<<

. 87
( 132 .)



>>


CDMA2000
1xRTT

CDMA2000
TDMA
1XEV-DO




Figure 6.141 CDMA2000 network evolution
418 UNIVERSAL MOBILE TELECOMMUNICATIONS SYSTEM


6.23.5 CDMA2000 1xRTT
The ¬rst step is referred to as 1x or 1xRTT (radio transmission technology), which allows
for signi¬cantly higher voice capacity as well as enabling data rates up to 153 kbps
in a 1.25 MHz carrier, with 307 kbps de¬ned for later releases. Voice capacity is dou-
bled. From the operator™s perspective there is little effort required in upgrading from
cdmaOne to a 1xRTT system, since much of the existing infrastructure simply requires
slight modi¬cations. It is available in both the 850 MHz and 1900 MHz bands.



6.23.6 CDMA2000 1xEV
The evolution of CDMA2000 beyond 1x is referred to as CDMA2000 1xEV. This actually
consists of two phases: 1xEV-DO (data only, Phase 1) and 1xEV-DV (data and voice,
Phase 2), both utilizing the standard 1.25 MHz carrier. In common with aspects of UMTS,
1xEV uses existing IP protocols extensively throughout the network to allow smooth
connectivity to external data networks. 1xEV-DO provides a new data channel, similar
in concept to the UMTS downlink shared channel (DSCH), which allows for downlink
data rates up to a theoretical 2.4 Mbps peak best effort service, with 153 kbps in the
uplink. 1xEV-DO networks are currently deployed extensively by operators in Korea.
1xEV-DV offers data rates of up to 3.09 Mbps and will support real-time applications
such as video streaming.



6.23.7 CDMA2000 3xMC
There is also a multi-carrier (MC) mode, which has a carrier bandwidth of 3.75 MHz. In
the forward link it actually consists of three consecutive 1.25 MHz channels, each with
a chip rate of 1.2288 Mcps. In the reverse link the chip rate is 3.6864 Mcps, which aids
in multipath reconstruction. There is also an alternative hybrid con¬guration whereby the
forward link is three consecutive carriers and the reverse link is a single 1x carrier using
the normal 1.2288 Mcps. It should be noted that the multi-carrier approach to CDMA is
not restricted to 3x, and higher chip rates over wider bandwidths (6x, 9x 12x) are also
possible. Due to spectrum limitations, and minimal system advantages, it is possible that
this system will never be widely deployed.



6.23.8 CDMA2000 network architecture
This system is also compatible with the ANSI-41 core network and can be implemented
using the existing frequency bands that an operator is licensed to use. It is therefore seen
as a simple and logical way of migrating to a 3G system. Like IS-95, CDMA2000 is
a synchronous system which requires the timing of these networks to be aligned within
a high degree. They are also aligned with the IS-95 networks for reasons of system
6.23 CDMA2000 419


Visited Network Home Network
BSC
SS7 Network
BTS

HLR
MSC/VLR
AAA
BTS
Mobile
Station

AAA
PDSN
BTS IP Network
BSC

FA HA

Figure 6.142 CDMA2000 example network

interoperability. This stringent timing is achieved through the use of the global positioning
system (GPS). In the forward link, each carrier is identi¬ed through a scrambling code.
Unlike UMTS, where each carrier has its own unique code, in CDMA2000, a single
code is used throughout the system, and each carrier is identi¬ed by its offset (phase
difference) from the reference code. In a similar fashion to UMTS on the reverse link,
the scrambling code (long code) is used to identify a particular mobile device which
has a dedicated channel. A simpli¬ed view of the CDMA2000 network architecture is
illustrated in Figure 6.142.
It can be seen that the radio network consists of CDMA BTSs and BSCs to control
them. The BSCs are connected to each other, enabling the soft handover mechanism to
function. The link between the BTS and the BSC is ATM, and both AAL2 and AAL5 are
used. The interface between the packet control function (PCF) located in the BSC and
the PDSN is referred to as the R-P interface and is used to transfer both packet data and
signalling messages.
The packet data serving node (PDSN) is used to connect to the external packet switched
networks. In CDMA2000 the point-to-point protocol (PPP) is used between the mobile
device and the PDSN to transfer both user data and signalling messages. This can be
compared to both GPRS and UMTS. In a GPRS system logical link control (LLC) is used
between the SGSN and mobile device, and the GTP protocol is implemented between the
SGSN and GGSN. In UMTS an RRC connection is established between the RNC and
mobile device and GTP tunnels are set up between the RNC and SGSN and between the
SGSN and GGSN. A single PPP session is allowed between a mobile device and the PDSN
and once established it is maintained while the mobile device is in the transmitting phase
but also when the mobile device is in the dormant state (equivalent to the idle state in
GSM/GPRS). The basic protocol stack for user data transfer is as shown in Figure 6.143.
The link access control (LAC) and media access control (MAC) play similar roles to the
UMTS RLC and MAC layers.
The functions of the PDSN are as follows:

• establish, maintain and terminate the PPP session with the subscriber;
• direct authentication, authorization and accounting (AAA) for the session to the AAA
server;
420 UNIVERSAL MOBILE TELECOMMUNICATIONS SYSTEM



IP IP IP


PPP PPP

Link Link
LAC LAC
layer layer
R-P R-P
MAC MAC

Air Air Physical Physical Physical Physical
interface interface layer layer layer layer

Mobile
Radio Network PDSN End host
device

Figure 6.143 CDMA2000 protocol stack


• collect usage data to be relayed to AAA server;
• support IP and mobile IP services;
• maintain the logical link;
• route data to and from the external packet network.

More details of the application of AAA and PPP to CDMA2000 can be found in
Chapter 5. The capacity of the PDSN can be measured in throughput or number of PPP
sessions that can be served. For a small-scale network this may be 50 000 PPP sessions and
for a larger implementation could be 400 000. The interfaces may be 100 Mbps Ethernet
toward the external packet network and ATM 155 Mbps OC-3 towards the radio network.
Since the PDSN is seen as a carrier class device redundancy and fail safe mechanisms
are also required.



6.23.9 Simple IP and mobile IP
Before any transfer of IP datagrams between the mobile device and the PDSN, the PPP
datalink must ¬rst be established. Once this is accomplished, the speci¬cation supports
two methods of accessing external networks, simple IP and mobile IP:

• Simple IP: here mobile devices requiring a change in PDSN as they roam through the
network will have their IP session terminated, since the new PDSN is required to assign
a new IP address. The mobile device may be assigned a static IP address or may be
assigned a dynamic address via the PDSN.
• Mobile IP: when a mobile device that has an established mobile IP session wishes to
transfer from one PDSN to another, the mobile device may do this and maintain its
originally assigned IP address. This is due to the added ¬‚exibility that the mobile IP
mechanism introduces, and this is discussed in Chapter 5.
6.24 TIME DIVISION-SYNCHRONOUS CDMA (TD-SCDMA) 421


It is requirement that the PDSN supports the following packet data transfers from a
single mobile device simultaneously:

• simple IPv4 and simple IPv6
• simple IPv4 and mobile IPv4
• mobile IPv4 and simple IPv6
• mobile IPv4, simple IPv6 and simple IPv4.

The mobile device may or may not support all of the above. IP and its related protocols
are discussed in more detail in Chapter 5.


6.23.10 Mobility management
Mobility is achieved through a system of handoffs (handovers). When this is between
PCFs which are connected to the same PDSN it is termed a PCF to PCF handoff whereas
when the PCFs are connected to different PDSNs a PDSN to PDSN handoff is required.

• PCF to PCF handoff: the connection over the R-P interface changes and as such a new
connection between the PDSN and the target RN needs to be established for each of
the packet service instances. This type of handoff may occur when the mobile device
is in either the active or dormant phase, and although the R-P connection changes, the
mobile device will maintain the PPP connection and the IP address(es). Allowing the
mobile device to maintain the PPP session even in the dormant state ensures that usage
of the airlink is kept to a minimum, thus reducing signalling etc.
• PDSN to PDSN handoff: if the mobile device has a mobile IP session activated during
the handoff then the IP address will be maintained even though the PDSN changes,
otherwise the connection will be released. To do this the mobile device needs to re-
register with its home agent (HA).



6.24 TIME DIVISION-SYNCHRONOUS CDMA (TD-SCDMA)

This system was jointly developed by the Chinese Academy of Telecommunications Tech-
nology (CATT) and Siemens, which have spent over $200 million on the standardization
process. It was proposed by the China Wireless Telecommunications Standards (CWTS)
group to the ITU in 1999 and was given its imprimatur as an of¬cial 3G standard under
the ITU-TC grouping, along with UMTS-TDD. China has reserved more spectrum for
this time division duplex (TDD) variant of the 3G system than to the FDD versions. In
fact, whereas 60 MHz has been reserved for CDMA2000 and UMTS, 155 MHz has been
reserved for TD-SCDMA.
It has also been adopted as part of the UMTS 3G R4 speci¬cation as UMTS-TDD
LCR (low chip rate, TR25.834). In common with UMTS-TDD, TD-SCDMA does not
422 UNIVERSAL MOBILE TELECOMMUNICATIONS SYSTEM


require separate uplink and downlink bands (i.e. paired spectrum) and offers speeds from
as low as 1.2 kbps up to 2 Mbps. Uplink and downlink traf¬c can be transferred in the
same frame but in different time slots, and there can be up to 16 codes allocated per slot,
enabling a number of simultaneous users. For asymmetric traf¬c such as web browsing
more time slots can be devoted to downlink transfer than in the uplink. This allocation of
time slots is dynamic and if a symmetric allocation is required, which is usually the case
for a telephone call, then this will also be allocated the required resources. The minimum
frequency band required for this system is 1.6 MHz and the chip rate is 1.28 Mcps. TD-
SCDMA does not have a soft handover mechanism but has a system similar to GSM
where the mobile devices are tightly synchronized to the network, and it is from here
that the term ˜synchronous™ is derived. It is designed to work with a GSM core network
in a similar way to WCDMA and can also use the UTRAN signalling stack when it
is deployed as a complementary technology. The frame is 5 ms rather than 10 ms in
WCDMA and is split into seven slots.
Although it is an of¬cial 3G standard, it is debatable whether TD-SCDMA will be
deployed outside of China. However, the large population of China does indeed form a
suf¬cient market to sustain the system.



6.25 SUMMARY
This chapter describes in considerable detail the architecture and operation of the UMTS
network, with particular emphasis on the protocols and signalling procedures. This are
enhanced by extensive use of trace captures to illustrate their operation. The UMTS log-
ical, transport and physical channels are described, as are the PDCP, RLC and MAC
layers which support them. Each of the control protocols “ RRC, NBAP, RNSAP and
RANAP “ are explained, with reference to the key procedures for establishment, mainte-
nance and release of radio access bearers. To assist in understanding the interoperation of
all these protocols, a typical user scenario is discussed at length, where the user receives a
phone call, and then proceeds to check their email. This is again augmented by reference
to live network traces. Finally, there is a brief examination of the other CDMA-based
IMT2000 technologies: CDMA2000 and TD-SCDMA.



FURTHER READING
O. Sallent, J. Perez-Romero, R. Agusti et al. (2003) ˜Provisioning multimedia wireless
networks for better QoS: RRM strategies for 3G W-CDMA.™ IEEE Communications
Magazine 41(2), 100“107.
3GPP TS22.105: Services and service capabilities.
3GPP TS23.060: General Packet Radio Service (GPRS) Service description; Stage 2.
3GPP TS23.101: General UMTS Architecture.
3GPP TS23.110: UMTS Access Stratum Services and Functions.
3GPP TS23.121: Architecture Requirements for release 99.
FURTHER READING 423


3GPP TS23.207: End to end quality of service concept and architecture.
3GPP TS23.821: Architecture Principles for Relase 2000.
3GPP TS24.008: Mobile radio interface Layer 3 speci¬cation; Core network protocols;
Stage 3.
3GPP TS25.101: UE Radio transmission and reception (FDD).
3GPP TS25.133: Requirements for support of radio resource management (FDD).
3GPP TS25.141: Base station conformance testing (FDD).

<<

. 87
( 132 .)



>>