Spectrum Allocation and Wireless Transmission Issues

B3G technology requires high bandwidth in order to provide multimedia services at a lower cost
than is presently the case. In the United States, B3G systems will likely migrate to the 5.2–5.9 GHz
range (assuming regulatory approval). It must be stressed, however, that there are serious spectrum
allocation issues associated with B3G technology, simply because today unallocated spectrum either
does not exist in some countries or is in short supply. Long-term planning is necessary to make
spectrum available for B3G applications [530]. In addition, worldwide standardization of spectrum
allocation for B3G systems would be desirable for maintaining connections when moving anyplace
in the world – the “always connected, anywhere” philosophy.
In the United States, there is a bright spot in the spectrum allocation arena. The FCC has noted
that there are large portions of allotted spectrum that are unused, and this is true both spatially and
temporally. In other words, there are portions of assigned spectrum that are used only in certain
geographical areas and there are some portions of assigned spectrum that are used only for brief
periods of time. Studies have shown that even a straightforward reuse of such “wasted” spectrum
can provide an order of magnitude improvement in available capacity. Thus, the issue is not that
spectrum is scarce – the issue is that we do not currently have the technology to effectively manage
access to it in a manner that would satisfy the concerns of the current licensed spectrum users.
The Defense Advanced Research Projects Agency (DARPA) is developing a new generation of
spectrum access technology that is not only ostensibly oriented toward military applications, but also
applicable to advanced spectrum management for communication services. The DARPA program is
pursuing an approach wherein static allotment of spectrum is complemented by the opportunistic use
of unused spectrum on an “instant-by-instant” basis in a manner that limits interference to primary
users. This approach is called opportunistic spectrum access spectrum management and the basic
parts of this approach are as follows: (1) sense the spectrum in which you want to transmit; (2) look
for spectrum holes in time and frequency; and (3) transmit so that you do not interfere with licencees. There are a number of research challenges to this adaptive spectrum management, including
(1) wideband sensing; (2) opportunity identification; (3) network aspects of spectrum coordination
when using adaptive spectrum management; (4) the need for a new regulatory policy framework;
(5) traceability so that sources can be identified in the event that interference does occur; and (6)
verification and accreditation.
The National Science Foundation (NSF) has a research program entitled Programmable Wireless
Networking (NeTS-ProWiN). This research program addresses issues that result from the fact that
wireless systems today are characterized by wasteful static spectrum allocations, fixed radio functions,
and limited network and systems coordination. This has led to a proliferation of standards that provide
similar functions – wireless LAN standards (e.g., Wi-Fi/802.11, Bluetooth) and cellular standards
(e.g., 3G, 4G, CDMA, and GSM) – which in turn has encouraged hybrid architectures and services
and has discouraged innovation and growth. Emerging programmable wireless systems can overcome
these constraints as well as address urgent issues such as the increasing interference in unlicensed
frequency bands and low overall spectrum utilization. The NSF research is based on the concept of
programmable radios. Programmable radio systems offer the opportunity to use dynamic spectrum
management techniques to help lower interference, adapt to time-varying local situations, provide
greater QoS, deploy networks and create services rapidly, enhance interoperability, and in general
enable innovative and open network architectures through flexible and dynamic connectivity [533].
Some of the proposed technologies for wireless transmissions in a B3G environment are detailed
in the subsequent text. Each has its own implications for spectrum allocation concerns.
6.1.1 Modulation Access Techniques: OFDM and Beyond
Multi-carrier modulation has been identified as a key technology for B3G, and Orthogonal Frequency
Division Multiplexing (OFDM) is the main technique under proposal. It is already present in IEEE
802.11a WLANs. OFDM was originally proposed for single users but extensions to multiusers, for
example OFDMA,1 support multiple access. Usually OFDM is combined with other access techniques,
typically CDMA and TDMA, to allow more flexibility in multiuser scenarios. Multi-carrier
Code Division Multiple Access (MC-CDMA) is another access technique with great potential. OFDM
and CDMA are robust against multipath fading, which is a primary requirement for high data rate
wireless access techniques. With overlapping orthogonal carriers, OFDM results in a spectrally efficient
technique. Each carrier conveys lower data rate bits of a high-rate information stream; hence it
can cope better with the intersymbol interference (ISI) problem encountered in multipath channels.
The delay-spread tolerance and good utilization of the spectrum has put OFDM techniques in a rather
dominant position among future communication technologies. OFDM, on the other hand, has strict
time and frequency synchronization requirements and is prone to the peak-to-average power ratio
(PAPR) problem.
6.1.2 Nonconventional Access Architectures
Wide coverage and local coverage are the two most distinctive B3G access components. It is expected
that the requirement for higher data throughput and support for a great number of users will result
in a shift to higher and less-congested frequency bands, for example the 5-GHz band, and wider
bandwidths (20–100 MHz). In cellular access, this would mean that the link budget would be seriously
degraded and unreasonable high power would have to be used to compensate for the higher attenuation
occurring in this frequency band. This could easily exceed the regulation for power emission from
base stations, and also it could dramatically reduce (the already challenged) battery life in terminals. Therefore, nonconventional access architectures for wide-area access are being considered to cope
with this problem. Multi-hop cellular, and particularly two-hop, approaches appear to be an effective
solution to the problem of achieving wide coverage and high data throughput. By using relaying
(repeating) stations, the equivalent distance between base station and mobile station can be reduced.
Efficient use of radio resources can also be attained since some resources can be reused in different
hops. In principle, the relay stations can be fixed (called infrastructure-based relaying) or mobile (ad
hoc relaying). In the distributed radio access approach, a base station has under its control a number
of remote access sites, each with its own antenna(s) and covering a small area. The small-sized cells
covering a large cell reduce the distance between the mobile terminal and its most suitable/closest
access point. The base station is connected to the remote radio access sites by using optical fiber
or radio links. Distributed radio access is a cost-effective approach to scalable networks. In localarea
access, several architectures can be used in addition to the single-hop cellular access approach.
Several ad hoc access concepts have shown their potential for short-range communications, including
multi-hop, peer-to-peer, and cooperative communications. Collaboration among users (or nodes) aims
to benefit either a single user or several (or all) collaborating users. Through cooperation (at intraand/
or interlayer level), the data throughput can be increased and signal quality can be enhanced.
Moreover, power efficiency can be boosted, which equates to an increased battery life in terminals.
6.1.3 Multiantenna Techniques
Multiantenna techniques2 are regarded as among the most important enabling technologies for B3G
technology. In principle, no technique other than the use of multiple antennas will easily permit
a high spectral efficiency. By exploiting these techniques, data throughput can be increased, link
quality improved, cell coverage extended, and network capacity enlarged. Three approaches can be
used, namely, diversity, beam-forming (smart antennas), and spatial multiplexing. Diversity techniques
require widely separated antenna elements (several wavelengths at least). Actual separation depends on
the type of channel. Directional channels (narrow angular spread) require large separation and vice
versa. Diversity techniques exploit the fact that the associated channels fade independently, while
diversity domains can be space, time, frequency, and polarization. Diversity gain will improve the
average signal-to-noise ratio. In beam-forming, signals are coherently combined (either in reception
or transmission) so as to enhance the array response in preferred directions. Nulls can also be spatially
controlled. Beam-forming allows the establishment of directional links. In beam-forming, it is assumed
that the channel or direction of arrival is known to the transmitter/receiver. Unlike with diversity, by
using beam-forming, the variability of the signal (e.g., fading statistics) is not affected. The array gain
is proportional to the number of elements of the array. Spatial multiplexing offers a linear increase
in capacity by exploiting the parallel transmission of different information from different antennas.
This is essential for attaining the high spectral efficiencies required by B3G. For the receiver to
separate and decode the parallel streams, it is assumed that the signal propagates in a rich scattering
channel and the number of reception antennas is at least equal to number of transmission antennas.
The term MIMO refers in principle to any technique exploiting multiple antennas at the receiver and
transmitter.
6.1.4 Adaptive Modulation and Coding
Adaptive Modulation and Coding (AMC) is a form of link adaptation that is used in response to the
changing characteristics of a radio channel. AMC jointly selects the most appropriate modulation and
coding scheme according to channel conditions. The better the radio conditions, the higher the modulation
rate and code rate combination, and vice versa. Clearly, AMC is more effective in packet networks – the networks envisioned for B3G. Conventional wireless services have mostly been
designed for constant rate applications, such as voice transmission. To combat channel fading, communication
systems have usually been designed to maximize time diversity with a combination of
interleaving and coding for better bit error rate performance. B3G wireless systems must target packet
data, and thus are usually designed to maximize throughput for a given battery energy budget while
allowing a certain delay.
6.1.5 Software Defined Radio
Since different wireless interfaces will be used in B3G, Software Defined Radio (SDR) appears to be
a cost-effective solution to implement several access approaches in one terminal. SDR uses a flexible
architecture that allows the wireless interface to be reconfigured. This allows multistandard wireless
interface operation with a common hardware platform, opening the door for forward compatibility.
Furthermore, SDR is an enabler for cooperative networks. SDR allows dynamic modifications of the
radio frequency, baseband processing, and even the MAC layer of the terminal (which can utilize a
particular wireless interface by reconfiguring the system). The degree of flexibility brought by realtime
reconfigurability opens up a new world of possibilities for users, operators, services providers,
and terminal manufacturers. Users can establish connection to any network, allowing simple local
and global roaming. Users can also benefit from the low-cost terminals that this technology can
entails. Hardware and software updates can easily and wirelessly be carried out by users or operators.
Manufacturers can also take advantage of SDR as large volumes of terminals with identical hardware
(and fewer components) are produced. Even upgrades or changes in the terminals can be easily
effected. In addition, service providers can exploit this flexibility to match their operation and services
to user demands better [532].
The shift in B3G toward IP-based, high-speed multimedia wireless traffic demands a high spectral
efficiency. A natural corollary to this is a need for cooperation across subnetworks and the use of
multi-hop relaying. Regulatory reforms could free up bandwidth currently used for analog broadcasting
– high-frequency bands – for B3G systems [534]. The more efficient modulation schemes
discussed above cannot be retrofitted into 3G architecture, which is one of the reasons B3G research
is being conducted before 3G systems are fully implemented (another reason is that 3G performance
may not be sufficient for future high-performance applications like full-motion video and wireless
teleconferencing). Spectrum regulation bodies must get involved in guiding the researchers by indicating
which frequency bands might be used for B3G. Along with regulatory reforms, a number
of spectrum allocation decisions, spectrum standardization decisions, spectrum availability decisions,
technology innovations, component development, signal processing, and switching enhancements,
plus intervendor cooperation have to take place before the vision of B3G will materialize. Standardization
of wireless networks in terms of modulation techniques, switching schemes, and roaming is
an absolute necessity for B3G technology. However, B3G is not an independent replacement architecture
for existing systems. Network architects must base their vision of B3G architecture on hybrid
network concepts that integrate wireless WANs, wireless LANs (IEEE 802.11a, IEEE 802.11b, IEEE
802.11g, IEEE 802.15, and IEEE 802.16), Bluetooth technology, and fiber-based backbones with
broadband wireless (B3G) networks. Moreover, B3G planning must allow for a smooth transition
from the current state of existing networks to their coexistence with B3G systems [535].