Digital Signal Processing Reference
In-Depth Information
Table 4.8
OFDMA parameters
Parameter
Value
Channel bandwidth [MHz]
8.75
Sampling factor
8/7
Sampling frequency (
F
s
) [MHz]
10
Sample time (1/
F
s
) [ns]
100
FFT size (
N
FFT
)
1024
Subcarrier spacing (
f
) [kHz]
9.76
Useful symbol time (
T
B
=
1
/
f
)[
μ
s
]
102.4
Guard time or cyclic prefix (
T
G
=
T
B
/
8) [
μ
s
]
12.8
OFDMA symbol time (
T
S
=
T
B
+
T
G
)[
μ
s
]
115.2
At 8.75 MHz channel bandwidth, the OFDMA symbol time is equal to
115.2 s; therefore, there are 43.4 symbols in the 5 ms TDD frame. Of these 43.4
symbols, 1.4 symbols are used for the TTG and RTG, leaving 42 symbols. If
n
of these 42 symbols are used for the DL subframe, then (42
n
) symbols
are available for the UL subframe. Since PUSC is used, one DL slot is one
subchannel by two OFDMA symbols and one UL slot is one subchannel by
three OFDMA symbols.
Therefore, the available 42 symbols can be divided such that (42
−
−
n
)is
a multiple of 3 and n is of the form (2
k
1). For a DL:UL ratio of 2:1,
these considerations would result in a DL subframe of 27 symbols and an
UL subframe of 15 symbols. In this case, the DL subframe will consist of
a total of 13
+
30 or 390 slots without preamble (one symbol column for
preamble). The UL subframe will consist of 4
×
35 or 140 slots excluding
one slot column for ranging, ACK/NACK, and CQICH. Table 4.9 lists the
OFDMA slot capacity (spectral efficiency) and the theoretical peak data rates
in the downlink direction for various MCSs, or in other words, for different
numbers of data bits per slot.
Dynamic radio resource allocation
or packet scheduling is an algorithm or a
set of algorithms that are implemented and executed at the MAC layer of BSs
within a mobile broadband network. Such algorithms have the main task
of allocating and de-allocating shared radio resources among users to data-
and control-plane packets at each time instant in the uplink and downlink
directions. The allocated and de-allocated resources can also include buffer
and processing resources [7]. The aim of packet scheduling algorithms is
to achieve as efficient a resource utilization as possible, while taking into
account the QoS requirements of the scheduled flows. Packet scheduling
algorithms perform user selection, radio resource allocation, and selection
of radio bearers whose packets are to be scheduled. A packet scheduling
decision typically takes into account the QoS requirements that are associated
with the radio bearers, the channel quality information fed back by the users,
the queuing delay of the packets, fairness indicators, buffer status, and
the interference situation, etc. In addition, it may also take into account
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