Signal Processing - September 2017 - 115

IEEE SIGNAL PROCESSING MAGAZINE

September 2017

...
Unsynchronized Paging
Channel Message

Zero
Padding
Message Message
Length
Body

Synchronized Paging
Channel Message

Message Message
Length
Body

...
Paging Chan.
Half-Frame
Body
Paging Chan.
Half-Frame
Body

CDMA Forward Link Modulation

-
sin(ωct )

cos(ωct )

Pulse-Shaping
Filter
Short Code
(Real)

FIGURE 2. Cellular CDMA forward link signal structure.

8 Bits

Short Code (Imaginary)

s (t )
+

m (t )

Message Body

SOM
=0

SOM
=1

2−1,146 Bits
30 Bits
Sync Channel Message Capsule

CRC

...
Sync Chan.
Frame Body
Sync Chan.
Frame Body

SOM
=0

Message Length

1.2 Ks/s

CRC

SCI = 0

SOM
=0

Sync Chan.
Frame Body

2.4 Ks/s

SOM
=0

Sync Chan.Superframe = 80 ms

4.8 Ks/s

SCI = 1

Zero Padding

64-Chip Walsh Code 32
1.2288 Mc/s

Sync Chan.
Frame Body

4.8 Ks/s
9.6 Ks/s

SCI = 0

Paging Chan.
Half-Frame
Body

Paging Channel Slot = 80 ms = Eight Half Frame

Symbol
Repitition
Paging Channel
Message
Convolutional
Encoder
r = 1/2, K = 9

Paging Channel p (1-7)
m (t )

4.8 Ks/s
Block
Interleaver

1.2288 Mc/s

9.6 Ks/s
19.2 Ks/s

SCI = 0

CRC

...

Paging Chan.
Half-Frame
Body

m (t )

Block
Interleaver

Decimator
1.2288 Mc/s

19.2 Ks/s

Symbol
Repitition

Sync Channel Encoding and Modulation

19.2 Ks/s

Long Code
Generator
Long Code Mask for

Paging Channel Encoding and Modulation

19.2 Ks/s

Navigation framework and
experimental results
By making pseudorange measurements to three
or more BTSs, one may estimate the 2-D position and clock bias of a cellular CDMA receiver,
provided that the BTS locations and their clock
biases are known. Unlike GNSS, the states of
cellular CDMA BTSs are unknown to a navigating receiver and need to be estimated. The
location of the BTSs can be obtained offline
via several methods: 1) accessed from a cellular CDMA BTS location database, 2) surveyed
using satellite imagery, or 3) estimated on the
fly individually or collaboratively [27], [28].
However, the clock error states of a BTS (clock
bias and clock drift) are stochastic and dynamic
and therefore must be continuously estimated.
A substantial part of the literature on navigation
using cellular signals considers positioning with
pseudorange measurements; however, certain
assumptions such as perfect synchronization
or negligible variations between the transmitter
and receiver clocks are made to eliminate the
clock biases of the BTS and the receiver from
the measurement model [29], [30]. Alternatively,
clock errors can be calibrated at the beginning
by knowing the initial position of the navigator
and can be recalibrated over the trajectory when

1.2288 Mc/s

64-Chip
Walsh Code p
1.2288 Mc/s

SCI = 1

Sync Channel
Message Convolutional
Encoder
r =1/2, K = 9

the prompt, early, and late correlations. The
prompt correlation is computed by correlating the received signal with a locally generated
PN sequence shifted by the code start time estimate. The early and late correlations are calculated by correlating the received signal with an
early and a delayed version of the prompt PN
sequence, respectively.
In a GPS receiver, the pseudorange is calculated based on the time a navigation message subframe begins to eliminate ambiguities
due to the relative distance between GPS SVs
[25]. This necessitates decoding of the navigation message to detect the start of a subframe.
These ambiguities do not exist in a cellular
CDMA system. This follows from the fact
that a PN offset of one translates to a distance
greater than 15 km between BTSs, which is
beyond the size of a typical cell [26]. The pseudorange can therefore be deduced by multiplying the code start time by the speed of light.
Figure 3 shows the receiver architecture along
with the intermediate signals produced with a
LabVIEW-based implementation of the SDR.
Note the C/N 0 of the received cellular CDMA
signal (60.8 dB-Hz), which is 15-20 dB higher
than that of a typical GPS signal.

115

Table of Contents for the Digital Edition of Signal Processing - September 2017