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Studying mobile wireless

Studying mobile wireless [Under Telecom]

Though well written, standards/specifications may not be the first study material when we want to study or learn about particular mobile wireless system. There is always a need for "that first tutorial" before we jump into standard/specification documents.

The article helps you here; it tells you "What to look for when you study mobile wireless communication standards/technologies". Assumed is digital communication.

Here is generic mobile network architecture that we will assume:

Access network provide wireless connectivity with mobile device. Service/core network provide connectivity with target or external networks. Access and service networks act like a bridge and provide transport for mobile device to access end user services/applications (e.g. voice call, web browsing, VoIP call, streaming audio/video, gaming, virtual private network etc.)

Here are study steps:

Step 1 - Targeted services/applications
Step 2 - Initial look at end-to-end communication
Step 3 - Air interface and network signaling procedures
Step 4 - Inside service/core network and its connectivity to external network
Step 5 - Complete look at end-to-end communication

» Step 1 - Targeted services/applications

First point to look for is: what kind of services the system is targeted for. You can look in terms of characteristics of service, typically known as QoS parameters. Services typically differ in: constant bit rate or variable bit rate, real time or non-real time, types of minimum bit rates required etc. We need to keep this at back of our mind while we study the system. Though 3G/4G technologies try to find a generic solution that would support all types of services, earlier technologies were tailored to only few types of services.

The information about targeted services/applications can be gained from standards/specifications or from industry forums promoting the system.

» Step 2 - Initial look at end-to-end communication

In step 2, we should look for "Network architecture, protocol layers and their functions in brief". The idea is *not* to get into details but rather get overall view of end-to-end connectivity - how various functions have been separated in network elements and protocol layers to realise end-to-end user service.

» Step 3 - Air interface and network signaling procedures

Next step is to study air interface (access technology and protocol).

Unlike wireline interface, air interface is limited. It is to be used efficiently and at the same time various kinds of services are to be supported for as many as users as possible. When studying radio access part, we need to see how these challenges are answered by the radio access technology and protocol. The initial set of topics to look at would be: underlying radio technology, multiple access and duplexing (FDD/TDD, half/full) techniques, frequency/time organisation (likely to be in terms of channels/bursts/slots etc.), modulation schemes, how the allocations are done (basic unit of allocation in DL/UL) etc. You can as well see how interference/multipath propagation and other radio issues are tackled.

Next topics to study would be:

a) Initial access procedures - how mobile device catch the signals and start talking to network.

First step for mobile device during power on would be to "catch", "synchronise" and "decode" the radio signals to get the information about the frequency/time organisation. Also it has to chose the "network" for further procedures. Home network is typically where mobile device user is originally subscribed to. When "roaming", network would typically be "partner network". Roaming is when mobile device has been moved or roamed to different place where home network coverage is not available. Good definition of roaming would be when connectivity between home network and roamed network is through another (typically) public network.

Upon receiving request from mobile device, access network would have certain algorithm to make initial estimation about the distance between the mobile device and base station antenna. In the allocation reply, access network would provide initial values of timing and power for mobile device to use. Note that adjustment of timing/power is required during initial access and later periodically (as the distance between mobile device and base station antenna is likely to change).

b) Network entry procedures

Once mobile device has got the signals and decided the network, it has to make an entry to network. To talk to network, mobile device has to typically make a request to get "allocation" over "contention" based "shared" channel/resources.

Different wireless systems suggest different "network entry" procedures but they will have common steps like: radio parameters/power corrections, frequency selection procedures, authentication, ciphering, registration, and capabality exchange/negotiation.

c) Idle state procedures

Mobile device will not be need radio resources all the time - in such case it just "idles". During idle state, typically mobile device listens to broadcast information from network. Broadcast information include information about frequency/time organisation. There should be certain way by which network should be able to contact mobile device (e.g. mobile terminated voice call). For this purpose, typically there would be certain shared radio resources in downlink direction to which mobile device has to listen to during idle state.

Also, during idle state, mobile device has to make sure that it is able to listen to signals all the time. Standard will typically specify measurement procedures for idle state. Based on these measurements, mobile device can select different frequency, which might belong to different base station antenna. Mobile device may even have to go for registration if the new frequency belong to different network. These procedures are typically called frequency re-selection procedures.

d) Allocation/Contention resolution procedures

For user application data to be transferred over air, mobile device has to request radio resources from network. Standard will specify detailed procedures about how mobile device is to make a request, how the allocated resources are to be used etc. These are called "contention resolution and "allocation" procedures.

d) Measurement/Handover procedures

When mobile device is having an active data connection with the network (the user application data transfer is in progress or expected), mobile device has to make sure that signal quality is up to the mark, power and time adjustments are correct etc. Standard will mention detailed measurement procedures for this. To maintain the data connection, it may be required at some point to switch to different frequency. Standard will mention detailed "handover" or "handoff" procedures to do the same. Measurements carried by both mobile station and base station are important here.

e) Mobility procedures

Tracking location of mobile is important in idle state. Different systems implement different methods to keep track of mobile so that it can be contacted back without using lot of radio resources. One approach is to "page" mobile device in all of coverage, other approach is to organise coverage area such that mobile device need not be paged in all of coverage area. In second approach, mobile device may have to inform location to network periodically or when required. Both approaches require certain amount of radio resources based on velocity (speed and direction) with which mobile device moves and the coverage area has been organised. A optimum approach is needed.

f) Data plane procedures

Aim here is to find out what kind of modes of user data transfer have been implemented and what mechanisms are in place for error detection and correction (including retransmission): Transparent, Non-transparent (Acknowledgement, Unacknowledgement) modes, ARQ mechanisms etc.

Responsibility of above procedures can be with phy layer or radio access layers or network signaling layers or partially by each layer. Next topics to study would be the interfaces between these layers.

» Step 4 - Inside service/core network and its connectivity to external network

Main intent of step 4 is know how mobile device is represented in target network by service/core network, how the user application data coming from user device are transported in service/core network to reach target network, is there any interworking function needed ? etc.

» Step 5 - Complete look at end-to-end communication

Now we repeat the step 2 with lot many details about the procedures executed by individual protocol layers and network elements.

This complete the brief on "Studying Mobile Wireless".

Copyright © Samir Amberkar 2010§ §