UMTS: Origins, Architecture and the Standard (Computer Communications and Networks)
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The operation of A-GPS requires the establishment of a GPS reference network, also known as a wide-area reference network WARN —whose receivers are placed at fixed known locations and with clear view of the sky, so that they can operate continuously [ 22 ]. Each fixed GPS receiver uses the satellites signals to estimate its position and compare this estimate with its ground-truth position, which has been previously surveyed to a high degree of accuracy.
This comparison yields differential correction factors that are provided to the SMLC when requested. DGPS improves the positioning accuracy to under meters. The WARN also provides information such as which satellites are visible over a given area at a given time, Doppler shifts, and ionospheric delays. The target MS uses this information to calculate the pseudoranges to the visible satellites.
If there is any error in the received messages, or any condition that prevents the MS from sending back the requested information, a Protocol Error message is sent to the SMLC. The MS receives the request and tries to provide the required information. If any problem occurs, a Protocol Error message is generated. When a receiving entity—either the SMLC or the MS—detects that some data is missing or receives erroneous data, a Protocol Error message is sent with an error code indicating the error type, as listed in Table 2. However, no authentication is used.
However, this is a violation of location privacy. It is even more serious as the subscribers may not even be aware that an RRLP session is taking place. Cell ID CID positioning, also known as cell of origin COO , is a proximity-based method, as it returns as the MS location estimates the geographic coordinates associated with the serving cell which is assumed to be the closest to the target MS [ 47 ]. These coordinates might be the location of the antennas—and not that of Node B equipment—or the centroid of the cell coverage area.
This improves the spatial resolution of RTT values: This better spatial resolution helps reducing the confidence region, which results in higher positioning accuracy. Oversampling factors down to of the chip period are supported [ 48 ]. This happens because, as all cells share the same downlink frequency, in the reception of the signals of a given cell the energy sum of signals from all other cells acts as noise.
If one cell—the near one—is received with a very high energy, the SNR of a distant cell—the far one—will be very low. The near-far problem might prevent the UE from achieving this minimum number of measurements required to obtain an unambiguous position fix. In fact, EOTD would be usable only at the border of each cell coverage area [ 46 ].
The idle periods of different cells might be time aligned or randomly distributed. During an idle period, a cell will transmit only the synchronization channel or nothing at all, depending on the idle period configuration—continuous mode or burst mode. In fact, this is caused by one of the most fundamental aspects of WCDMA networks planning—that each cell must have a dominant area, where its signal overcomes the signals from neighbor cells. Please refer to Section 3. AoA positioning is also known as multiangulation.
Its deployment in cellular networks requires the installation of antenna arrays at Nodes B [ 51 — 54 ], which might be quite expensive and time-consuming. As previously mentioned, this positioning method suffers heavily from NLOS propagation [ 55 ], being of little use in dense urban environments. PEs are handheld-sized elements accessible only through the air interface. PEs coordinates must be accurately known. They must be placed at reference points other than Nodes B locations. PEs transmit in the downlink channel and their signals, upon reception at the UE, provide TOA measurements to be used in the hyperbolic multilateration [ 46 ].
As previously mentioned in Section 3. To obtain an unambiguous position estimate, the TDOA between at least three pairs of cells must be calculated and therefore a minimum of four geographically dispersed cells is needed. Their transmissions are not synchronous, so the control channel frames transmission at BTS 2 and BTS 4 start at instants and , respectively. It is detected at the MS at instant. Note that the PEs transmissions are carried out during the serving Node B idle period. As already mentioned in Sections 3. However, RRC is not a protocol designed solely for positioning.
It includes many other functions, such as outer loop power control and paging notification. As it is not a positioning specific protocol, we are not going to get into details of RRC here. For further information, please refer to [ 58 ]. Essentially, the same specific positioning related network elements introduced in 2G and 3G networks are also present in 4G networks. It has the same basic functions of the 2G and 3G SMLC, but with support to enhanced positioning features, such as hybrid localization and geofencing [ 59 ].
As already stated in Sections 3. However, this requires the installation of antenna arrays in the eNBs. Besides that, AoA is too sensitive to multipath propagation, which is predominant in the heavily NLOS conditions in dense urban areas. A discussion on the FCC Enhanced E [ 1 ] emergency call locating regulations and possible related applications can be found in [ 62 ]. Some aspects of the E evolution to accommodate Voice-over-Internet Protocol VoIP emergency calls, which will certainly increase with the widespread deployment of 4G networks no support to circuit-switched voice calls , are explored in [ 63 ].
The possibility of using signals from different constellations improves both positioning availability and precision. UE-based type, where the UE receives assistance data from the network and calculates its location, and UE-assisted type, where the UE receives assistance data from the network, makes position related measurements e.
Reference signals in LTE do not convey any higher layer information, existing only at the physical layer. To improve the hearability i. The techniques to combine those measurements are open to different implementation [ 40 ]. It is designed to be forward-compatible with future access networks to prevent piling up several positioning protocols through the generations to come.
Control plane positioning uses dedicated control channels to convey location related information assistance data and position estimates. It is considered more reliable and fast, so it is used in emergency call location [ 67 ]. Control plane positioning can be carried out without user intervention or even without user awareness refer to Section 3.
User plane positioning transfers location data into IP Internet Protocol datagrams using end-user applications. SUPL , however, does not reinvent the wheel. LPPa has two modules: LPP uses six types of procedures or transactions, grouped into three groups [ 64 ]: Capabilities Transfer and Capabilities Indication Procedures. LPP is also capable of detecting and reporting several specific error conditions, mostly in the positioning assistance data. An example of a full LPP session is shown in Figure It shows a control plane network initiated location request NI-LR.
UMTS: Origins, Architecture and the Standard : Pierre Lescuyer :
The server then ends this transaction with an acknowledgment ACK message. The server must then reply with one or more Provide Assistance Data Messages. The UE then replies with one or more Provide Location Information Messages, containing its estimated coordinates in the case of UE-based positioning or position related measurements in the case of UE-assisted positioning. The same applies to the Location Information Delivery Procedure, when the UE sends unsolicited location information to the server.
LPPa has the following functions: It enables the exchange of location related information information that will be used to improve CID positioning, e.
It is reporting error situations for which no specific error message is defined. Each of these functions comprises one or more elementary procedures, which in turn encompass a set of messages that define the information exchange. Table 3 maps the LPPa functions to its elementary procedures [ 69 ]. This review paper has presented the positioning capabilities in cellular networks, starting with the intrinsic localization support, inherent to any cellular system, and then passing through the specific features added by 3GPP—network elements supported functions and interconnection with other elements of the architecture , supported positioning methods position calculation details, related air interface parameters, autonomous and assisted operation modes, and assistance data provided by the network , and protocols functions, elementary procedures and more relevant message exchanges —from the second until the fourth generation.
It has also brought a section explaining the organization of 3GPP Technical Specifications and provided a quick review on the cellular networks evolution path. Wireless Communications and Mobile Computing. Indexed in Science Citation Index Expanded. Subscribe to Table of Contents Alerts.
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Abstract This review paper presents within a common framework the mobile station positioning methods applied in 2G, 3G, and 4G cellular networks, as well as the structure of the related 3GPP technical specifications. Introduction At first, the main drive behind the development of positioning techniques to support location services LCS in cellular networks was the need to locate any mobile station MS originating emergency calls.
Evolution path of the two cellular technologies families: This timeline is greatly simplified; many intermediary standards have been ruled out. Only the evolution cornerstones are represented. The figure also brings some key features introduced by each standard, such as maximum achievable data rates, multiple access, and modulation techniques.
Location resolution hierarchy within a GSM cellular network. This scenario represents a mobile terminated location request MT-LR [ 22 ]. The message flow shown here is a higher layer abstraction. Some specific position related protocols and messages are described in Sections 3. An example is the prolonged activity around creating network—based selective IP flow mobility between cellular and Wi—Fi networks, i.
With the advent of smartphones, the increased complexity of endpoint devices is a foregone conclusion. Thus the cellular mobile network should follow the principle that was proven successful in the Internet: The current cellular network architecture employs many protocols that involve four or more parties and numerous round trips.
There are 10s of roundtrips depending on previous states required to complete this process. It is doubtful that this is an ideal or unavoidable situation, yet some aspects appear inherent in the current architecture. It also presents a rich target for hacking or DoS attacks. Thus, simplicity of protocols should be one of the prime objectives of any new protocol design, along with modularity of protocols and soft—failure under errors. Most of 3GPPs recent activities, such as latency reduction, local traffic off—loading, flattening the network architecture, and peak rate improvements, have been in reaction to seemingly unexpected growth of demands from reality: Web centric content, Smartphones, push applications, etc.
It should be obvious that the basic network architecture standards should be designed for uncertainty and flexibility [ 16 ], rather than specific service scenarios. Also, considering the long delays in responding to market demands through the standard—setting process, the basic standards should be independent of the details of features and services, even if such a separation sacrifices some of the benefits of tight integration. The use of host—centric protocols is also consistent with this principle, since new services or features can typically be implemented on host devices and servers, and should not involve changes deep inside networks.
With the recent increase of the aggregate throughput and the peak rates of radio links, the high latency of cellular data networks has become a major performance issue. The use of layer 2 tunnels, connection—oriented centralized routing, protocol encapsulations and translations, and complex protocols have all contributed to the high latency that is now making the improvements in bandwidth less useful and noticeable.
Under high latency, the impact of dropped packets while very rare in cellular networks by design is very severe, congestion collapse is more likely, TCP congestion control becomes less effective, and temporary performance disparity among users is more pronounced. VoIP and other real—time applications show noticeably poor performance compared to the wired Internet, even when the cellular throughput for bulk transfers approaches that of some wired broadband access technologies.
Considered necessary in the past for large—area mobility, operator control, and roaming, they have increasingly become a source of inefficiency, poor scaling, and complexity. Alternative approaches as suggested below may better address the needs of present and future cellular networks.
Instead of tunnels, direct use of Ethernet switching in small areas and plain IP routing over bigger areas, similar to the rest of the Internet, could harness the innovations and competition existing in the wired networking industry. The basic functions that tunnels used to provide, such as mobility, transport over circuit networks, and route control, are now unnecessary, or can be provided by other means as described below.
The base stations would handle localized fast mobility in a distributed manner, without centralized control. This is a step in the right direction, but having layer 2 mobility managed by base stations, rather than the MME, may be the ultimate solution. With the inevitable trend toward using a large number of small cells in the future, more distributed protocols and technologies should be favored, even if they are not needed at present.
Current cellular core networks are largely private networks, with user devices typically given dynamic private IP addresses. They access operator—provided services within private networks and access the rest of the Internet via multiple layers of NAPTs. Most of the widely believed benefits, such as the alleged greater security of using private networks, are either untrue, not worth the price, or can be achieved via different means [ 22 ].
Many decisions made about cellular network architecture in the early days have remained in effect and influenced many subsequent changes and additions with numerous unintended interactions and limitations. Considering our general inability to predict technology and economic changes for any significant stretch of time, the basic network architecture should not be designed based on any set of specific usage models or requirements that might be asserted at design time.
Instead, the architecture should be designed to allow quick addition and removal of overlay of protocols and services for diverse applications [ 23 ]. Host—based on—demand IP mobility, instead of network—based always—on IP mobility. At present, the network handles most aspects of mobility and does so at all times for all traffic, at great cost and complexity. Also, many devices use the network in a mostly static context, including some applications of M2M services. Treating every device uniformly for high speed mobility and always—on connectivity is clearly not optimal, considering the overhead needed for such features.
The network need only provide IP mobility anchoring services as hosts request them on demand. In principle, IP applications on user devices require two fundamental functions from the cellular networks they attach to: Session Continuity and Reachability. Session continuity refers to the survival of layer 2 and layer 3 connectivity across handoffs between base stations, while there is active traffic to and from user devices, such as streaming video, ftp, etc.
This is what the cellular network provides by using the user—plane tunnels that combine layer 2 and 3 and follow wherever user devices go. In effect, user devices never see changes in their IP subnet over large geographical areas, unless they have to switch between different gateways, roaming partner networks or different wireless link technologies. Reachability is provided when a mobile device is reachable by its communication peers, even when idle. For voice calls, this is achieved by paging, but for data apps, the current cellular data networks do not provide this functionality, because they use dynamic private addresses.
This technique provides the appearance of reachability while the actual connections are initiated by clients and maintained through firewalls and NAPT by both communicating parties. Given this reality, a new architecture should not suffer from excessive network control signaling triggered by these long—lived connections coming in and out of idle states, as the current architecture does [ 24 ]. One possible approach is for the network to be largely stateless beyond base stations with respect to user devices.
In light of this development, IP mobility should be per—flow on—demand rather than always on, and also locally distributed to allow flexible network deployment choices, considering various utilities of caching, CDNs, and peer—to—peer communications. Substitute hard—state rigid protocols with soft—state, soft—fail protocols. These protocols often require that three or more parties remain synchronized in their protocol states for correct operation.
Perhaps not surprisingly, these complex protocols are rarely verified for correctness and race conditions. They fail hard, recover slowly, require large memories and processing power, and are difficult to interoperate and debug. Most of them could well be replaced with soft—state, soft—fail protocols with better results. Each new generation of the cellular air interface has been accompanied by a large overhaul of the wired core and RAN.
That approach appears no longer sustainable for keeping pace with different rates of innovation in wired networking, mobile computing, and wireless link technologies. Arguably, innovations and changes in the network and application layers are more rapid and unpredictable, as evidenced by the rapid rise of the smartphone ecosystem, compared to wireless link layer evolution that proceeds more slowly and requires industry—wide coordination. This separation also widens the market for air interface technology beyond cellular mobile technologies into fixed wireless access, indoor private networks, etc.
The current tight integration of standards for the air interface and the wired network should be separated, with clear interfaces to allow their separate evolution. Based on the preceding discussion, an example architecture for a future cellular network can be imagined in a straightforward manner. This, of course, is not the only possible solution, nor entirely original, but serves as a concrete example for discussion. As more and more cellular base station sites are connected with high—speed backhaul to handle increasing demand, Ethernet is becoming the de facto choice.
For this example, we assume all base stations are connected via wide—area Ethernet services commonly called Metro Ethernet [ 25 ]. They are connected via Metro LAN service with a topology that fits the local conditions rather than point—to—point logical connections between base stations and MTSO as done in the present architecture.
On this layer 2 network there are one or more IP routers with a small set of mobility—related features, such as dynamic distributed IP mobility, to be discussed later. This familiar IP subnet forms a unit of RAN and connects to the Internet directly, instead of connecting back to a cellular core network as is done currently. There is no cellular core network for user traffic. Instead of near—permanent tunnels for each user device, tunnels or VPNs are sparingly used for groups of devices, and only for special functions such as lawful intercept, hot—lining for emergency access, etc.
This unit of RAN can be as big as a metropolitan area or as small as one base station. The same logical unit of RAN can be used to build a large city network, a venue network, or a small hotspot. The current air interface technologies do not have MAC layer addresses for user devices that can be directly used to switch user packets in the RAN Ethernet connecting base stations. They can be added by user devices or by base stations, and there is little risk of address spoofing since the identities of user devices are verified cryptographically per existing air interface security measures [ 26 ].
They may even assist layer 2 mobility directly [ 29 ]. The handoffs between base stations are handled directly between base stations using Ethernet switching along with light—weight L2 such as MACinMAC temporary tunnels when needed for forwarding buffered packets. It is commonly envisioned that there will be increased direct communication between base stations for various reasons such as multipoint coordinated transmission [ 30 ], or plain local user—to—user traffic. The tunnel—less dynamic auto—configuring layer 2 network described above would make transition to such scenarios much simpler.
Some user devices will cross the coverage boundary between two RAN units while still engaged in active data flow at the IP layer. Some will cross with data flows, such as push notification connections, that are designed to survive IP network changes by reconnecting in the background. Some will cross while idle and some, such as meter readers and stationary modems, will never cross.
The actual mixture of these different types will keep changing and the best place to understand the need is at the devices. In light of this observation, the most sensible and scalable IP mobility protocol should be host—based, on—demand, distributed mobility [ 31 ].
One possible implementation is depicted in Figure 2 where three RAN units, each with its own router, are serving a geographic area. These RAN units can each be covering a city and its surrounding areas.
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A mobile device always uses the locally valid IP address for any new IP flows. This flow will remain anchored at the origin router A until the flow is terminated by the user device and reinitiated if needed as a new flow in RAN unit B , or when a maximum life span is reached as determined by mAR A [ 34 ]. While this flow is anchored at the previous router A, any new flow originated by the user device uses a locally valid IP address corresponding to the new network B, thus, as shown in Figure 3, it is not tunneled.
The key assumption for this particular example is that most long—lived user flows, if not all, are reachability flows that can reconnect after attaching to a new subnet. Other flows only need temporary session continuity minutes at most or not at all. Temporary session continuity maintains the call while necessary signaling is performed to transfer the call to the new IP address [ 37 ].
Observe that this distributed mobility management also uses tunnels, even per—flow tunnels in some cases, so it appears to be not much different from the current architecture at first glance. The key difference is that these tunnels are temporary, localized, and on—demand, instead of being permanent and global for every device. Even for highly mobile user devices, mobility tunnels are used temporarily only when they cross boundaries between potentially large RAN units [ 38 ].
One of the main sources of handoff delay across IP subnets is IP stack configuration in addition to security—related signaling, particularly when the UE can only communicate with one network at a time. There are many approaches to reduce this delay such as the use of Signal Forwarding Function [ 39 ] in mARs and other radio interface—specific optimizations. Some features only available to the lower layers of cellular radio links, such as paging, may be of great use for some applications, if they can be safely exposed to the higher layers.
For example, even when many devices handle their own reachability issues like most smartphones, it may be desirable to keep some devices idle with no layer 3 connectivity and rely on layer 2 radio paging to wake them up for infrequent activities. An application on a user device can negotiate with a server that has back—end connections with the cellular paging system and provide authorization for another trusted party from the Internet to trigger cellular paging. Details of such a system are omitted here for space concerns, but this service can bridge the application layer reachability and cellular radio paging capabilities to enable efficient operation for certain applications.
This approach to network features where end hosts and the network interact on the application layer over the normal IP network can be used to deploy and experiment with new features rapidly with low risk. Currently, data roaming is handled by a visited network forwarding APN—indicated tunnels to a home network. The current cellular architecture is in fact designed with roaming in its foundations, which may not be ideal for some network operators with low roaming activities. The current architecture generally assumes that all the traffic of a visiting roamer except for circuit or packet voice must go back to its home network [ 40 ].
Due to the resulting inefficiency and complexity, such an assumption is no longer appropriate for many operators, especially those with small amount of roaming business and those experiencing strong pressure to lower roaming fees. In the future, roaming should be handled via local accounting, while all user traffic is routed locally and efficiently without tunneling back to home networks.
This can be augmented with rapid accounting information exchange [ 41 ], device self—reporting, third—party audit, or micro—transactions directly between roamers and local visited networks. Thus, the basic architecture should reflect this trend and the old roaming model may be accommodated as special cases only for very high value traffic.
The main function of a mobile network is to handle a large quantity of packet traffic efficiently and cheaply while user devices move across its coverage area. Other features such as Mobile Broadcast Multimedia Service [ 42 ] or layer 2 VPN for enterprise networks are important for some market segments, but should not dictate how the basic architecture is devised.
Most of these services can be implemented at the IP layer, sometimes with some loss of efficiency from the perspective of those specific services. For example, emulating layer 2 VPN over user IP layer over radio links is less efficient than forming layer 2 VPN over a transport network connected to a radio bearer. However, that is not a reason enough to justify changing the basic architectural goal, which is to cover the majority of users and services efficiently. This principled approach should be maintained in the face of many competing interests from subsets of users and services.
Fortunately, there appear to be several voices [ 43 ] in industry and academia that generally agree with this paper on the problems of current cellular architecture and the general directions for solutions. It is a valuable exercise to design a new architecture unencumbered by existing networks and user devices, but it is obviously necessary to examine how to transition from the current architecture to the new, and what kind of compromise is necessary.