Bi-directional channel analysis

Details Bi-directional channel analysis Bi-directional channel analysis

1999-12-13

2002-02-05

Patel, Ajit (Department: 2661)

Multiplex communications

Communication techniques for information carried in plural...

Combining or distributing information via time channels

C370S514000, C375S365000

Reexamination Certificate

active

06345057

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to methods for signal detection in general and to methods for signal detection using known preambles in particular.
BACKGROUND OF THE INVENTION
Methods for detecting a signal and evaluating a channel using a known preamble, are known in the art. Such methods are used in timed framed communication methods such as TDMA communication standard, CDMA communication standard, and the like.
Reference is made to
FIG. 1
which is a schematic illustration of a timed framed TDMA sequence, referenced
1
, known in the art. Frame sequence
1
includes a plurality of slots
2
,
4
,
6
,
8
and
10
. Each of these slots includes bits of data, lasting a predetermined period of time.
In TDMA, several users may use a predetermined communication channel, each at predetermined intervals in time. In the present example, slot
2
is assigned to a first user, slot
4
is assigned to a second user, and so on.
According to common TDMA standards, such as IS-136, IS-54, RCR-27, the preamble (i.e. the first few bits of information) of each slot represents a known synchronization sequence, also known as a sync-word. Each of slots
2
,
4
,
6
,
8
and
10
has a sync-word
12
,
14
,
16
,
18
and
20
, respectively.
Sync-words are used to determine what the channel looks like with regard to several aspects such as amplitude, phase, timing, frequency offset, reflections which produce echoes, represented by channel taps, distortions, interference and the like.
Multiple channel taps cause inter-symbol interference. For example, the value of a sample si sampled at the output of the multi-path channel, is given by
s
i
=
∑
n
=
-
L
1
L
2
⁢
h
n
⁢
a
i
-
n
wherein the a
i
are the transmitted samples (−∞<i<∞); and
h
n
are the gains of the channel taps (−L
1
≦n≦L
2
). L
1
and L
2
define the length of channel memory which, in turn, defines the number of neighboring symbols which affect each sample.
Conventional methods use the sync-word of each slot to determine the appropriate synchronization to this slot. Thus, the receiver of the first user, receiving slot
2
, will analyze sync-word
12
(S
1
), determine the channel characteristics and proceed analyzing the rest of slot
2
, accordingly.
It will be appreciated that a communication channel may develop in time, which affects the form of a received slot. Thus, an analysis, based on a sync-word at the beginning of a slot, might not be accurate for the last part of the slot.
Accordingly, conventional receivers execute dynamic tracking procedures, which track the channel development in time, while decoding the data contained therein. Temporary fading of a channel may cause considerable degradation in such channel tracking procedures.
Another method known in the art analyzes the channel of a received slot according to the sync-word of the slot and the sync-word of the next adjacent slot. For example, the receiver of the first user, receives slot
2
as well as the sync-word
14
of slot
4
and analyzes the channel from sync-word
12
(S
1
) and
14
(S
2
).
Analyzing the channel for slot
2
using sync-word
12
(S
1
), combined with forward in time channel tracking, is also called forward analysis. Analyzing the channel for slot
2
using sync-word
14
(S
2
), combined with backward in time channel tracking, is also called backwards analysis. According to prior art methods, this can be performed only when S
1
and S
2
are known, fixed sequences.
It will be appreciated that in some communication standards, such as TDMA IS-54, IS-136, RCR-27 and the like, the sync-word of the next slot is not a fixed predetermined sequence, which make the above prior art methods inefficient.
These TDMA standards define a frame which includes six slots. A TDMA standard also defines two modes of channel usage. A first mode is called half-rate, in which a user is assigned one slot (i.e. every sixth frame is reserved for his use). A second mode is called full-rate, in which a user is assigned two slots in a frame (i.e., every third slot).
Reference is now made to
FIG. 2
which is a schematic illustration of a combined full-rate half-rate frame TDMA sequence, generally referenced
48
. Frame
48
includes a plurality of data slots,
50
,
52
,
54
,
56
,
58
,
60
and
62
, each having a preamble
70
,
72
,
74
,
76
,
78
,
80
and
82
, respectively.
The first six slots define a frame, operative for a number of users, which will be repeated further sync-words. This frame is assigned to two half-rate users and two full-rate users.
Slots
54
and
60
are assigned to the first and second half-rate users, respectively. Slots
50
and
56
are assigned to the first full-rate user and slots
52
and
58
are assigned to the second full-rate user.
In the present example, preambles
70
and
76
of slots
50
and
56
which are both assigned to the first full-rate user, have the same sync-word S
1
. Preambles
72
and
78
of slots
52
and
58
which are both assigned to the second full-rate user, have the same sync-word S
2
. Preambles
74
and
80
of slots
54
and
60
, each assigned to a different half-rate user, have different sync-words S
3
and S
6
, respectively.
Accordingly, S
1
is followed by S
2
, while S
2
is followed by either S
3
or S
6
.
Yow-Jong Liu, “Bi-Directional Equalization Technique for TDMA Communication systems over Land Mobile Radio Channels”,
GLOBECOM
1991 IEEE, p 1458-1462, describes a method for using the sync-word of the next slot which decodes the data of a slot according to its sync-word (forward analysis) and according to the sync-word of the next slot (backward analysis). When the sync-word of the next slot may have more than one option, then the data of the current slot is backward analyzed according to each of these options, thereby providing a plurality of data hypotheses. Finally, the most likely hypothesis is selected.
It will be appreciated that an analysis according to this method requires a considerable amount of computing resources and power, which can be critical for mobile communication devices having limited power resources.


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