Current Wireless Systems

Current Wireless Systems

1. Cellular Telephone Systems:
Cellular telephone systems, also referred to as Personal Communication Systems (PCS), are extremely
popular and lucrative worldwide: these systems have sparked much of the optimism about the future
of wireless networks. Cellular systems today provide two-way voice and data communication at vehicle
speeds with regional or national coverage. Cellular systems were initially designed for mobile terminals inside vehicles with antennas mounted on the vehicle roof. Today these systems have evolved to support
lightweight handheld mobile terminals operating inside and outside buildings at both pedestrian and
vehicle speeds.
The basic premise behind cellular system design is frequency reuse, which exploits path loss to reuse
the same frequency spectrum at spatially-separated locations. Specifically, the coverage area of a cellular
system is divided into nonoverlapping cel ls where some set of channels is assigned to each cell. This
same channel set is used in another cell some distance away, as shown in Figure 1.4, where fi denotes
the channel set used in a particular cell. Operation within a cell is controlled by a centralized base
station, as described in more detail below. The interference caused by users in different cells operating
on the same channel set is called intercell interference. The spatial separation of cells that reuse the
same channel set, the reuse distance, should be as small as possible to maximize the spectral efficiency
obtained by frequency reuse. However, as the reuse distance decreases, intercell interference increases, due
to the smaller propagation distance between interfering cells. Since intercell interference must remain
below a given threshold for acceptable system performance, reuse distance cannot be reduced below
some minimum value. In practice it is quite difficult to determine this minimum value since both the
transmitting and interfering signals experience random power variations due to path loss, shadowing,
and multipath. In order to determine the best reuse distance and base station placement, an accurate
characterization of signal propagation within the cells is needed. This characterization is usually obtained
using detailed analytical models, sophisticated computer-aided modeling, or empirical measurements.
Initial cellular system designs were mainly driven by the high cost of base stations, approximately
one million dollars apiece. For this reason early cellular systems used a relatively small number of cells
to cover an entire city or region. The cell base stations were placed on tall buildings or mountains and
transmitted at very high power with cell coverage areas of several square miles. These large cells are called
macrocells. Signals propagated out from base stations uniformly in all directions, so a mobile moving in a
circle around the base station would have approximately constant received power. This circular contour of constant power yields a hexagonal cell shape for the system, since a hexagon is the closest shape to a
circle that can cover a given area with multiple nonoverlapping cells.
Cellular telephone systems are now evolving to smaller cells with base stations close to street level or
inside buildings transmitting at much lower power. These smaller cells are called microcells or picocells,
depending on their size. This evolution is driven by two factors: the need for higher capacity in areas with
high user density and the reduced size and cost of base station electronics. A cell of any size can support
roughly the same number of users if the system is scaled accordingly. Thus, for a given coverage area a
system with many microcells has a higher number of users per unit area than a system with just a few
macrocells. Small cells also have better propagation conditions since the lower base stations have reduced
shadowing and multipath. In addition, less power is required at the mobile terminals in microcellular
systems, since the terminals are closer to the base stations. However, the evolution to smaller cells has
complicated network design. Mobiles traverse a small cell more quickly than a large cell, and therefore
handoffs must be processed more quickly. In addition, location management becomes more complicated,
since there are more cells within a given city where a mobile may be located. It is also harder to develop
general propagation models for small cells, since signal propagation in these cells is highly dependent on
base station placement and the geometry of the surrounding reflectors. In particular, a hexagonal cell
shape is not a good approximation to signal propagation in microcells. Microcellular systems are often
designed using square or triangular cell shapes, but these shapes have a large margin of error in their
approximation to microcell signal propagation [7].
All base stations in a given geographical area are connected via a high-speed communications link
to a mobile telephone switching office (MTSO), as shown in Figure 1.5. The MTSO acts as a central
controller for the network, allocating channels within each cell, coordinating handoffs between cells when
a mobile traverses a cell boundary, and routing calls to and from mobile users. The MTSO can route
voice calls through the public switched telephone network (PSTN) or provide Internet access for data
exchange. A new user located in a given cell requests a channel by sending a call request to the cell’s
base station over a separate control channel. The request is relayed to the MTSO, which accepts the call
request if a channel is available in that cell. If no channels are available then the call request is rejected.
A call handoff is initiated when the base station or the mobile in a given cell detects that the received
signal power for that call is approaching a given minimum threshold. In this case the base station informs
the MTSO that the mobile requires a handoff, and the MTSO then queries surrounding base stations to
determine if one of these stations can detect that mobile’s signal. If so then the MTSO coordinates a
handoff between the original base station and the new base station. If no channels are available in the
cell with the new base station then the handoff fails and the call is terminated. False handoffs may also
be initiated if a mobile is in a deep fade, causing its received signal power to drop below the minimum
threshold even though it may be nowhere near a cell boundary.
The first generation of cellular systems were analog and the second generation moved from analog to
digital technology. Digital technology has many advantages over analog. The components are cheaper,
faster, smaller, and require less power. Voice quality is improved due to error correction coding. Digital
systems also have higher capacity than analog systems since they are not limited to frequency division
for multiple access, and they can take advantage of advanced compression techniques and voice activity
factors. In addition, encryption techniques can be used to secure digital signals against eavesdropping.
Third generation cellular systems enhanced the digital voice capabilities of the second generation with
digital data, including short messaging, email, Internet access, and imaging capabilities (camera phones).
There is still widespread coverage of first generation cellular systems throughout the US, and some rural
areas only have analog cellular. However, due to their lower cost and higher efficiency, service providers
have used aggressive pricing tactics to encourage user migration from analog to digital systems. Digital
systems do not always work as well as the old analog ones. Users can experience poor voice quality,
frequent call dropping, short battery life, and spotty coverage in certain areas. System performance
will certainly improve as the technology and networks mature. Indeed, in some areas cellular phones
provide almost the same quality as wireline service, and a segment of the US population has replaced
their wireline telephone service inside the home with cellular service. This process has been accelerated
by cellular service plans with free long distance throughout the US.
Spectral sharing in digital cellular can be done using frequency-division, time-division, code-division
(spread spectrum), or hybrid combinations of these techniques (see Chapter 14). In time-division the
signal occupies the entire frequency band, and is divided into time slots ti which are reused in distant
cells [8]. Time division is depicted by Figure 1.4 if the fis are replaced by tis. Time-division is more
difficult to implement than frequency-division since the users must be time-synchronized. However, it is
easier to accommodate multiple data rates with time-division since multiple timeslots can be assigned
to a given user. Spectral sharing can also be done using code division, which is commonly implemented
using either direct-sequence or frequency-hopping spread spectrum [9]. In direct-sequence each user
modulates its data sequence by a different pseudorandom chip sequence which is much faster than the
data sequence. In the frequency domain, the narrowband data signal is convolved with the wideband chip
signal, resulting in a signal with a much wider bandwidth than the original data signal - hence the name
spread spectrum. In frequency hopping the carrier frequency used to modulate the narrowband data
signal is varied by a pseudorandom chip sequence which may be faster or slower than the data sequence.
Since the carrier frequency is hopped over a large signal bandwidth, frequency-hopping also spreads the
data signal to a much wider bandwidth. Typically spread spectrum signals are superimposed onto each
other within the same signal bandwidth. A spread spectrum receiver can separate each of the distinct
signals by separately decoding each spreading sequence. However, since the codes are semi-orthogonal,
the users within a cell interfere with each other (intracell interference), and codes that are reused in other
cells also cause interference (intercell interference). Both the intracell and intercell interference power is
reduced by the spreading gain of the code. Moreover, interference in spread spectrum systems can be
further reduced through multiuser detection and interference cancellation.
In the U.S. the standards activities surrounding the second generation of digital cellular systems
provoked a raging debate on multiple access for these systems, resulting in several incompatible standards
[10, 11, 12]. In particular, there are two standards in the 900 MHz (cellular) frequency band: IS-54, which
uses a combination of TDMA and FDMA, and IS-95, which uses semi-orthogonal CDMA [13, 14]. The
spectrum for digital cellular in the 2 GHz (PCS) frequency band was auctioned off, so service providers
could use an existing standard or develop proprietary systems for their purchased spectrum. The end
result has been three different digital cellular standards for this frequency band: IS-136 (which is basically the same as IS-54 at a higher frequency), IS-95, and the European digital cellular standard GSM, which
uses a combination of TDMA and slow frequency-hopping. The digital cellular standard in Japan is
similar to IS-54 and IS-136 but in a different frequency band, and the GSM system in Europe is at a
different frequency than the GSM systems in the U.S. This proliferation of incompatible standards in
the U.S. and abroad makes it impossible to roam between systems nationwide or globally without using
multiple phones (and phone numbers).
All of the second generation digital cellular standards have been enhanced to support high rate
packet data services [15]. GSM systems provide data rates of up to 100 Kbps by aggregating all timeslots
together for a single user. This enhancement was called GPRS. A more fundamental enhancement, called
Enhanced Data Services for GSM Evolution (EDGE), further increases data rates using a high-level
modulation format combined with FEC coding. This modulation is more sensitive to fading effects, and
EDGE uses adaptive modulation and coding to mitigate this problem. Specifically, EDGE defines six
different modulation and coding combinations, each optimized to a different value of received SNR. The
received SNR is measured at the receiver and fed back to the transmitter, and the best modulation and
coding combination for this SNR value is used. The IS-54 and IS-136 systems currently provide data rates
of 40-60 Kbps by aggregating time slots and using high-level modulation. This new TDMA standard is
referred to as IS-136HS (high-speed). Many of these time-division systems are moving toward GSM, and
their corresponding enhancements to support high speed data. The IS-95 systems support higher data
using a time-division technique called high data rate (HDR)[16].
The third generation of cellular phones is based on a wideband CDMA standard developed within
the auspices of the International Telecommunications Union (ITU) [15]. The standard, initially called
International Mobile Telecommunications 2000 (IMT-2000), provides different data rates depending on
mobility and location, from 384 Kbps for pedestrian use to 144 Kbps for vehicular use to 2 Mbps for
indoor office use. The 3G standard is incompatible with 2G systems, so service providers must invest in
a new infrastructure before they can provide 3G service. The first 3G systems were deployed in Japan,
where they have experienced limited success with a somewhat slower growth than expected. One reason
that 3G services came out first in Japan is the process of 3G spectrum allocation, which in Japan was
awarded without much up-front cost. The 3G spectrum in both Europe and the U.S. is allocated based
on auctioning, thereby requiring a huge initial investment for any company wishing to provide 3G service.
European companies collectively paid over 100 billion dollars in their 3G spectrum auctions. There has
been much controversy over the 3G auction process in Europe, with companies charging that the nature
of the auctions caused enormous overbidding and that it will be very difficult if not impossible to reap a
profit on this spectrum. A few of the companies have already decided to write off their investment in 3G
spectrum and not pursue system buildout. In fact 3G systems have not yet come online in Europe, and
it appears that data enhancements to 2G systems may suffice to satisfy user demands. However, the 2G
spectrum in Europe is severely overcrowded, so users will either eventually migrate to 3G or regulations
will change so that 3G bandwidth can be used for 2G services (which is not currently allowed in Europe).
3G development in the U.S. has lagged far behind that of Europe. The available 3G spectrum in the U.S.
in only about half that available in Europe. Due to wrangling about which parts of the spectrum will be
used, the spectral auctions have been delayed. However, the U.S. does allow the 1G and 2G spectrum
to be used for 3G, and this flexibility may allow a more gradual rollout and investment than the more
restrictive 3G requirements in Europe. It appears that delaying 3G in the U.S. will allow U.S. service
providers to learn from the mistakes and successes in Europe and Japan.
Efficient cellular system designs are interference-limited, i.e. the interference dominates the noise
floor since otherwise more users could be added to the system. As a result, any technique to reduce
interference in cellular systems leads directly to an increase in system capacity and performance. Some methods for interference reduction in use today or proposed for future systems include cell sectorization
[6], directional and smart antennas [19], multiuser detection [20], and dynamic channel and resource
allocation [21, 22].