WCDMA: Broadcast and multicast services

Sunday, July 27, 2008

Broadcast and multicast services

Broadcast and multicast services
Multimedia Broadcast and Multicast Service (MBMS) provides support for multicast and broadcast services such as mobile TV. With MBMS, the same content is transmitted to multiple users in a unidirectional fashion, typically by multiple cells, to cover a large area where the service is provided. To a user at the cell edge, the signal from the neighboring base station can be used to improve the received signal quality. In essence, this is a form of downlink soft handover and leads to a significant improvement of data rates and coverage for broadcast services.

As with HSPA, strong emphasis was put on supporting MBMS on the same carrier as other (speech and data) traffic to provide the operator with maximum flexibility. In Cdma2000/EV-DO, a separate carrier is required, which also necessitates aseparate receiver in the mobile device.

References : http://www.ericsson.com/

History of spread spectrum and CDMA

This brief historical explanation begins with a look at Spread Spectrum (SS)
principles, since they are fundamental to the multiple access principle known as
CDMA.
The frequency bandwidth needed to transmit a given amount of information is directly
related to the information rate – the amount of information transmitted per unit of
time. The term “bandwidth” is now often used interchangeably with “bit rate” so this
appears intuitively clear to most people.
SS techniques are methods by which more frequency bandwidth is deliberately used
than motivated by the actual information rate (spectrum spreading). On one hand,
this implies a certain waste of valuable frequency spectrum; but on the other hand,
these techniques have advantages such as increasing the resistance to interference,
jamming and interception.

The SS technique Frequency-Hopping Spread Spectrum (FH-SS) involves
transmitting a signal with a “normal” bandwidth over a carrier frequency that rapidly
changes within a wider frequency spectrum, see Figure 2. To decode the signal, the
receiver in the FH-SS system needs to know the frequency-hopping sequence – the
“code.” FH-SS was first developed by scientist Nicola Tesla around 1900, but a more
popular early reference is the system proposed by actress Hedy Lamarr and her
pianist George Antheil in 1942, as a means to avoid jamming of the guidance signal
to a radio-directed torpedo [1].

Direct Sequence Spread Spectrum (DS-SS), illustrated in Figure 3, is another SS
principle wherein the carrier frequency stays the same, but instead the signal to be
transmitted is multiplied with a high bandwidth signal. This multiplying signal looks
like a random signal, or noise, but is actually completely reproducible. The resulting
signal obtains the same high bandwidth as the multiplying signal, which increases its
robustness against interference. On the receiving end, the reproducible high
bandwidth signal – the “code” – is multiplied with the received signal to recover the
original signal.

The received signal is made up of a number of identical but time-shifted copies.
These echoes can lead to improved reception by using a so-called “rake” receiver
structure, described in the 1950s by Price and Green and others [2, 5, 6]. The rakelike
receiver structure gathers and combines the signal energies of the various
echoes or paths.
Not just the rake receiver but all the SS concepts described above including CDMA
were invented in the 1950s or earlier, mainly as part of the early Cold War effort to
find secure communication methods [2].

During the 1960s and 1970s, CDMA was primarily employed for military use and it
was not until the late 1970s and early 1980s that CDMA was proposed as an MA
technology for digital cellular mobile telephony [3, 4]. An advantage of CDMA for
cellular use is avoidance of frequency planning, because the same carrier
frequencies can be used in all cells, which is typically not possible in early FDMAbased
systems due to higher sensitivity to interference. Using CDMA for cellular was
not without new challenges, however. At a given base station, signals from different
mobile devices could have extremely large differences in received power levels, due
to radio propagation phenomena and the fact that the devices are located at different
distances from the base station. Since all mobile devices use the same carrier
frequency, weaker signals would be “drowned” by stronger ones. Even the built-in
interference robustness of CDMA would not be enough to compensate. This was
known as the so-called “near-far problem”. Power control was a commonly proposed
solution to combat this problem2.
It is not widely known that concepts including SS and CDMA were proposed in the
mid-1980s as candidates for the pan-European Group Special Mobile (GSM) system
(later called the Global System for Mobile Communication). GSM was then in the
early phase of selecting the MA concept and a number of candidate proposals had
been put forward by various companies and research institutes. The “SFH-900”
system included FH-CDMA technology; the “CD-900” system included DS-SS
technology; the “Mats-D” system included DS-CDMA technology; and the “DMS-90”
system proposed by Ericsson included TDMA technology [7]. In 1986, a “beauty
contest” was held in Paris, after which the TDMA-based solution was selected for
GSM. At the time, TDMA technology was considered to have several implementation
advantages over other technologies.
Although GSM is based on TDMA, a slow form of FH-CDMA was actually
incorporated into the standard through the use of a technology denoted “slow
random frequency hopping,” which increased robustness against interference.
In the late 1980s, Qualcomm proposed a cellular system based on DS-CDMA and a
paper published in 1991 further analyzed the theoretical benefits of CDMA for cellular
use [8]. The proposed system was a voice-centric (i.e. 2G), relatively narrowband
CDMA system with a bandwidth of 1.25MHz. By comparison, the WCDMA 3G
standard employs 5MHz of bandwidth. Through trial systems Qualcomm was able to
demonstrate a functioning system that the Telecommunications Industry Association
(TIA) later standardized as “IS-95” in the US. The first release of this standard was in
July 1993.

Technologies in WCDMA

In this section we describe some of the important innovations that constitute the WCDMA standard, developed in response to the challenging 3G requirements placed on it. We also include descriptions of how the technologies differ from those used in the IS-95 family of standards. In particular we discuss functions related to the air interface, which is the interface between the mobile device and the network.

Asynchronous network operation
Several of the requirements for WCDMA did not exist in legacy standards, including IS-95. One such requirement is asynchronous network operation. Early on, one of the fundamental technical principles shared by the Japanese and European WCDMA concepts (including CoDiT) was asynchronous operation; that is, the base stations should not have to be synchronized to each other. This was an opposite requirement to that of the IS-95 (and later Cdma2000) standard, which relies on GPS (Global Positioning System) for its proper operation. The UMTS requirement for an asynchronous system was fundamentally important because it led to a standalone and easily deployable system attractive to operators around the world. But it also gave rise to several new technological challenges for WCDMA, including cell search and handover, described below.

Cell search
Before engaging in any kind of communication with the network, a mobile device must first synchronize to the signals sent from base stations. Mobile devices need to find out which base station code to look for and then synchronize to it. Searching for all possible base station codes at all possible times is an arduous task that could potentially affect the time from power-on to placing a call. Being asynchronous, WCDMA could not employ the principle adopted in the IS-95/Cdma2000 standards, where a common base station code with unique GPS time-shifts was used throughout the system. Clearly there was a need for innovation in this area. The early Japanese version of WCDMA included a structure based on dual synchronization channels, but it was generally considered too complex. Extensive research was conducted to find an information encoding principle that would minimize the cell search time while still keeping the complexity of the search at a negligible level. Solutions were progressively proposed to the standard and soon the goals had been achieved [13]. One of the early major technical challenges of asynchronous CDMA-based 3G had been overcome.

Soft handover
In most older mobile systems, the mobile devices are connected to one base station at a time and handover is the process of seamlessly shifting a mobile device’s connection from one base station to another. In “soft handover” however, mobile devices communicate the same information through two or more base stations simultaneously (see Figure 7). In practice, soft handover is a prerequisite for any high-capacity DS-CDMA-based system operating with the same carrier frequency in neighboring cells. As early as 1988, Jan Uddenfeldt et al., of Ericsson, pioneered the fundamental principles of soft handover in some of Ericsson’s early patents in the digital cellular field [14].

In soft handover, the signals transmitted from different base stations need to arrive at
the device at the same time. This posed some interesting design challenges,
because the relative timing of the base stations was not known beforehand due to
the asynchronous operation requirement in WCDMA. To solve this problem, WCDMA
embraced a kind of “per-call synchronization” principle, earlier adopted in CoDiT [9,
15]. Prior to connecting to a new base station, mobile devices measure a time
difference related to the new base station and the already connected base stations.
This measurement is reported to the network controller, which adjusts the
transmission timing of the new base station accordingly.

Compressed mode

Already from the beginning, virtually all WCDMA mobile devices were expected to bedual-mode (to support GSM) as the initial WCDMA coverage would probably not beas wide as that of GSM. Therefore, handover of ongoing connections betweenWCDMA and GSM was crucial for a smooth early rollout of WCDMA4. For a securehandover from WCDMA to GSM, the mobile device needs to perform signalmonitoring on potential GSM target base stations in the area, even during an activecall on the WCDMA system. The problem is to find time for this, because the mobiledevice is “busy all the time” with sending and receiving on a given WCDMA carrierfrequency. A technology denoted “Compressed Mode,” originally developed in CoDiT[9, 16], was adopted in WCDMA to solve this problem.

Time for monitoring is made available to the mobile device by, for example changing
the so-called Spreading Factor (SF) and hence “compressing” the information to be
received and transmitted into a shorter time span, leaving idle time for
measurements on other frequencies, including other systems. These measurements
are then reported to the network so a decision on the WCDMA to GSM handover can
be made. Compressed mode is also used for handover within the WCDMA system
from one carrier frequency to another.

WCDMA

Sunday, July 27, 2008

Broadcast and multicast services

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History of spread spectrum and CDMA

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