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Since WiMAX* is slated to operate in the 2.5, 3.5, or
5.8 GHz bands, it may require more cells than 3G
(which typically has frequencies < 2 GHz) due to the
higher frequencies. The main impact will be to
operators planning to deploy in the unlicensed 5.8 GHz
spectrum. However, the costs associated with the
licensed spectrum for 3G and 2.5/3.5 GHz spectrum
may offset the cost for additional cell sites, so this cost
impact must be considered as well.
OFDM/OFDMA performs much better than CDMA* in a
multipath environment since it is better at overcoming
Inter Symbol Interference (ISI), which happens when
reflected signals overlap with the transmitted signal.
Frequency Selective Fading
OFDMA is more resistant to frequency selective fading
since its parallel nature allows errors in sub-carriers to
Frequency Offset and Phase Noise
OFDMA is more sensitive to frequency offset and phase
noise which results in Inter Carrier Interference (ICI),
although this is somewhat mitigated by the use of guard
Impulse Noise Rejection
Since OFDMA symbols are longer in duration than
CDMA symbols, an impulse noise may not cause an
increase in the error rate. For CDMA, a few CDMA
symbols may be lost, and that could lead to an increase
in the coded Bit Error Rate (BER).
Adaptive Modulation and Coding (AMC)
OFDMA better utilizes AMC, so it achieves higher
throughput (9.6 Mbps) when compared to WCDMA (3
Mbps). This test was performed using textbook OFDM
and only up to 16 QAM (IEEE 802.16e supports 64
QAM). In addition, OFDMA may be able to utilize higher
order modulation (higher data rates) at greater ranges.
AMC and OFDMA
OFDMA may be able to further improve its advantage
over CDMA by applying AMC at the sub-channel level.
This is known as Space Division Multiple Access
(SDMA) and could allow the optimization of sub-
channel selection based on geographical location.
CDMA employs interference averaging, which allows it
to maintain a frequency re-use of 1. OFDMA typically
needs a frequency reuse of 1 to 3, which means the
achievable throughput per cell for a specific bandwidth
must be divided by 3. Advanced Antenna Systems
(AAS) may allow OFDMA to overcome this limitation,
although AAS may be expensive.
Due to the limitations of code availability and client
complexity, most HSDPA clients will be limited to 5 of
the maximum 15 codes. Furthermore, since each user
will need at least one code for voice or data, this could
have a significant impact on the number of users
supported by each system, especially when compared
to the high number of sub-carriers employable by
Quality of Service
WiMAX has a data oriented MAC compared to the
essentially circuit-switched MACs of HSDPA and
WCDMA. WiMAX can also take advantage of multiple
duplexing modes, including TDD dynamic asymmetry;
this allows the uplink/downlink bandwidth to be
allocated according to current traffic conditions.
CDMA systems are much better suited to handle mobile
voice calls because they support multiple voice coding
schemes, seamless handoffs and roaming.
OFDMA-based systems may be easier to implement
since they don’t require the higher complexity of a
RAKE receiver needed in CDMA. It may also be simpler
to implement equalization, interference cancellation and
adaptive antenna array algorithms with OFDMA, where
the algorithms are done in the frequency domain.
WiMAX (OFDMA) is based on IEEE 802.16e, an
upcoming industry standard, allowing it to avoid the
costly proprietary interfaces found in 3G networks. This
also allows it to take advantage of other standardized
technologies, including work being done in the
proposed 802.21 IEEE Handoff Group.
Advanced Radio Techniques
WiMAX (OFDMA) may be able to better take advantage
of diversity techniques (Space Time Coding, Maximum
Ratio Combining), MIMO and smart antenna