22-11-2012, 05:16 PM
Introduction to CPM-SC-FDMA: A Novel bMultiple-Access Power-Efficient Transmission Scheme
Introduction to CPM-SC-FDMA.pdf (Size: 384.83 KB / Downloads: 65)
ABSTRACT
This paper presents a novel multiple-access modulation scheme which combines
key characteristics of single carrier frequency division multiple accesses (SC-FDMA)
with continuous phase modulation (CPM) in order to generate a power efficient waveform.
CPM-SC-FDMA is developed based upon the observation that the samples from
a CPM waveform may be treated as "data symbols" taken from a constant-envelope
encoder. As with any encoder output, these samples may be pre-coded using the Discrete
Fourier Transform and transmitted using SC-FDMA. Having originated from a
constant envelope CPM waveform, CPM-SC-FDMA can potentially retain much of the
power efficiency of CPM thus resulting in a lower peak-to average power ratio (PAPR)
than conventional SC-FDMA. In this paper, the power efficiency, bit error rate (BER)
performance and spectral occupancy of CPM-SC-FDMA are discussed.
INTRODUCTION
Wireless communications is moving rapidly towards small, low cost devices. However,
the mobility and value of these devices is often limited by battery life since device
miniaturization is progressing at a faster rate than battery technology optimization.
Thus, the issue of battery life represents a key concern in the next generation of wireless
communication systems.
For mobile communication systems offering high data rates, orthogonal frequency
division multiplexing (OFDM) has received a lot of attention in the past few
years. OFDM is a popular broadband wireless system, which is currently in use in
wireless LAN , fixed broadband wireless access and in digital video and audio broadcasting.
The spectral efficiency of this system is based on the salient observation that
the orthogonality of subcarriers provides a way to pack more subchannels into the same
channel spectrum. In OFDM, the subcarriers are generated using the computationally
efficient inverse discrete Fourier transform(IDFT) and can thus exploit the well-known
circular convolution properties of the DFT in order to implement low complexity frequency
domain equalization techniques.
Orthogonal frequency division multiple access (OFDMA) is the multi-user
counterpart of OFDM. In its simplest form, OFDMA users transmit information symbols
using one or more subcarriers which maintain their othogonality with the other
usersŠ subcarriers, even under multipath fading channel conditions. As a result, the
multiuser interference is deterministically suppressed, regardless of the underlying radio
channel environment. However, the use of multiple subcarriers is generally done
at the expense of non-constant modulus signaling, which reduces the efficiency of the
power amplification stage of the transmitter.
Application of CPM to SC-FDMA
The modulated data symbols that are typically used in SCFDMA are drawn
from BPSK, QPSK or M-QAM constellations. However, in this paper we consider
modulated data symbols that are obtained from the sampled output of a CPM modulator.
Cyclic Prefix
As in conventional SC-FDMA, when the channel is frequency-selective, we
can add a cyclic prefix to the post- IDFT sequence by copying the last GN samples
of ~su and appending them to the beginning of the data block under the constraint that
the impulse reponse of the channel is less than GT seconds. After this operation, the
sequence of samples, including the guard interval, is now denoted as su;n where
In order to retain as much of the good envelope properties of CPM as possible,
the data symbols coming into the CPM modulator may be designed such that a valid
state transition occurs at the (circular) boundary between the end of the data block and
the beginning of the data block. By introducing a small number of data-dependent
symbols into each transmission block, one can assure that there are no abrupt jumps in
the signal samples when the cyclic prefix is appended. This, in turn, helps to maintain
more of the low envelope variations of the underlying CPM waveform.
SIMULATION RESULTS
To illustrate the flexibility of CPM-SC-FDMA between BER performance and
PAPR reduction, we use two different M = 4-ary CPM-SC-FDMA schemes; the first
scheme is a 3RC scheme with modulation index h = 5=16 (Scheme 1) and the other
scheme uses a larger modulation index h = 5=8 and a Gaussian pulse with L = 3 and
BT = 0.25 (Scheme 2). For both CPM-SC-FDMA schemes, we sample the underlying
CPM waveform at a rate of N = 2 samples per symbol. As discussed above, the modulator
followed by the sampler can be viewed as a constant-envelope encoder; for these
CPM-SC-FDMA examples, the effective coding rate is 1 information bit/symbol. To
check the flexibility of CPM-SC-FDMA, comparing the performance of these CPMSC-
FDMA schemes(1 and 2) to that of a convolutionally encoded QPSK modulated
SC-FDMA (CC-QPSK-SC-FDMA) with rate 1/2 (Rc = 1/2), so that 3 schemes having
same complexities.
CONCLUSION
This paper presented a novel framework which provisions multiple access of CPMlike
transmissions within an FDMA framework. This new, power efficient modulation
- CPM-SC-FDMA - is appealing for uplink applications wherein battery power or long
range communications are the presiding concerns. Although CPM-SC-FDMA is a nonlinear
modulation scheme, we have shown that the same low complexity techniques
used in FDMA (such as frequency domain equalization) are applicable and, in fact, the
complexity of such operations is the same as for a conventional FDMA transmission.
Since the signal parameters for the CPM-SC-FDMA are derived from conventional
CPM, this scheme exhibits robust error performance, as the signal memory, modulation
index and phase shaping function may be selected to meet a desired error performance
criterion.