users by studying the effects of each technical parameter. Some of the technical factors may have more influence on the negotiation or spectrum sharing requirements than others. As a result, they will play an important role in the classification process. The following are some of the factors that could be used in the process:

- Types of spectrum use (communications, sensing) - Types of wireless services (fixed, mobile, fixed satellite, mobile satellite, etc.) - Transmission characteristics (broadcast, multicast, unicast, bursty, continuous, etc.) - Quality of service/Values of communications (BER, data rate, delay, etc.) - Channel multiplexing schemes (FDM, TDM, CDM spread spectrum, CDM multi-carrier) - Device characteristics (fixed, portable, transmit power, receiver sensitivity, etc.) - Service areas (urban, rural, etc.) - Types of spectrum access (licensed, unlicensed, leasing, opportunistic, underlay) - Types of spectrum sharing (by frequency, by time, by geographical space, uplink band,

downlink band, etc.)

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Figure 10: Matrix of application/service types

With such a classification, we can develop a matrix of service behaviors of primary and secondary user and systematically analyze the potential interactions associated with each spectrum-dependent application and with each type of spectrum sharing (Figure 10). Identifying the problems or conditions for each scenario can enlighten the reality of secondary use and is a crucial step towards the development of secondary spectrum market for a more efficient use of radio spectrum.

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