22-04-2014, 02:46 PM
3GPP LTE: Introducing Single-Carrier FDMA
3GPP LTE.pdf (Size: 490.02 KB / Downloads: 24)
Close on the heels of IEEE’s new 802.16e standard—better
known as Mobile WiMAX TM — follows the response from the
Third-Generation Partnership Project (3GPP) in the form of its
Long-Term Evolution (LTE) project. We featured WiMAX TM
in Issue Three of Agilent Measurement Journal and in this
article we explore what LTE aims to bring to the wireless
ecosystem. After considering the broader aspects of LTE, we
take a closer look at the uplink, which uses a new modulation
format called single-carrier frequency-division multiple access
(SC-FDMA). These are interesting times because it is rare that
the communications industry rolls out a new modulation format.
From both a technical and practical point of view, there is much
to understand, examine and evaluate in the capabilities and
benefits that SC-FDMA brings to LTE. SC-FDMA is a hybrid
modulation scheme that combines the low peak-to-average
ratio (PAR) of traditional single-carrier formats such as GSM
with the multipath resistance and in-channel frequency
scheduling flexibility of orthogonal frequency-division
multiplexing (OFDM).
Acronyms galore: LTE history and context
LTE’s study phase began in late 2004. The overall goal was to
select technology that would keep 3GPP’s Universal Mobile
Telecommunications System (UMTS) at the forefront of mobile
wireless well into the next decade. Key project objectives were
set in the following areas: peak data throughput; spectral
efficiency; flexible channel bandwidths; latency; device
complexity; and overall system cost. The main decision was
whether to pursue the objectives by continuing to evolve the
existing W-CDMA air interface (which incorporates HSPA*) or
adopt a new air interface based on OFDM. At the conclusion
of the study phase, 3GPP decided that the project objectives
could not be entirely met by evolving HSPA. As a result, the LTE
evolved radio access network (RAN) is based on a completely
new OFDM air interface.
LTE objectives and timing
The sidebar LTE at a glance (page 25) describes the major
objectives of the LTE project and some of the key system
attributes. Figure 1 shows an overall timeline for the LTE
project. Compared to UMTS, the overall timescale is shorter,
due largely to a much smoother standardization process. The
development of LTE will avoid the 8000-plus change requests
ultimately applied over a four-year period to the “frozen” UMTS
Release 99 specifications. The instability and subsequent delays
in the UMTS standard led to commercial deployment of a
proprietary system in Japan before the worldwide standard was
available. It is expected that the surprises and delays of UMTS
will be averted with LTE, meaning its introduction should be
more predictable and better able to avoid a proprietary launch.
The dates in Figure 1 are acknowledged as aggressive and may
slip; however, progress is solid and, as UMTS proved, trying to
rush the process can be counterproductive.
Assessing the advantages of OFDM
The primary advantage of OFDM is its resistance to the damaging
effects of multipath delay spread (fading) in the radio channel.
Without multipath protection, the symbols in the received signal
can overlap in time, leading to inter-symbol interference (ISI).
In OFDM systems designed for use in multipath environments,
ISI can be avoided by inserting a guard period, known as the
cyclic prefix (CP), between each transmitted data symbol. The
CP is a copy of the end of the symbol inserted at the beginning.
By sampling the received signal at the optimum time, the
receiver can avoid all ISI caused by delay spread up to the
length of the CP.
Comparing OFDM and SC-FDMA
Figure 2 shows how a series of QPSK symbols are mapped into
time and frequency by the two different modulation schemes.
Rather than using OFDM, we will now shift to the term OFDMA,
which stands for orthogonal frequency-division multiple access.
OFDMA is simply an elaboration of OFDM used by LTE and
other systems that increases system flexibility by multiplexing
multiple users onto the same subcarriers. This can benefit the
efficient trunking of many low-rate users onto a shared channel
as well as enable per-user frequency hopping to mitigate the
effects of narrowband fading. For clarity, the example here
uses only four (N) subcarriers over two symbol periods with the
payload data represented by QPSK modulation. Real LTE signals
are allocated in units of 12 adjacent subcarriers (180 kHz) called
resource blocks that last for 0.5 ms and usually contain seven
symbols whose modulation can be QPSK, 16QAM or 64QAM.