B. P. Lathi, Zhi Ding - Modern Digital and Analog Communication Systems-Oxford University Press (2009)

708 DIGITAL COMMUNICATIONS UNDER LINEARLY DISTORTIVE CHANNELS
Figure 12. 17
Frequency and
subcarrier
allocation in
ADSL services.
Channel
gain dB
-10
Upstream
subcarriers
Downstream
I
Subchannel
bandwidth
4.3175 kHz
0 4 25.875 138 1104 Frequency, kHz
J.-------- 255 subc arriers -------
TABLE 12.2
Basic ADSL Upstream and Downstream Subcarrier Allocations and Data Rates
Upstream
Downstream
Modulation (bit loading)
DMT frame transmission rate
Pilot subcarrier
Typical subcarriers
Typical bits per frame
Maximum possible subcarriers
Maximum bits per frame
Maximum data rate
QPSK to 64-QAM (2-6 bits per symbol)
4kHz
No. 64 No. 96
6 to 32 33 to 255
Up to 162 bits
1 to 63
Up to 378 bits
4 kHz x378 = 1.512 Mbit/s
Up to 1326 bits
1 to 255 (excluding 64 and 96)
Up to 1518 bits
4 kHz x 1518 bits = 6.072 Mbit/s
nominal band of 25.875 to 1104 kHz (subcarrier 6 to subcarrier 255). These 250 available
subcarriers are divided between downstream data transmission (from DSL server to homes)
and upstream data (from homes to DSL server).
In today's internet applications, most individual consumers have a higher downstream
need than upstream. Unlike business users, these "asymmetric" data service requirements
define the objective of ADSL. Therefore in ADSL, the number of downstream subcarriers is
greater than the number of upstream subcarriers. In ADSL, subcarriers 6 to 32 ( corresponding
to 25.875-138 kHz) are generally allocated for upstream data. Subcarrier 64 and subcarrier
96 are reserved for upstream pilot and downstream pilot, respectively. Excluding the two
pilot subcarriers, subcarriers 33 to 255 (corresponding to 138-1104 kHz) are allocated for
downstream data. The typical carrier allocation and data rates are summarized in Table 12.2.
Notice that this table applies only to the basic DSL recommendations by ITU-T (G.992.1).
Depending on the channel condition, various service providers may choose to increase the
data rate by using higher bandwidth and even more subcarriers above subcarrier 255.
In AD SL, the DMT frame transmission rate is 4 kHz. Upstream DMT utilizes 64-point realvalued
IFFT that is equivalent to 32-point complex IFFT. The upstream cyclic prefix has length
4. On downstream, 512 real-valued IFFT is applied, equivalent to 256-point complex IFFT.
The downstream cyclic prefix has length 32 (equivalent to 16 complex numbers). Because
the channel delay spread is usually larger than the prescribed cyclic prefix, TEQ channel
shortening is commonly applied in ADSL with the help of several thousand training symbols
(e.g., in downstream) to adapt the TEQ parameters.