U.S. patent application number 13/405377 was filed with the patent office on 2013-06-06 for control channel acquisition.
This patent application is currently assigned to BROADCOM CORPORATION. The applicant listed for this patent is Arkady Molev-Shteiman. Invention is credited to Arkady Molev-Shteiman.
Application Number | 20130142057 13/405377 |
Document ID | / |
Family ID | 48523580 |
Filed Date | 2013-06-06 |
United States Patent
Application |
20130142057 |
Kind Code |
A1 |
Molev-Shteiman; Arkady |
June 6, 2013 |
Control Channel Acquisition
Abstract
Disclosed are various embodiments of control channel acquisition
systems and methods. A baseband processor in communication with the
RF transceiver performs an initial detection of a carrier frequency
based on GMSK symbols in the FCCH. The initial detection is refined
and verified by passing a signal through a mathematical filter and
comparing an energy of the filtered signal to a threshold.
Inventors: |
Molev-Shteiman; Arkady;
(Cliffwood, NJ) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Molev-Shteiman; Arkady |
Cliffwood |
NJ |
US |
|
|
Assignee: |
BROADCOM CORPORATION
Irvine
CA
|
Family ID: |
48523580 |
Appl. No.: |
13/405377 |
Filed: |
February 27, 2012 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61565864 |
Dec 1, 2011 |
|
|
|
61568868 |
Dec 9, 2011 |
|
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Current U.S.
Class: |
370/252 |
Current CPC
Class: |
H03M 13/3723 20130101;
H04W 36/18 20130101; H04W 56/002 20130101; H03M 13/3972 20130101;
H03M 13/3905 20130101; H04L 1/005 20130101 |
Class at
Publication: |
370/252 |
International
Class: |
H04W 24/00 20090101
H04W024/00 |
Claims
1. A mobile device comprising: a radio frequency (RF) transceiver;
a baseband processor in communication with the RF transceiver, the
baseband processor executing a frequency correction channel (FCCH)
acquisition module, the FCCH acquisition module comprising: logic
that performs an initial detection of a carrier frequency based on
a plurality of GMSK symbols in the FCCH; logic that refines the
initial detection of the carrier frequency, wherein the refined
initial detection downsamples the GMSK symbols by at least a factor
of four; and logic that verifies the refined initial detection of
the carrier frequency by passing a signal through a mathematical
filter and comparing an energy of the filtered signal to a
threshold.
2. The mobile device of claim 1, wherein a set of samples is
selected from the GMSK symbols.
3. The mobile device of claim 2, wherein the logic that performs an
initial detection of the FCCH transmission frequency further
comprises: logic that performs a fast Fourier transform on the set
of samples; logic that calculates a total signal energy associated
with a plurality of frequency bins resulting from the fast Fourier
transform, wherein each of the frequency bins is associated with a
frequency; logic that selects a subset of the frequency bins; and
logic that calculates an energy of each of the selected subset of
frequency bins and selects one of the selected subset of frequency
bins associated with an energy value above a threshold as an
initial frequency estimate.
4. The mobile device of claim 3, wherein the logic that selects one
of the frequency bins further comprises logic that determines
whether a pair of consecutive frequency bins is associated with an
energy value above the threshold and designating a frequency range
associated with the pair of consecutive frequency bins.
5. The mobile device of claim 3, wherein the plurality of frequency
bins comprises thirty-two frequency bins, and the logic selects a
subset of the frequency bins further comprises logic that selects a
fourth through the twelfth frequency bins.
6. The mobile device of claim 1, wherein the logic that refines the
initial detection of the carrier frequency further comprises: logic
that derotates the initial detection; and logic that downsamples
the initial detection by summing a plurality of four consecutive
GMSK symbols to produce a set of samples of the GMSK symbols.
7. The mobile device of claim 6, wherein the logic that downsamples
the initial detection downsamples the initial detection by a factor
of four.
8. The mobile device of claim 6, further comprising logic that
performs an autocorrelation of the downsampled initial
detection.
9. The mobile device of claim 8, further comprising logic that
generates a first frequency estimation of the autocorrelated
downsampled initial detection by executing a frequency estimation
function: F EST = F SYM 2 .pi. M arctan [ 1 N n = 0 N - 1 Y ( n ) Y
* ( n + M ) ] ##EQU00008## where F.sub.EST is the first frequency
estimation, F.sub.SYM=270.8333 KHz, Y is a first vector
representing the autocorrected downsampled initial detection, and M
and N are frequency estimation parameters.
10. The mobile device of claim 9, wherein M=4 and N=26.
11. The mobile device of claim 10, further comprising: logic that
performs a derotation of the first frequency estimates; logic
executes the frequency estimation function to generate a second
frequency estimate, wherein M=10 and N=20; and logic that performs
a derotation of the second frequency estimate to generate the
refined initial estimate.
12. The mobile device of claim 11, wherein the mathematical filter
comprises a mathematical filter function: 1 N n = 0 N - 1 X ( n )
exp ( - 2 j .pi. FrEstim n ) 2 1 N n = 0 N - 1 X ( n ) 2 > Thr
##EQU00009## where X(n) is the downsampled initial detection, N is
a number of samples in the downsampled initial detection, FrEstim
is the refined initial estimate, and Thr is the threshold.
13. A method executed in a mobile device for acquiring a carrier
frequency from a frequency correction channel (FCCH), comprising:
performing an initial detection of the carrier frequency based on a
plurality GMSK symbols in the FCCH; refining the initial detection
of the carrier frequency, wherein the refined initial detection
downsamples the GMSK symbols by at least a factor of four; and
verifying the refined initial detection of the carrier frequency by
passing a signal through a mathematical filter and comparing an
energy of the filtered signal to a threshold. Attorney Docket:
50228-7220
14. The method of claim 13, wherein a set of samples is selected
from the GMSK symbols and the step of performing an initial
detection of the carrier frequency further comprises: performing a
fast Fourier transform on the set of samples; calculating a total
signal energy associated with a plurality of frequency bins
resulting from the fast Fourier transform, wherein each of the
frequency bins is associated with a frequency; selecting a subset
of the frequency bins; and calculating an energy of each of the
selected subset of frequency bins and selects one of the selected
subset of frequency bins associated with an energy value above a
threshold as an initial frequency estimate.
15. The method of claim 14, wherein the step of selecting one of
the frequency bins further comprises the step of determining
whether a pair of consecutive frequency bins is associated with an
energy value above the threshold and designating a frequency range
associated with the pair of consecutive frequency bins.
16. The method of claim 14, wherein the plurality of frequency bins
comprises thirty-two frequency bins, and the step of selecting a
subset of the frequency bins further comprises logic that selects a
fourth through the twelfth frequency bins.
17. The method of claim 13, wherein the step of refining the
initial detection of the carrier frequency further comprises:
derotating the initial detection; and downsampling the initial
detection by a factor of four.
18. The method of claim 17, further comprising: performing an
autocorrelation of the downsampled initial detection; generating a
first frequency estimation of the autocorrelated downsampled
initial detection by executing a frequency estimation function;
performing a derotation of the first frequency estimation;
generating a second frequency estimation by executing the frequency
estimation function; and performing a derotation of the second
frequency estimation to generate the refined initial estimate.
19. The method of claim 13, wherein the mathematical filter
comprises a mathematical filter function: 1 N n = 0 N - 1 X ( n )
exp ( - 2 j .pi. FrEstim n ) 2 1 N n = 0 N - 1 X ( n ) 2 > Thr
##EQU00010## where X(n) is the downsampled initial detection, N is
a number of samples in the downsampled initial detection, FrEstim
is the refined initial estimate, and Thr is the threshold.
20. A system, comprising: means performing an initial detection of
the carrier frequency based on a plurality GMSK symbols in a
frequency correction channel (FCCH); means for refining the initial
detection of the carrier frequency, wherein the refined initial
detection downsamples the GMSK symbols by at least a factor of
four; and means for verifying the refined initial detection of the
carrier frequency by passing a signal through a mathematical filter
and comparing an energy of the filtered signal to a threshold.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims priority to co-pending U.S.
provisional application Ser. No. 61/565,864, entitled "Cellular
Baseband Processing," filed Dec. 1, 2011, which is incorporated
herein by reference in its entirety. This application also claims
priority to co-pending U.S. Provisional application Ser. No.
61/568,868, entitled "Cellular Baseband Processing," filed Dec. 9,
2011, which is incorporated herein by reference in its
entirety.
BACKGROUND
[0002] Cellular wireless communication systems support wireless
communication services in many populated areas of the world. While
cellular wireless communication systems were initially constructed
to service voice communications, they are now called upon to
support data communications as well. The demand for data
communication services has exploded with the acceptance and
widespread use of the Internet. While data communications have
historically been serviced via wired connections, cellular wireless
users now demand that their wireless units also support data
communications. Many wireless subscribers now expect to be able to
"surf" the Internet, access their email, and perform other data
communication activities using their cellular phones, wireless
personal data assistants, wirelessly linked notebook computers,
and/or other wireless devices. The demand for wireless
communication system data communications will only increase with
time. Thus, cellular wireless communication systems are currently
being created/modified to service these burgeoning data
communication demands.
[0003] Cellular wireless networks include a "network
infrastructure" that wirelessly communicates with wireless
terminals and/or mobile devices within a respective service
coverage area. The network infrastructure typically includes a
plurality of base stations dispersed throughout the service
coverage area, each of which supports wireless communications
within a respective cell (or set of sectors). The base stations
couple to base station controllers (BSCs), with each BSC serving a
plurality of base stations. Each BSC couples to a mobile switching
center (MSC). Each BSC also typically directly or indirectly
couples to the Internet.
[0004] In operation, each base station communicates with a
plurality of wireless terminals operating in its cell/sectors. A
BSC coupled to the base station routes voice communications between
the MSC and a serving base station. The MSC routes voice
communications to another MSC or to the PSTN. Typically, BSCs route
data communications between a servicing base station and a packet
data network that may include and/or couple to the Internet.
Transmissions from base stations to wireless terminals are referred
to as "forward link" transmissions while transmissions from
wireless terminals to base stations are referred to as "reverse
link" transmissions. The volume of data transmitted on the forward
link typically exceeds the volume of data transmitted on the
reverse link. Such is the case because data users typically issue
commands to request data from data sources, e.g., web servers, and
the web servers provide the data to the wireless terminals.
[0005] Wireless links between base stations and their serviced
wireless terminals typically operate according to one (or more) of
a plurality of operating standards. These operating standards
define the manner in which the wireless link may be allocated,
setup, serviced and torn down. One popular cellular standard is the
Global System for Mobile telecommunications (GSM) standard. The GSM
standard, or simply GSM, is predominant in Europe and is in use
around the globe. While GSM originally serviced only voice
communications, it has been modified to also service data
communications. In GSM, wireless terminals are informed of the need
to service incoming communications via pages from base stations to
the wireless terminals. GSM General Packet Radio Service (GPRS)
operations and the Enhanced Data rates for GSM (or Global)
Evolution (EDGE) operations coexist with GSM by sharing the channel
bandwidth, slot structure, and slot timing of the GSM standard.
GPRS operations and EDGE operations may also serve as migration
paths for other standards as well, e.g., IS-136 and Pacific Digital
Cellular (PDC).
[0006] According to the GSM standard, a BSC transmits various
signaling channels that facilitate communication with a wireless
terminal, or mobile device. For example, Broadcast Channels can
include a Broadcast Control Channel (BCCH), a Frequency Correction
Channel (FCCH), and other channels as defined by the standard.
These various channels facilitate the BSC and a mobile device to
establish communications with one another. For example, the BCCH
allows the BSC to broadcast information about the identity of a
network to which it corresponds, such as a Mobile Network Code
(MNC), Location Area Code (LAC), and other information. The FCCH
includes an FCCH burst, which is an all-zero or all-one sequence
that produces a fixed GMSK tone, which enables a mobile device to
lock its local oscillator to the BSC clock.
BRIEF DESCRIPTION OF THE DRAWINGS
[0007] Many aspects of the invention can be better understood with
reference to the following drawings. The components in the drawings
are not necessarily to scale, emphasis instead being placed upon
clearly illustrating the principles of the present invention.
Moreover, in the drawings, like reference numerals designate
corresponding parts throughout the several views.
[0008] FIG. 1 is a system diagram illustrating a portion of a
wireless communication system that supports mobile devices and/or
wireless terminals operating according to the present
invention.
[0009] FIG. 2 is a block diagram functionally illustrating a mobile
device according to an embodiment of the disclosure.
[0010] FIG. 3 is a drawing illustrating a state machine executed by
the baseband processor to determine a carrier frequency from the
FCCH according to one embodiment of the disclosure.
[0011] FIG. 4 represents a flow of how an initial frequency
estimation is calculated by the baseband processor according to one
embodiment of the disclosure.
[0012] FIG. 5 illustrates a sliding window employed by the baseband
processor to calculate an initial frequency estimation according to
one embodiment of the disclosure.
[0013] FIG. 6 represents a flowchart that illustrates one way in
which the baseband processor can refine the initial estimation
according to one embodiment of the disclosure.
DETAILED DESCRIPTION
[0014] Embodiments of the present disclosure are directed to
algorithms that facilitate acquisition of control channels in the
GSM standard, such as, but not limited to, a Frequency Correction
Channel (FCCH). During wireless data communication, the base
stations transmit data bursts to the mobile devices or wireless
terminals in TDMA frames. Each TDMA frame has eight time slots
corresponding to eight data bursts, equivalently to data bursts for
each multiframe. The data bursts belong to frequency correction
channels FCCH, synchronization channels SCH, broadcast control
channels BCCH, or common control channels CCCH.
[0015] The frequency correction channel FCCH data bursts do not
contain training sequences, and comprises data burst of "zeros" or
"ones" so that the mobile station can correct the local oscillator
frequency error. The synchronization channel SCH is a downlink
channel comprising regular sequences of bits that enables the
mobile stations to synchronize received frame boundaries with the
base stations on registration. The common control channel CCCH
transfers data bursts containing training sequences known to the
mobile stations for timing synchronization, supporting common
procedures to establish a dedicated link between the base station
and the mobile station. In the GSM specification, eight training
sequences for normal bursts are specified, each base station
utilizes a fixed training sequence thereof on all channels. The
training sequence in the CCCH data burst is shorter than that of
the synchronization burst in SCH, thus the timing synchronization
provided by the CCCH data is less accurate than that of the SCH
data. In the GSM systems, the common control channel CCCH includes
RACH (Random Access Channel) for initial access to the GSM network,
PCH (Paging Channel) indicating incoming calls or messages on
waiting for the mobile station, and AGCH (Access Grant Channel)
assigning the GSM network resource to another mobile station
requesting the network access. The broadcast control channel BCCH
is a downlink channel containing specific parameters required by
the mobile station to identify the base station and obtain network
access through the base station.
[0016] Accordingly, embodiments of the disclosure are related to
acquisition algorithms for control channel data that reduce
computational complexity relative to other implementations. To
begin, a general architecture of an example GSM environment is
shown and discussed. FIG. 1 is a system diagram illustrating a
portion of a cellular wireless communication system 100 that
supports wireless terminals operating according to the present
invention. The cellular wireless communication system 100 includes
a Mobile Switching Center (MSC) 101, Serving GPRS Support
Node/Serving EDGE Support Node (SGSN/SESN) 102, base station
controllers (BSCs) 152 and 154, and base stations 103, 104, 105,
and 106. The SGSN/SESN 102 couples to the Internet 114 via a GPRS
Gateway Support Node (GGSN) 112. A conventional voice terminal 121
couples to the PSTN 110. A Voice over Internet Protocol (VoIP)
terminal 123 and a personal computer 125 couple to the Internet
114. The MSC 101 couples to the Public Switched Telephone Network
(PSTN) 110.
[0017] Each of the base stations 103-106 services a cell/set of
sectors within which it supports wireless communications. Wireless
links that include both forward link components and reverse link
components support wireless communications between the base
stations and their serviced wireless terminals. These wireless
links support digital data communications, VoIP communications, and
other digital multimedia communications. The cellular wireless
communication system 100 may also be backward compatible in
supporting analog operations as well. The cellular wireless
communication system 100 supports the Global System for Mobile
telecommunications (GSM) standard and also the Enhanced Data rates
for GSM (or Global) Evolution (EDGE) extension thereof. The
cellular wireless communication system 100 may also support the GSM
General Packet Radio Service (GPRS) extension to GSM. However, the
present invention is also applicable to other standards as well,
e.g., TDMA standards, CDMA standards, etc. In general, the
teachings of the present invention apply to digital communications
that combine Automatic Repeat ReQuest (ARQ) operations at Layer 2,
e.g., LINK/MAC layer with variable coding/decoding operations at
Layer 1 (PHY).
[0018] Wireless terminals 116, 118, 120, 122, 124, 126, 128, and
130 couple to the cellular wireless communication system 100 via
wireless links with the base stations 103-106. As illustrated,
wireless terminals may include cellular telephones 116 and 118,
laptop computers 120 and 122, desktop computers 124 and 126, and
data terminals 128 and 130. However, the cellular wireless
communication system 100 supports communications with other types
of wireless terminals as well. As is generally known, devices such
as laptop computers 120 and 122, desktop computers 124 and 126,
data terminals 128 and 130, and cellular telephones 116 and 118,
are enabled to "surf" the Internet 114, transmit and receive data
communications such as email, transmit and receive files, and to
perform other data operations. Many of these data operations have
significant download data-rate requirements while the upload
data-rate requirements are not as severe. Some or all of the
wireless terminals 116-130 are therefore enabled to support the
GPRS and/or EDGE operating standard as well as supporting the voice
servicing portions the GSM standard.
[0019] FIG. 2 is a block diagram functionally illustrating a mobile
device 200 constructed according to the present invention. The
mobile device 200 of FIG. 2 includes an RF transceiver 202, a
baseband processor 206, a central processing unit (CPU) 208, and
various other components contained within a housing. The baseband
processor 206 can perform physical layer processing, include a
speech COder/DECoder, and other baseband functions that interact
with the RF transceiver 202. In one embodiment, the baseband
processor 206 can comprise a Digital Signal Processor (DSP). The
CPU 208 can interact with data provided by the baseband processor
206, which represents decoded information received via the RF
transceiver 202 as well as interact with the various other systems
and components in the mobile device 200, such as a display 220,
microphone 226, speaker 228, user input device 212, camera 214,
LED's 222 and other components as can be appreciated that might be
incorporated into a mobile device. The user input device 212 can
include a capacitive touchscreen that is integrated within the
display 220, a keypad, other buttons or switches integrated into
the mobile device 200, or any other user input device as can be
appreciated.
[0020] The mobile device 200 can also include a battery 224 or
other power source that can provide power to the various components
in the terminal. The terminal can also include one or more
Subscriber Identification Module (SIM) port 213, a flash RAM 216,
an SRAM 218, or other system resources. The mobile device 200 can
also include one or more ports 210, which can comprise a universal
serial bus (USB) port and its variants (e.g., micro-USB, mini-USB,
etc.), a proprietary port, or any other input/output ports that can
provide for data operations as well as power supply that can
facilitate charging of the battery 224.
[0021] Accordingly, reference is now made to FIG. 3, which
illustrates one embodiment of a state machine corresponding to the
FCCH acquisition algorithm disclosed herein that is executed by the
baseband processor 206 in response to an RF signal received by the
RF transceiver 202. The various stages of the state machine are
discussed in further detail in the text as well as drawings that
follow. In box 301, the baseband processor 301 performs an initial
frequency estimation of a carrier frequency, or presence of an FCCH
signal. Generally, speaking, the process of box 301 is performed
with a low probability of miss detection and a high probability of
false alarm. Accordingly, in box 303, the baseband processor 206
refines the initial frequency estimation to arrive at a more
precise frequency estimation at which the FCCH tones are broadcast.
In box 305, the baseband processor 206 performs a final test that
verifies the refinement of the initial estimation performed in box
303. The FCCH acquisition method illustrated in FIG. 3 is generally
executed by the baseband processor 206 every 156 GMSK symbols that
are received, where the symbol rate, or F.sub.SYM, as referred to
herein, is approximately 270.8333 kHz.
[0022] Reference is now made to FIG. 4, which illustrates one
representation of the logic executed by the baseband processor 206
to perform an initial estimation of a carrier frequency
corresponding to the FCCH as referenced in box 301 of the state
machine shown in FIG. 3. As noted above, the FCCH acquisition
algorithm is executed every 156 GMSK symbols that are received by
the baseband processor 206. Accordingly, the baseband processor 206
receives 156 complex samples and divides the samples into four
portions comprising thirty-nine samples. On each vector of
thirty-nine samples, a subset of the samples is chosen (e.g.,
thirty-two samples) and a fast Fourier transform is executed as
shown in box 401 to produce thirty-two frequency bins as shown
below:
X.sub.F(k)=FFT(X.sub.T(k))
Where:
X.sub.T(k)={x.sub.T(k,0),x.sub.T(k,1), . . . , x.sub.T(k,N-1)}
X.sub.F(k)={x.sub.F(k,0),x.sub.F(k,1), . . . , x.sub.F(k,N-1)}
[0023] In the above formulation, N can equal 32, or the number of
samples of the GMSK symbols for which a fast Fourier transform is
executed. A subset of the frequency bins can then be selected in
box 403. In one embodiment, the fourth through the twelfth bins can
be selected as shown below:
{circumflex over (x)}.sub.F(k,n)=x.sub.F(k,n+4)
Where:
n=0,1, . . . , 8
[0024] This subset of the frequency bins can be selected because it
can be assumed that the FCCH signal frequency is
F SYM 4 ##EQU00001##
as well as that the maximum frequency error is
.+-. 4 F SYM 32 . ##EQU00002##
Accordingly, the energy of each selected bin can also be calculated
as a square of its absolute value as shown below:
E.sub.F(k,n)=|{circumflex over (x)}.sub.F(k,n)|.sup.2
n=0,1, . . . , 8
[0025] In box 405, each of the three last output vectors can be
summed as shown below:
E.sub.OUT(k,n)=E.sub.F(k,n)+E.sub.F(k-1,n)+E.sub.F(k-2,n)
n=0,1, . . . , 8
[0026] In box 407, a sliding window comprising sets of pairs of
frequency bins can be generated as shown below:
E.sub.OUT(k,n)=E.sub.OUT(k,n)+E.sub.OUT(k,n+1) n=0,1, . . . , 7
[0027] Such a sliding window 501 is further illustrated in FIG. 5
and is generated in the event that the carrier frequency may exist
between neighbor bins, in which case the carrier frequency
potentially manifests its presence both bins. Accordingly, a
frequency bin associated with a maximum energy level is selected as
is illustrated in the following pseudo code and represented in
box:
Pntr ( k ) = arg max n ( E OUT ( k , n ) ) ##EQU00003##
[0028] where, Pntr(k) represents a pointer to a frequency bin and
n=0,1, . . . , 7, and
E.sub.MAX(k)=E.sub.OUT(k,Pntr(k))
[0029] If (E.sub.MAX(k)>Threshold) and
(E.sub.MAX(k)>E.sub.MAX(k+1)) as shown in boxes 415 and 417, the
frequency bin corresponding to k is selected as the initial
frequency estimation and a pointer to that bin is returned as shown
in the following pseudocode:
if(E.sub.OUT(k,Pntr(k))>E.sub.OUT(k,Pntr(k)+1)) return
Pntr(k)
else return Pntr(k)+1
[0030] The threshold to which the energy level associated with a
respective bin is compared can represent a minimum energy level
associated with an FCCH signal.
[0031] Reference is now made to FIG. 6, which illustrates one
representation of the logic executed by the baseband processor 206
to perform an initial estimation of a carrier frequency
corresponding to the FCCH as referenced in box 303 of the state
machine shown in FIG. 3. In other words, FIG. 6 illustrates a
flowchart that shows one way in which the initial frequency
estimation can be refined by the baseband processor 206 according
to an embodiment of the disclosure.
[0032] In box 601, the initial frequency estimation can be
derotated as shown below:
X ^ ( n ) = X ( n ) exp ( - 2 p j n Pntr 32 ) ##EQU00004## n = 0 ,
1 , 2 , , 5 39 - 1 ##EQU00004.2##
[0033] Where Pntr is a pointer to a frequency bin associated with
the initial frequency estimation. Next, in box 603, the derotated
initial frequency estimation can be downsampled to reduce
computational complexity of the FCCH acquisition algorithm. In one
embodiment, the signal sampling rate can be reduced by a factor of
four and potentially even more without performance degradation. In
one example, the derotated initial frequency estimation can be
downsampled by a factor of four using an accumulator as a decimater
as shown below:
Y ( k ) = n = o 3 X ^ ( 4 k + n ) ##EQU00005## k = 0 , 1 , 2 , , 5
39 4 - 1 ##EQU00005.2##
[0034] In box 603, the downsampled and derotated initial frequency
estimation can be autocorrelated for the purpose of improving the
timing of the initial frequency estimation. In one embodiment, the
correlator can be implemented as a sliding window in which thirty
samples in the middle of the FCCH burst can be employed.
[0035] Next, in box 605, the baseband processor 206 can execute a
frequency estimation function on the result of the autocorrelation.
The frequency estimation can be performed as shown below:
F EST = F SYM 2 .pi. M arctan [ 1 N n = 0 N - 1 Y ( n ) Y * ( n + M
) ] ##EQU00006##
[0036] Where F.sub.EST is the first frequency estimation,
F.sub.SYM=270.8333 KHz, Y is a first vector representing the
autocorrected downsampled initial detection, and M and N are
frequency estimation parameters. To generate the first frequency
estimation, the frequency estimation parameter M can be set to 4
and N can be set to 26. However, it should be appreciated that
other frequency estimation parameters can also be employed. In box
605, the baseband processor 206 can derotate the first frequency
estimation and in box 607, the derotated frequency estimation can
be used to generate a second frequency estimation by using
frequency estimation parameters of M=20 and N=30. The second
frequency estimation can be derotated to generate a refined
frequency estimation.
[0037] Accordingly, as shown in box 305 of FIG. 3, the baseband
processor 205 can perform a final test of the refined frequency
estimation to verify its accuracy as well as its timing.
Accordingly, the baseband processor 206 can execute a mathematical
filter that passes a signal through a filter and compares an energy
of the filtered signal with a total received energy. A mathematical
expression of such a final test is shown below:
1 N n = 0 N - 1 X ( n ) exp ( - 2 j .pi. FrEstim n ) 2 1 N n = 0 N
- 1 X ( n ) 2 > Thr ##EQU00007##
[0038] Where X(n) is the downsampled initial detection, N is a
number of samples in the downsampled initial detection, FrEstim is
the refined initial estimation, and Thr is a threshold to which the
value resulting from execution of the filter is compared. If the
refined initial estimation when passed through the filter exceeds
the threshold, then the baseband processor 206 can declare that a
carrier frequency has been acquired from the FCCH.
[0039] Any logic or functionality illustrated herein, if embodied
in software, each block may represent a module, segment, or portion
of code that comprises program instructions to implement the
specified logical function(s). The program instructions may be
embodied in the form of source code that comprises human-readable
statements written in a programming language or machine code that
comprises numerical instructions recognizable by a suitable
execution system such as a processor in a computer system or other
system. The machine code may be converted from the source code,
etc. If embodied in hardware, each block may represent a circuit or
a number of interconnected circuits to implement the specified
logical function(s).
[0040] Although the flowcharts show a specific order of execution,
it is understood that the order of execution may differ from that
which is depicted. For example, the order of execution of two or
more blocks may be scrambled relative to the order shown. Also, two
or more blocks shown in succession may be executed concurrently or
with partial concurrence. Further, in some embodiments, one or more
of the blocks shown may be skipped or omitted. In addition, any
number of counters, state variables, warning semaphores, or
messages might be added to the logical flow described herein, for
purposes of enhanced utility, accounting, performance measurement,
or providing troubleshooting aids, etc. It is understood that all
such variations are within the scope of the present disclosure.
[0041] Also, any logic or application described herein that
comprises software or code can be embodied in any non-transitory
computer-readable medium for use by or in connection with an
instruction execution system such as, for example, a processor 803
in a computer system or other system. In this sense, the logic may
comprise, for example, statements including instructions and
declarations that can be fetched from the computer-readable medium
and executed by the instruction execution system. In the context of
the present disclosure, a "computer-readable medium" can be any
medium that can contain, store, or maintain the logic or
application described herein for use by or in connection with the
instruction execution system. The computer-readable medium can
comprise any one of many physical media such as, for example,
magnetic, optical, or semiconductor media. More specific examples
of a suitable computer-readable medium would include, but are not
limited to, magnetic tapes, magnetic floppy diskettes, magnetic
hard drives, memory cards, solid-state drives, USB flash drives, or
optical discs. Also, the computer-readable medium may be a random
access memory (RAM) including, for example, static random access
memory (SRAM) and dynamic random access memory (DRAM), or magnetic
random access memory (MRAM). In addition, the computer-readable
medium may be a read-only memory (ROM), a programmable read-only
memory (PROM), an erasable programmable read-only memory (EPROM),
an electrically erasable programmable read-only memory (EEPROM), or
other type of memory device.
[0042] It should be emphasized that the above-described embodiments
of the present invention are merely possible examples of
implementations, merely set forth for a clear understanding of the
principles of the invention. Many variations and modifications may
be made to the above-described embodiment(s) of the invention
without departing substantially from the spirit and principles of
the invention. All such modifications and variations are intended
to be included herein within the scope of this disclosure and the
present invention and protected by the following claims.
* * * * *