Pulse or digital communications – Equalizers
Reexamination Certificate
2000-03-08
2004-04-06
Chin, Stephen (Department: 2634)
Pulse or digital communications
Equalizers
Reexamination Certificate
active
06717985
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to transmission channel equalization and, more particularly, to a technique for efficiently equalizing a transmission channel in a data transmission system.
BACKGROUND OF THE INVENTION
As is well known in the telecommunication arts, data transmission in accordance with the Global System for Mobile Communications (GSM) standard is accomplished using a combined Time Division Multiple Access (TDMA) and Frequency Division Multiple Access (FDMA) scheme. The TDMA component of the combined TDMA and FDMA scheme is based upon a TDMA frame structure, as shown in FIG.
1
. Each TDMA frame
10
has a duration of 4.615 ms and is divided into eight time slots (TS)
12
. Each time slot
12
has a duration of 0.577 ms and comprises a first 3-bit tail bits (TB) section
14
, a first 57-bit coded data section
16
, a first 1-bit control bit (C) section
18
, a 26-bit training sequence code (TSC) section
20
, a second 1-bit control bit (C) section
22
, a second 57-bit coded data section
24
, a second 3-bit tail bits (TB) section
26
, and an 8.25-bit long guard period (GP) section
28
. Thus, each time slot
12
comprises 156.25 total bits, including the 8.25-bit long guard period (GP) section
28
.
As is also well known in the telecommunication arts, data reception in accordance with the GSM standard is accomplished, in part, using a channel equalizer for creating a model of a transmission channel over which a message, or burst, has been transmitted. This transmission channel model is necessary to compensate for any changes which may occur in a message, or burst, as a result of having been transmitted over the transmission channel. That is, referring to
FIG. 2
, a transmission channel may be represented as a filter (H)
30
encompassing all of the characteristics of the transmission channel. More particularly, the filter
30
encompasses all of the characteristics of the transmission channel that affect a message, or burst, that is transmitted over the transmission channel, such as, for example, fading, time dispersion, multipath distribution, and inter-symbol interference (ISI). As described in more detail below, a channel equalizer in a GSM receiver creates a model (H
m
) of the transmission channel. The channel equalizer then creates an inverse of the transmission channel model, which is represented as inverse filter (H
m
−1
)
32
in FIG.
2
. Thus, an original message, or burst, represented by I in
FIG. 2
, that is passed through filter
30
and inverse filter
32
, represents an original message, or burst, that is transmitted over a transmission channel and through an inverse model of the transmission channel created by a channel equalizer in a GSM receiver. A resulting message, or burst, represented by I
m
in
FIG. 2
, emerges from inverse filter
32
. If the model, and hence the inverse model, of the transmission channel created by the channel equalizer in the GSM receiver is accurate, then I and I
m
should be identical.
As is further well known in the telecommunication arts, a channel equalizer in a GSM receiver creates a model, as well as an inverse model, of a transmission channel using the 26-bit training sequence code (TSC) described above, which is embedded in a message, or burst, that has been transmitted over the transmission channel. The 26-bit training sequence code (TSC) contains a known bit pattern, which the channel equalizer in the GSM receiver uses to predict the transmission channel over which a message, or burst, containing the 26-bit training sequence code (TSC) has been transmitted. More particularly, and with reference to
FIG. 3
, the channel equalizer in the GSM receiver “equalizes” the transmission channel over which the message, or burst, containing the 26-bit training sequence code (TSC) has been transmitted by first receiving the message, or burst, containing the 26-bit training sequence code (TSC), as shown in step
40
. In step
42
, the channel equalizer correlates the received 26-bit training sequence code (TSC) with the known bit pattern and, based upon this correlation, predicts the transmission channel over which the message, or burst, containing the 26-bit training sequence code (TSC) has been transmitted. In step
44
, the channel equalizer creates a filter model of the transmission channel based upon the prediction. In step
46
, the channel equalizer applies the received message, or burst, to an inverse of the filter model so as to obtain an estimation of the actual transmitted message, or burst. Finally, in step
48
, the estimated message, or burst, is output for further processing within the GSM receiver.
At this point it should be noted that the quality of the filter model can be measured by applying the received 26-bit training sequence code (TSC) to the inverse of the filter model, and then comparing the result with the known bit pattern.
The above-described transmission channel equalization procedure is carried out regardless of when in time messages, or bursts, are received, due to the fact that each message, or burst, is considered to be not correlated to any other message, or burst. This is the case even with some of the new GSM phase 2+ services that are being defined such as, for example, high speed circuit switched data (HSCSD) and general packet radio service (GPRS), which operate using more than one consecutive time slot for a single connection, i.e. multiple slot operational modes. That is, channel equalization is still performed on each received message, or burst, in each corresponding time slot despite the fact that some connections utilize more than one consecutive time slot. This is due to the fact that all messages, or bursts, are still considered to be not correlated to each other despite the fact that they might be consecutively received, and thereby might have been transmitted over a common or similar transmission channel. Thus, none of the transmission channel information from a previous transmission channel equalization procedure is passed on to a subsequent transmission channel equalization procedure despite the fact that the messages, or bursts, upon which the transmission channel equalization procedures are based are consecutively received. Given these circumstances, the transmission channel equalization procedures for two consecutively received messages, or bursts, during a multiple slot mode operation cannot be performed any faster than the sum of the times required to perform each individual transmission channel equalization procedure.
The above-described multiple slot operational mode is illustrated in
FIG. 4
, for example, wherein four messages, or bursts, are consecutively received in each TDMA frame
10
during multiple slot mode operation, and wherein C represents the correlation/prediction and channel model building steps (i.e., steps
42
and
44
in
FIG. 3
above) and E represents the message estimation steps (i.e., steps
46
and
48
in
FIG. 3
above). As can be seen in
FIG. 4
, the correlation/prediction and channel model building steps and the message estimation steps are performed for each received message, or burst, despite the fact that the messages, or bursts, upon which the transmission channel equalization procedures are based are consecutively received. Thus, none of the transmission channel information from a previous transmission channel equalization procedure is passed on to a subsequent transmission channel equalization procedure. This is very inefficient, particularly due to the fact that the correlation/prediction and channel model building steps take up a substantial part of the overall transmission channel equalization procedure time.
In view of the foregoing, it would be desirable to provide a technique for equalizing a transmission channel in a data transmission system in a more efficient and cost effective manner.
OBJECTS OF THE INVENTION
The primary object of the present invention is to provide a technique for efficiently equalizing a transmission channel in a data transmission system.
The above-stated p
Chin Stephen
Jenkens & Gilchrist P.C.
Kim Kevin
Telefonaktiebolaget LM Ericsson (publ)
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