High-Speed Data

 
The introduction of multimedia services into mobile communications will require mobile transmission
speeds of up to 100 Mbps. Therefore, a wider frequency band than that in 3G will have to be assigned
to B3G mobile communication systems. Generally speaking, mobile communication systems should
be assigned the lowest available frequency band when taking into account path loss in radio channels.
However, it will probably be impossible to assign a lower frequency band than that of 3G to B3G
systems because of the fact that these bands are already regulated and in use. Therefore, techniques
that enable high-speed data transmission within a limited frequency band will become important
in B3G systems. Simply put, techniques for increasing the efficiency of frequency utilization will
play a great role in B3G systems. In addition, it is indispensable to combat severe selective fading
in a mobile communication environment where such a high-speed data will be transmitted. While
some techniques that satisfy the above requirement have been proposed and verified, the spatial
signal processing technique has been recognized as one that can potentially increase the efficiency of
frequency utilization and system capacity. Among the techniques, MIMO systems have attracted signal
processing researchers since MIMO raises the possibility of increasing system capacity in proportion to
the number of antennas installed in a transmitter and receiver, using spatial multiplexing. For instance,
Bell Labs Layered Space Time (BLAST) Code has been experimentally verified to achieve highcapacity
transmission rates in indoor scenarios. On the other hand, Space Division Multiple Access (SDMA), utilizing spatial multiplexing as well as MIMO systems, is also considered a promising
technique for improving system capacity. In these techniques, the orthogonalization of channels plays
an important role in attaining high capacity. In contrast with these, Multiuser Detection (MUD)
separates a user’s signals, which are superposed at the top of a receiver. Therefore, MUD makes
it possible to improve frequency utilization efficiency in cellular mobile communication systems.
However, MUD with multiple antennas is considered to be a type of MIMO system without channel
knowledge at a transmitter. Therefore, MUD also shows promise for improving channel capacity.
While many types of MUDs have been investigated in CDMA, MUD is possible to implement in
other systems, such as single-carrier systems. For instance, a MIMO turbo equalizer has been proposed
that deploys a linear equalizer with iterative decoding (Turbo decoding) in addition to array signal
processing. Besides, a multibeam interference canceler (MIC) has been proposed that deploys both
MLSE (Maximum Likelihood Sequence Estimation) and an array antenna. MIC was shown to achieve
the optimum transmission performance without any assistance from coding. Although MIC achieves
excellent performance even in fading channels, it has a drawback in high hardware complexity, which
grows exponentially as the number of the beams increases [540].
It will be technically challenging to enable high-speed data transfers in B3G mobile networks
precisely because B3G systems will really be a means to integrate a variety of technologies, including
cellular, cordless, WLAN, WMAN, and wired networks, with seamless global access among them.
Planners aspire to achieve higher bit rates, higher spectral efficiency, and lower costs per bit than
in 3G systems – all with lower power usage. Proposed B3G transmission protocols include OFDM,
Wideband Orthogonal Frequency Division Multiplexing (W-OFDM), MC-CDMA, and Large-Area-
Synchronized Code Division Multiple Access (LAS-CDMA). OFDM is good for high-bandwidth
data transmission; it multiplexes thousands of orthogonal waves in one time waveform. W-OFDM
enables data to be encoded on multiple high-speed radio frequencies concurrently. This allows for
greater security, increased amounts of data transmission, and the industry’s most efficient use of
bandwidth. W-OFDM permits the implementation of low power multipoint RF networks that minimize
interference with adjacent networks. This allows independent channels to operate within the same
band, enabling multipoint networks and point-to-point backbone systems to be overlaid in the same
frequency band. MC-CDMA is actually OFDM with a CDMA overlay. Like single-carrier CDMA
systems, the users are multiplexed with orthogonal codes to distinguish users in (multi-carrier) MCCDMA.
However in MC-CDMA, each user can be allocated several codes, where the data is spread
in time or in frequency. LinkAir Communications is the developer of LAS-CDMA, a patented B3G
wireless technology. LAS-CDMA enables high-speed data transmission and increases voice capacity,
using SDRs, and is advertised as the most spectrally efficient, high-capacity duplexing system available
today [539].