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4G
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4G Wireless
Systems in Virtex-II
Designers
of the 4G wireless systems infrastructure are confronted with
challenging product development issues, including the uncertainty
about fundamental system architectural standards such as the air
interface, encryption protocols, planetary interoperability, and so
on. Because there are unresolved uncertainties, you must pay close
attention to risk management — making sure your designs can evolve
with the changing standards.
Virtex®-II FPGAs are an ideal platform for designing with ambiguous
or evolving standards. Because of the inherent flexibility,
reprogrammability, and extremely high performance (approximately 0.6
TeraMACs) of Virtex-II devices, you can easily test different
air-interface schemes and variants in-system, and you can quickly
assess the system performance. In particular, Virtex-II FPGAs make
it easy to develop hybrid systems such as multi-carrier CDMA or QAM-modulated
OFDM.
Orthogonal
Frequency Division Multiplexing (OFDM)
Currently, there are two principal
4G development technologies contending for attention: CDMA and OFDM.
Code Division Multiple Access is a well-known standard and has been
used for several years. However, OFDM is relatively new.
OFDM, with many technical variants, is endorsed by Nokia, Cisco,
Lucent, and Philips Semiconductor, and is represented as the
successor to frequency hopping and direct sequence CDMA. It is also
positioned as the technique of choice for next generation wireless
LANs and metropolitan networks. The capability of OFDM to cancel
multipath distortion in a spectrally efficient manner without
requiring multiple local oscillators has won adherents in the IEEE
802.11a and 802.16 working groups. However, despite the support of
many key industry players, OFDM is not actually deployed in
mainstream wireless systems.
Todd Carothers, vice president of marketing for Adaptive Broadband,
recently stated "We've developed a commercial OFDM system for
one application, and we think OFDM has real advantages in the mobile
arena, but we don't see it for fixed point. We think that adaptive
time division multiple access is still the best solution for fixed
point-to-multipoint, and I'll state that we still have the fastest
system out there and the most extensively deployed."
Philip Gee of WiLAN said recently, "There is no question that
OFDM and CDMA are in contention for some of the same wireless
markets. We believe that OFDM enjoys a number of significant
advantages, however."
How it Works
OFDM is fundamentally different from other modulation schemes. In
fact, it should probably not be considered a modulation scheme at
all, because it may be transmitted via AM, FM, QAM (Quadrature
Amplitude Modulation), and so on. OFDM is properly defined as a
mathematically elegant technique for the generation and demodulation
of radio waves. Although its origins date back to the second World
War, its application to wireless communications is new.
In OFDM the subcarrier pulse shape is a square wave. The task of
pulse forming and modulation can be performed by a simple Inverse
Discrete Fourier Transform (IDFT) which can be implemented very
efficiently in Virtex-II FPGAs as an Inverse Fast Fourier Transform
(IFFT). To decode the transmission, a receiver need only implement
an FFT.
As you can see in Figure 1, the spectra of the subcarriers overlap.
By using an IFFT, the spacing of the subcarriers is varied in such a
way that, at the target frequency of the received signal (indicated
as arrows), all other signals are zero. This is known as
"frequency orthogonality." This contrasts with Direct
Sequence CDMA, which uses a Walsh code to achieve code orthogonality.
Figure 1: OFDM allows for greater special efficieny
OFDM
and the Virtex-II Architecture Virtex-II FPGAs offer several
architectural advances
that allow you to create extremely efficient implementations
of OFDM systems.
Multipliers
Virtex-II FPGAs contain a number of 18x18 2's complement signed
multipliers associated with the block SelectRAM™ memory. This
association allows high-speed access to complex multiplicand
coefficients, thus supporting extremely high-performance arithmetic.
To see why the multipliers are so valuable, consider the nature of
the FFT algorithm itself. It essentially decomposes into a series of
multiply-accumulate functions.
Digital Clock Management
To successfully implement OFDM, the receiver and the transmitter
must be in perfect synchronization. Synchronizing to the
transmitter's data clock is always necessary, whereas, carrier
recovery is only necessary in coherent detection receivers. The data
clock must be recovered so that the receiver will sample the
transmitted data symbols at the appropriate time.
An algorithmic approach such as times-two, early-late, or
zero-crossing clock recovery can be implemented in a Virtex-II
device; all of these functions are performed in the digital domain.
These algorithmic approaches are perfect applications for the Virtex-II
Digital Clock Manager (DCM). For example, the DCMs in the Virtex-II
devices, along with a DDS (Direct Digital Synthesis) core, can
provide the complex sinusoids necessary for demodulating the
incoming data. The timing/phase of these complex sinusoids is
directed by the data recovery clock and easily adjusted by the DCM's
timing controls. The DCM can also perform other functions vital to
synchrony of the transmitter and receiver including clock deskew and
frequency synthesis.
The DCM can also de-skew the received signal relative to the local
receiver frequency by adding digital delay. This results in a signal
that is delayed but has perfect phase alignment to the local
receiver frequency. Virtex-II DCMs can drive global clock resources,
general logic interconnect, and I/O pads simultaneously. This
provides maximal flexibility when placing logic.
High Performance
The most valuable feature of the Virtex-II family, for
implementation of advanced wireless systems, is the extremely
high-performance. This gives you a great degree of freedom that is
not available with alternative implementations, such as ASICs. To
understand the value of this advantage, consider the following
scenario.
OFDM
Field Deployment Example
In this example, an OFDM system is deployed on an
experimental basis by a wireless service provider. It is located in
an urban market, but there are many business factors that must be
addressed if the venture is to succeed, including:
- Which of the emerging broadband
wireless services will generate high demand?
- What is the peak bandwidth per
subscriber?
- What is the average bandwidth
per subscriber per service?
- What Quality of Service factors
can be used to differentiate the new service?
There is no substitute for field trials to answer these
questions, and there is no better platform for field trials than
Virtex-II FPGAs.
Consider a case where a standards body adopts a new variant of OFDM,
a very likely scenario. With conventional ASICs, the upgrade path is
painful. Utility workers must climb telephone poles, climb to the
tops of buildings, and so on, and manually upgrade circuit boards in
the base stations. Keep in mind that many of these base stations are
deployed in regions of the world that suffer climactic extremes.
This expensive and potentially dangerous upgrade path is part of the
cost of ownership of a base station constructed around conventional
ASICs.
Now consider a base station constructed with a Virtex-II Platform
FPGA; the configuration of the cellular base station may be
completely altered simply by transmitting a new Virtex-II
configuration file from the comfort of a central office. This
technique is extensible, so that an entire network can be
reconfigured, automatically, without replacing any hardware. This
capability allows you to more rapidly introduce the product to the
market and helps to protect the base station architecture against
obsolescence.
There is another degree of flexibility which the Virtex-II-based
OFDM base station provides, the ability to trade off silicon area
for performance. Consider the market success factors described
above. If it became evident through field trials and customer
testing that the average customer was not a heavy consumer of
bandwidth, the OFDM algorithm could be re-targeted to use more
general purpose logic. Properly done, this would result in the
ability to support more channels in the same device. Essentially,
the Virtex-II Platform FPGA allows you to dynamically trade off
silicon area for performance.
Conclusion
The Virtex-II product family is uniquely suited to the
demanding digital signal processing that will be required to roll
out next-generation broadband wireless services. Its powerful suite
of dedicated high-performance logic functions such as the high-speed
multipliers and DCM, along with extremely versatile high-performance
general logic, define an optimal solution for wireless designs.
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