Signal Processing - September 2017 - 114

The sequences transmitted on the forward link channel, i.e.,
from BTS to receiver, are known. Therefore, by correlating
the received cellular CDMA signal with a locally generated
sequence, the receiver can produce a pseudorange measurement.
This technique is used in GPS. With enough pseudorange measurements and knowing the states of the BTSs, the receiver can
localize itself within the cellular CDMA environment.

mobile operators also transmit the BTS latitude and longitude
on the paging channel, which can be exploited for navigation.
The major cellular CDMA providers in the United States (Sprint
and Verizon) do not transmit the BTS latitude and longitude.
The sync and paging channel structures and the cellular CDMA
forward link signal modulator are summarized in Figure 2.

Overview of cellular CDMA forward link structure

The goal of a cellular CDMA receiver is to acquire and track
the signal parameters, specifically 1) the code phase or code
start time and 2) the carrier phase, which can be constructed
from the apparent Doppler frequency. To this end, a CDMA
receiver consists of three main stages: signal acquisition, signal
tracking, and message decoding.

In a cellular CDMA communication system, several logical
channels are multiplexed on the forward link channel, including
a pilot channel, a sync channel, and seven paging channels as
described next.

Modulation of forward link CDMA signals
The data transmitted on the forward link channel in cellular
CDMA systems are modulated through quadrature phase-shift
keying (QPSK) and then spread using direct-sequence CDMA
(DS-CDMA). However, the in-phase and quadrature components of
the channels of interest carry the same message m(t). The spreading sequences, called the short code, are maximal-length pseudorandom noise (PN) sequences that are generated using 15 linear
feedback shift registers (LFSRs). Hence, the length of the short code
components is 2 15 - 1 = 32, 767 chips [22]. An extra zero is added
after the occurrence of 14 consecutive zeros to make the length of
the short code a power of two. To distinguish the received data from
different BTSs, each station uses a shifted version of the PN codes.
This shift, known as the pilot offset, is unique for each BTS and is
an integer multiple of 64 chips. Each individual logical channel is
spread by a unique 64-chip Walsh code [22]. Spreading by the short
code enables multiple access for BTSs over the same carrier frequency, while the orthogonal spreading by the Walsh codes enables
multiple access for logical channels over the same BTS.
The CDMA signal is subsequently filtered using a digital
pulse-shaping filter that limits the bandwidth of the transmitted
CDMA signal according to the CDMA200 standard. The signal
is finally modulated by the carrier frequency to produce s(t).

Pilot channel
The message transmitted by the pilot is nothing but the short
code. A CDMA receiver utilizes the pilot signal to detect the
presence of a CDMA signal and then tracks it. The fact that
the pilot signal is dataless allows for longer integration time.
The receiver differentiates between the BTSs based on their
pilot offsets.

Sync channel
The sync channel is used to provide time and frame synchronization to the receiver. The cellular CDMA system uses GPS
as the reference timing source, and the BTS sends the system time as well as the PN offset to the receiver over the sync
channel [23].

Paging channel
The paging channel transmits all the necessary overhead parameters for the receiver to register into the network [23]. Some
114

CDMA SDR architecture

Acquisition
The objective of this stage is to determine which BTSs are in the
receiver's proximity and to obtain a coarse estimate of their corresponding code start times and Doppler frequencies. For a particular PN offset, a search over the code start time and Doppler
frequency is performed to detect the presence of a pilot signal.
The frequency spacing must be a fraction of the inverse of the
integration period, which is 0.08/3 ms if it is assumed to be one
PN code period. The code start time search window is naturally
chosen to be 0.08/3 ms with a delay spacing of one sample.
Similar to GPS signal acquisition, the search could be implemented either serially or in parallel, which, in turn, could be
performed over code phase or Doppler frequency. The proposed
receiver performs a parallel code phase search by exploiting the
optimized efficiency of the fast Fourier transform (FFT) [24]. If
a signal is present, a plot of the squared magnitude of the correlation will show a high peak at the corresponding code start time
and Doppler frequency estimates, as shown in Figure 3.

Tracking
After obtaining an initial coarse estimate of the code start time
and Doppler frequency of the pilot signal, the receiver refines
and maintains these estimates via tracking loops. In the proposed design, a phase-locked loop (PLL) is employed to track
the carrier phase, and a carrier-aided delay-locked loop (DLL)
is used to track the code phase.
The PLL consists of a phase discriminator, a loop filter, and
a numerically controlled oscillator (NCO). Since the receiver
is tracking the dataless pilot channel, an atan2 discriminator,
which remains linear over the full input error range of ! r,
could be used without the risk of introducing phase ambiguities.
In contrast, a GPS receiver cannot use this discriminator unless
the transmitted data-bit values of the navigation message are
known [25]. Furthermore, while GPS receivers require secondor higher-order PLLs due to the high dynamics of GPS SVs,
lower-order PLLs could be used in cellular CDMA navigation
receivers. The receiver could easily track the carrier phase with
a second-order PLL.
The first-order carrier-aided DLL employs the noncoherent
dot-product discriminator followed by an amplifier. To compute the code phase error, the dot-product discriminator uses

IEEE SIGNAL PROCESSING MAGAZINE

September 2017

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