Round up 2012

Here is a brief round up for what we covered in 2012.

We started with the history of wireless communication. After understanding the basic concepts we moved to the different generations the world of wireless has seen. After getting into the details of the requirements/standards of a particular generation, we also studied the techologies that actually help in fulfilling requirements. This list will include, LTE technologies like OFDM/SCFDMA etc ir 3G technologies of multiple access.

Then we dived deep into LTE. Tried to understand the concepts of LTE. Followed by VoLTE discussions.. Challenges in trends for LTE. And here we are at the end of the first year of living wireless!!


Wish you all a very happy holiday season!

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VoLTE Part 2

In this post, I intend to cover VoLTE architecture.

VoLTE Architecture
There are three components that need to be understood before understanding the VoLTE architecture , Evolved packet core,Access network and control portion of the network. Its in line with the architecture concept of single box radio thats getting popular from LTE. This flat architecture, reduces the latencies and improves the signalling speeds.

Evolved Packet Core : EPC contains a total of three entities. One in control place (MME) and two Gate ways in user plane. (S-GW/P-GW). MME , mobile management entity has controls the user authentication authorization functions.Other MME functions include NAS signalling , security , tracking area lists , CSFB and SRVCC inter technology handovers. S-GW assists the UE in inter/intra enodeB handovers , packet forwarding , routing etc, P-GW on the other hand manages gateways towards PDN. It is also in charge of policy and charging enforcements.

Access Network : Its the User Equipment and the eNodeB.

Control Network : The control network has three major componets. IMS (IP multimedia system) , PCRF (Policy and charging rule function) , HSS (Home subscriber server). As the names are self explanatory lets not get into too many details. In short IMS is responsible for IP connectivity , roaming , QoS , IP policy and service control. PCRF creates CDRs using the programmed call charging functions. Last but not least HSS maintains subscriber and service related data.

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VoLTE Part 1

We had discussed the need and challenges for voice over LTE .. Now lets understand it by digging deeper..

Although all the operators will be moving to 3GPP eventually. The underlying 3G technologies are different. Some operators have CDMA, some have GSM and hence it is going to be a difficult but necessary transition. Theoretically most of the solutions look very very attractive.. Its when the real situations come, the challenges are visible. There are two angles to the whole LTE deployment strategy. CDMA angle and GSM/WCDMA angle.


GSM/WCDMA :

Initially the operators will have to support circuit switched fall back. In the later stages, when LTE coverage is obtained throughout the region, Single radio voice call continuity needs to be achieved to reach the end result of all IP networks !


CDMA : 

I always feel that GSM is a very very complex system and difficult to understand. :(
These operators need to support Simultaneous Voice and LTE (SV-LTE) or Simultaneous Voice and Data (SV-DO) or evolved high rate packet data (eHRPD) and achieve the end goal of all IP networks controlled by IMS.

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The "FRAME"

Lets discuss the LTE frame structure. Depending upon TDD/FDD , there are two types of frame structures.


Frame structure 1 : 
Let me start with FDD, as I am more familiar with FDD systems and yet to work on any TDD system

Smallest slot here is called as Tslot = 15360Tb = 0.5ms. Two consecutive slot is a subframe. Each slot has some OFDM symbols and cyclic prefixes.Every frame will have 10 subframes i.e 10ms.

The diagram shows one frame structure. In these systems , the uplink and downlink systems are separated in the frequency domain each with 10 sub frames.

Every slot will have 6 symbols in case of Normal cyclic prefixes and 5 in case of extended cyclic prefixes. 

Frame Structure 2 : 

Here is frame is 10ms and consists of two sub frames of 5ms each. These 5ms subframes are further divided into smaller frames of 1ms. Each of these smaller sub frames have three fields.

Every subframe will have following fields.

a. DwPTS - Downlink Pilot TimeSlot - Normal downlink channel with 3-12 OFDM symbols.
b. UpPTS - Uplink Pilot TimeSlot - Normal uplink channel with 1 or 2 OFDM sysmbols.
c. GP Field - Guard Period - It has remaining symbols!

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LTE :PRB : The actual transmission!

To begin with, Lets start with Physical resource blocks.. Before we move into what to transmit and how to transmit, Let's look at the actual transmission..

The physical channel: means of actual transmission , is made of multiple resource blocks and resource elements.. A resource element is one single carrier over single OFDM symbol. 
Resource block is the smallest portion of the resource that can be allocated.

Here are the different channels for uplink and downlink

Downlink Physical Channels

There are six different types of downlink physical channels.
1. Physical Downlink Control Channel : PDCCH - It contains information regarding transport format and resource allocation.

2. Physical Downlink Shared Channel : PDSCH - It contains information user data and signalling.

3. Physical Downlink Broadcast Channel : PDBCH - It carries system information

4. Physical Multicast Channel : PMCH - multicast or broadcast information is carried.

5. Physical Hybrid ARQ Indicator Channel : PHICH - It carries ACK/NAKs associated with data transmission

6. Physical Control Format Indicator Channel : PCFICH -  Indicates number of OFDM symbols used.


Uplink Physical Channels 

There are just three different uplink channels.

1. Physical Uplink Control Channel : PUCCH - It carries ACK/NAK information along with the CQI 

2. Physical Uplink Shared Channel : PUSCH - It is similar to PDSCH - It carries user data along with some signalling.

3. Physical Random Access Channel - PRACH - It has the random access permeable sent by UE.

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LTE :RIP

Yes, RIP - Radio Interface Protocols..LTE follows a layered protocol approach. The 5 layers can further be divided into two planes.

The following diagram will help us understand the layers, hierarchy and division of planes.

Now lets look at every single layer

1. PHY : It is also called as Layer 1 as per OSI model. This the layer where actual data is transmitted and received. This data transfer is using resource blocks. Some control mechanisms are also implemented as a function of the physical layer.

2. MAC : The second layer of the OSI model. This layer is mainly responsible for the error corrections. Terms like MIMO rank, code rate, modulation etc are related to this layer. It proves how appropriate the name "Media Access Control" is !

3. RLC : The radio link control layer. For people like me, who are working on network optimization/performance, layer 3 rings a bell! Well sometimes, it can turn out to be a nightmare.. Anyways , this layer is responsible for segmentation and concatenation of data units. Automatic repeat requests is an error correction mechanism implemented in this layer.

4. PDCP : Packet Data Convergence Protocol. : Header compression and decompression in the packet is done on this layer.

5. RRC : Radio Resource Control : pagin, maintenance, release/setup connection , QoS are the functions of this layer sitting at the top.

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LTE - Digging deeper!

Now that we have covered basics and enabling technologies for LTE.. Let's understand try to understand the standards better!

3GPP's official website is the best source to understand the standard.. It is like learning from the "GURU's" or directly getting it from the owners!

3GPP has a very well defined, organised way of working. Every project is a "Technical Specifications Group". It is divided into multiple working groups. As an example RAN TSG is subdivided into RAN planery, Radio layer 1 etc..

Again the standard is divided into multiple series.

Series 23 : SAE
Series 36 : EUTRAN
Series 24 : Signalling protocols (UE to Network)
Series 26 : CODECs
Series 37 : Multiple Radio Access.


I wish I was an expert on all this , but I am still reading and understanding them. For now, Lets focus on Series 36 : LTE (Evolved UTRA) and LTE-Advanced radio technology

Again the series is subdivided into multiple "SUB - series"! This is going to be an adventurous journey.

36.1 : It is all about Base stations conformance and radio
36.2 : Physical Layer
36.3 : Radio Access Network - Layer 2 , Layer 3
36.4 : Architecture
36.5 : UE conformance Testing
36.8 : Background and more technical reports

Now that we have a clear understanding of the nitty-gritties we are all set to start the journey!

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Follow UP : LTE and Voice

This month it is LTE and Voice!!  In the previous blog we have seen the problems/developments in voice over LTE.  Lets dig deeper into the basics!!
What exactly is Voice over LTE.

As previously said, LTE is an all IP network and voice call in GSM, UMTS and CDMA2000 are circuit switched. Currently all the calls go on 2G/3G systems.

There are several proposed approaches. The chart below shows these.

There are four approaches. Lets discuss them one by one.

1. Circuit Switched Fall Back (CSBF): This is the simplest option. In this case LTE is data only network and whenever there is a voice call to be received/initiated , it will fall back to circuit switched domain(2G/3G).

2. Simultaneous voice LTE (SV-LTE) : This is a handset based solution. The device works simultaneously in the Circuit switched and  LTE(packet switched) 
modes providing voice services and data services respectively. Can you think of the battery life of such a device??

3. Voice over LTE via GAN (VoLGA) :  This concept came up to unable LTE users to receive voice/SMS as they transition between LTE/GSM/UMTS. This was an interim solution.


4. One Voice/Voice Over LTE (VoLTE) : This approach eliminates the dependency on the circuit switched networks. This will facilitate LTE networks to be stand alone networks. It is based on IP Multimedia Subsystem (IMS).

The key technical requirements for VoLTE :
1.  IP Multimedia System (IMS) 
2. SMS over IP
3. Multimedia telephony.
4. Single Radio Voice call continuity 

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LTE and Voice

I was telling my father about LTE the other day, my father is not really a super tech-savvy person. According to him, cellphones are devices invented to make and receive calls. So he asked me a very simple question, what does LTE offer to me , a person who doesn't use any data/Apps. Then I started thinking..I am missing on a big part.. LTE and Voice

Since the beginning, whenever anyone starts talking about LTE everyone starts talking data..Is the "LTE = higher data rates" only valid equation. What about LTE and Voice. Does LTE even support voice? Can the LTE networks be stand alone networks and function without the existing 2G, 3G networks? Some valid and some invalid questions!!

I started searching for articles.This week I read an amazing article on the qualcomm blog by Mr Prakash Sangam , "Evolving your voice – VoLTE, WCDMA+ and more"..And all these questions started occupying my mind..The author starts with a similar question and the best part is he provides us with the list of current and future solutions/developments clarifying the equation between Voice and LTE!!
Do read this article on qualcomm blog page.

After reading this article I actually started reading more about VoLTE. VOIP over LTE. The operators have a BIG say here, they are currently satisfied with the present 2G/3G network for voice telephony and secondly the vendors and devices are not ready yet!!

Another interesting report is from the LTE World summit (May 2012) "4G Voice Still Just a Whisper"!! The title clearly explains the current status of Voice and LTE. So there is still a great deal of efforts needed in VOIP /Voice in LTE. It is predicted that Voice over LTE will not be widely used till 2013-2014..


Now lets see why is Voice over LTE such a big fuss word! As we are fully aware that the LTE standard only supports packet switching with its all-IP network. The voice calls in the previous standards, like GSM, UMTS and CDMA2000 are circuit switched, so with the adoption of LTE, carriers will have to re-engineer their voice call network. The devices should be capable of roaming hubbing. In no LTE-coverage areas, the device should fall back to 2G/3G networks..

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Trends and Challenges in LTE

Now we have spent enough time in discussing how we got to 4G and the basics of wireless. It is time to know whats in store or what is the scope for improvement. As we all know there are going to be some huge technical challenges to cope up with the exploding number of connected devices. These challenges are going to be on both the fronts, network and device. From the network side people have come up with solutions like mobile data offloading to WiFi or Femtocell offloading..

It is interesting to read about challenges in LTE predicted a couple of years ago.
I am writing some of the challenges listed in 2009 in wirelessweek article.

1. Spectrum Harmonization
2. 2G/3G Spectrum Re-farming
3. Voice over LTE
4. Backhaul :  Adding raw capacity, Realizing full mesh backhaul
5. Self Organizing Networks
6. Traffic Management

Surprisingly, many of these are still challenges and many more new challenges have been seen/tackled in these years as networks have been rolled out.

From the device manufacturers side, performance, cost and re-usability of devices remain the major challenges.

Now as we have discussed challenges speculated when LTE was in an infant stage and now when it is rolled out in a large part of the United States.

1. A looming spectrum shortage!! Every operator "just" gets 500MHz band.
2. Bandwidth issues
3. Unlimited data plans : lead to enormous data usage.
4. 2G and 3G networks are still there..It is not entirely LTE.

Here now on,  I am trying to take one problem a month and  read/write as much as I can,

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Understanding LTE

There are several new key enabling technologies in the design of LTE. These are
1. OFDM
2.Multiple antenna techniques
3. SC-FDMA
4. Channel dependent resource sharing
5. IP based Network

These technologies enable this evolution. It is important to know about these technologies if one wants to understand this standard. This is the brief discussion about some of these.

1. OFDM : Hmm..OFDM stands for Orthogonal Frequency Domain Multiplexing. Basically it is a multicarrier system which uses multiple subcarriers for transmitting single data stream.. Now thats something!! :) We have multicarrier system, multiple subcarriers and just single data stream!! Isn't that unfair..The picture after this section will explain it well. This is the flow chart I had created for one of my class projects..This helped me understand the working of OFDM.

2. Multiple Antenna Techniques : Everything related to LTE has to have "multiple" things!! Basically it means that this standard can use multiple antennas to transmit and receive. There are several techniques available to be used. We will discuss these in detail. These techniques include, Transmit diversity, beamforming, multiuser MIMO and spatial multiplexing!!


3. SC-FDMA : SC-FDMA is a multiuser version of SC FDE(Single Carrier Frequency Division Equalization). It is implemented at the uplink.

4. Channel dependent resource sharing : OFDMA provides great deal of flexibility in LTE. Allocation both in frequency and time is permitted here.

5. IP Based Network : LTE standard has been designed to reduce the hierarchical structure of networks. In the previous generations, the levels were more and so was the cost and latency. In this particular standard, there is just MME between SAE Gateway and the eNodeB. This has drastically reduced the latency and increased the cost. 

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Follow Up : Multiple Access Techniques

Now the number of connecting devices exploding!! So the capacity of the systems should grow in the same proportions!! Here comes another savior technique :  Multiple access..
These schemes are used to allow a number of mobile users to share simultaneously a finite amount of radio spectrum. The sharing of spectrum is essential as the available spectrum is not going to increase with the number of devices! High capacity can be achieved by simultaneously allocating the available bandwidth/channels to multiple users.

Lets start withe basics! The duplexing techniques. There are two ways 
1. Time Division Duplexing : Here multiple users share single radio channel by taking turns in the time domain. Individual users are allowed to access the channel in assigned time slots, and each duplex channel has both a forward time slot and a reverse time slot to facilitate bidirectional communication.   

2. Frequency Division Duplexing : Here every user will have two distinct bands of frequencies. The forward band provides traffic from the base station to the mobile, and the reverse band provides traffic from the mobile to the base station.

Now the "Multiple Access".. The following diagram shows the multiplexing techniques.

1. FDMA : Frequency Division Multiple Access
Individual channels are assigned to every user.

2. TDMA :  Time Division Multiple Access
First the whole radio spectrum into slots, one user in each slot is allowed to                  
transmit or receive at any given time.

3. CDMA : Code Division Multiple Access
This is a type of spread spectrum techniques.All the users can use a wide band for entire time but each has a different psuedo-random code assigned.

I hope now some of the terms I mentioned while writing about 3G and 4G standards make more sense.

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The long term Evolution

So after looking at the first three generations, here we are. I do not want to call LTE as a 4G standard yet as according to International Telecommunications Union-Radio communications sector (ITU-R) for any 4G system, enhanced peak data rates to support advanced services and applications (100 Mbit/s for high and 1 Gbit/s for low mobility )..Now thats something really huge..From 10s of Mbps we are going to 1Gbps!! I am not quite sure if any of the service provider actually provides these data rates. Well it is advertised as 4G though..There were couple of other standards beyond the third generation such as HSPA+ and WiMAX.  So the operators had to choose between these three. For now we will be focusing only on LTE, the Long Term Evolution. I really like the name given to this standard, it actually suggests that atleast this generation will be able to cater our needs for a longer time. As we have seen in the previous posts, the wireless communication industry has seen rapid growth over the years. The forecasts say that 3GPP LTE will enable the growth of cellular systems and might be able to meet the need till 2017. Now thats a long time considering the growth patterns seen in last 5 years. The number of connected devices will reach 15 billions by 2015!! So we will need robust systems to support these big number of users and speed and coverage will be the decisive factors in competitors..

It is said that "Necessity id the mother of inventions"..Well same goes for the development of LTE. There were some important driving factors in the development of LTE.

1. Ever increasing demand of higher data rates.
2. Increasing number of connected devices
3. Flexibility for the operators (LTE supports flexible bandwidths)
4. Availability of smart phones

There are several requirements for LTE published by ITU. (I am using the official ITU document for reference)

1. Peak data rate : 100 Mbps DL/ 50 Mbps UL within 20 MHz bandwidth 
(DL : 5bps/Hz, UL : 2.5bps/Hz, 2x2 MIMO default)
2. Co-existance with 2G and 3G systems.
3. Latency : upto 100ms
3. Mobility : Optimized for 0 ~ 15 km/h, 15 ~ 120 km/h supported with high     performance.Can support upto 350km/h.
4. Spectrum usage:Flexible.Permissible channel bandwidths:1.4, 3, 5,10, 20 MHz
5. Supported duplexing : FDD, TDD

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Follow Up : Understanding 3G Systems

For a better understanding of 3G systems and basics of 4G systems, it is essential to understand the concept of modulation and different modulation techniques.


Lets understand the word first!! I traced back to the origin of this word, It seems that people from 15th and 16th century defined it as  "regulate, measure off properly, measure rhythmically; play, play upon," or "act of regulating according to measure or proportion" . This word was used in context of music those days. Though we are discussing it in telecommunication systems, the purpose and meaning of ''modulation remains the same."

It is necessary to understand modulation before we start discussing 4G as we are going to encounter words like orthogonal frequency modulation in that standard.

Lets start with analog modulation. Two basic techniques of modulation.

1. Amplitude modulation : Here the frequency of the modulated signal is kept constant while the amplitude is varied by the message signal. Thus, AM signals have all their information in the amplitude of the carrier.

2. Frequency Modulation : In this technique, the amplitude of the modulated carrier signal is kept constant while its frequency is varied by the modulating message signal. Thus, FM signals have all their information in the phase or frequency of the carrier.FM is a part of a broader technique called Angle Modulation.

Now lets move on to Digital Modulation. In digital systems, the modulating signal is represented as a sequence of pulses/symbols where each symbol has m finite states Each symbol represents n bits of information, where n = log2m bits/symbol.

There are four major classes of Digital modulation techniques.
1. Linear Modulation Techniques : The amplitude of the transmitted signal varies linearly with the modulating digital signal. These techniques are bandwidth efficient. 

2. Constant Envelope Modulation : The amplitude of the carrier is constant, regardless of the variation in the modulating signal. These are power efficient.

3. Combined Linear and Constant Envelope Modulation Techniques :  Here  digital baseband data may be sent by varying both the envelope and phase (or frequency) of an RF carrier. These are efficient techniques in terms of power and bandwidth.

4. Spread Spectrum Modulation Techniques : Spread spectrum techniques employ a transmission bandwidth that is several orders of magnitude greater than the minimum required signal bandwidth.

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Here comes 3G!!

The second generation had a significant increase in the capacity and hence the number of users supported. But the third generation was a big jump in terms of data rate, services and applications. Besides these enhanced features, 3G systems also provide better Quality of Service for voice telephony, internet browsing, multimedia, location based services,  etc.

There were certain requirements(laid by ITU) every 3G system has to fulfill. These are in terms of data rates.

1. 2 Mbps in fixed/building environment
2. 384 kbps in pedestrian environment
3. 144 kbps in vehicular environment

There are four major 3G standards

1. WCDMA
2. CDMA 2000
3. EV-DO
4. HSPA

Lets discuss each standard.

1. WCDMA : It's actually same as UMTS (Universal Mobile Telephone System). Initially I had a confusion between WCDMA and UMTS unless I got to know that UMTS is a standard based on WCDMA.(Why not to call it UMTS then !! :)) 

The basic specs include frequency bands 850/900MHz, 1.8/1.9/2.1GHz, channel bandwidth of 5MHz, peak data rate 384-2048 kbps (but typical user rate is just 150-300 kbps).The user plane latency was 100-200 ms and CDMA, FDD was implemented. DS SS - QPSK modulation scheme was used.
UMTS retains the basic architecture fron GSM networks  but the air interface (WCDMA) is way different than that of GSM. Its is a DIrect sequence spread spectrum, here data is multiplied with psuedo random codes that provide channelization and scrmbling. It is specified for TDD and FDD both!! (although FDD is by far the most widely deployed). 

Some of the important features of W-CDMA are
1. Use of Alamouti space-time coding for transmit diversity
2. Wider choice of spreading factors
3. multi code use by a single user is supported(higher data rates!!)

2.CDMA2000 : Well strictly speaking CDMA2000-1X did not meet the data rate requirements but CDMA2000 -3X did.Sometimes it is mentioned as CDMA-1X-EVDO. i.e. CDMA-1X EVolution Data Only. The specifications are frequency bands 450/850MHz 1.7/1.9/2.1GHz , 1.25MHz channel bandwidth, Data rates of DL:2.4–4.9Mbps UL:800–1800kbps, latency upto 200ms was observed. DS-SS: QPSK, 8PSK and 16QAM was the modulation scheme with Frequency division duplexing. 


3. HSPA : High Speed Packet Access was another 3G standard. There were number of techniques developed for increasing the data speed like adaptive modulation , dynamic scheduling etc. Basically, HSPA is a combination of HSDPA and HSUPA . These were the two enhancements applied to WCDMA to further increase the data rate. Except for the modulation scheme that HSPA standard is same as WCDMA. The modulation scheme is adaptive and hence DS-SS: QPSK, 16QAM and 64QAM is used. The latency was reduced to 50-90 ms and the data rates were drastically increased to DL:3.6–14.4Mbps UL:2.3–5Mbps!!

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Follow Up - Understanding 2G systems

Now that we have reviewed the second generation systems, lets understand all the concepts that play a role in making these systems work!

In the last blog, I used words like FDD/TDD/FDMA etc.. Lets go into details of all these words..

In this blog we will dig deeper into Frequency division duplexing and time division duplexing

To begin with, lets see what is a duplex system.. A duplex system can send and receive functionality. Depending upon whether send and receive functions are done simultaneously, a system can be FULL duplex or half duplex. For point to multipoint networks like cellular systems , it is necessary to separate inward and outward signals. 

1. Time division duplexing : It is a method of transmitting and receiving at the same frequency but having synchronized switches at both the ends. Although it is intuitive that TDM systems are synchronous, statistical time division multiplexing has been successfully implemented. The advantages include lower spectrum usage, lower cost of implementation than FDD, asymmetrical or dynamic UL/DL allocation.

2. Frequency Division duplexing : In this case there are two separate frequency channels are required. The cons include higher spectrum usage , no dynamic allocation. Advantages include lower(theoretically zero) latency.

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The second One!

The second generation of cellular system was introduced after a decade of first generation. The last decade of 19th century started off with this digital generation. 2G systems were primarily aimed towards the voice market.

There were several shortcomings of the 1G systems, like limited coverage and  lower capacity. As the popularity of cellular phones started increasing , the capacity of cellular systems was a challenge for the service providers.
Another shortcoming of 1G systems was the limited ability of the controller and the phones to exchange information during the course of a call. The control information was passed through the voice channel in a "blank and burst" basis. That means speech was briefly( 100 ms or may be more) interrupted whenever  
the control message moves phone and the base station. I am sure this was a major issue as QoS was degraded. 

The shift from analog to digital enhanced the performance of cellular systems. 
Secondly to meet the increasing demands following techniques were used
1. The reuse of spectrum efficient digital speech codecs.
2. multiplexing techniques (frequency / time)
3. higher(tighter) frequency reuse by better performance of digital modulation and coding

There were several major 2G standards like GSM, IS-95 CDMA, IS- 135 TDMA.

1. GSM : This system originated in Europe in 1990. The frequency bands used are 850/900 MHz, 1.8/1.9 GHz with channel bandwidth of 200kHz. These systems were either TDMA or FDMA. FDD duplexing technique was implemented. The peak data rates supported by the system was 107 kbps(GPRS) and 384 kbps(EDGE). Typically user data rates were 20-40 kbps(GPRS) and 80-120 kbps(EDGE). The permissible latency was 600- 700 ms.

2. IS-95 : This was the first CDMA system using 850MHz/1.9 GHz frequency bands with 1.25 MHz channel bandwidth. The peak data rate was 115 kbps but typically a user could get a data rate upto 64 kbps. The user plane latency was greater than 600ms. (FYI : for LTE the latency is less than 100ms).

3. IS-54 : This system was later modified to IS-136. The frequency used was 850MHz/1.9 GHz with 30kHz channel bandwidth. TDMA/FDMA were the multiple access techniques. The data rate supported was very low, a peak of 12 kbps and typical user data rate was around 9 kbps.


The 2G systems not only provided improved voice quality but also supported some new applications like SMS , updates, alerts, bill payments. Later these systems were equipped to provide low data rates as well.

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Cellular Systems

Now that we will be talking about 2G systems next month,and move to 3G soon, it is important to understand the cellular concept in order to understand how is the capacity of systems is improved. The problem of limited spectrum Vs higher capacity was solved using this concept.
In a very simple language, one single high power transmitter was replaced by multiple low power transmitters!! To answer the question how exactly the system capacity is increased by increasing the number of transmitters when the allocated spectrum still remains the same?
The answer is frequency reuse!! Every base station is allocated certain number of radio channels, and nearby base stations are assigned different groups of channels so that all the available channels are assigned to a relatively small number of neighboring base stations. Neighboring base stations are assigned different groups of channels so that the interference between base stations (and the mobile users under their control) is minimized.

This channel assignment can be fixed or dynamic. Another concern in cellular systems (now we know why these are called "cellular systems!!")  is handoffs. When a user moves from one cell to another, the call should be transferred to another base station this is called handoff..


People have come up with creating different sized cells in order to improve the capacity further. The picture shows an example of the same.

The factor that limits cell sizes and hence capacity is "Interference". Sources of interference are other calls in same or different cells, other base stations and systems. It is necessary to place co-channel cells(cells using same set of frequencies) far apart so as to minimize the interference caused by them. This distance (lets say D) can not be decreased beyond certain limit and hence the capacity can not be increased.

If the interference is caused due to adjacent channels (mostly due to imperfect RF filters) following steps are taken to minimize it.
1. Use of better filters
2. careful channel assignment
3. More frequency separation between channels

Nest month we will start with 3G systems and some concepts related to Modulation techniques.

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Wireless Propagation Basics

We have already looked at the First generation of telecommunication system. I think it is necessary to understand the basic principles of Mobile radio Propagation. Without the basic understanding of the principles of mobile radio propagation it is impossible to understand the practical problems related any communication system. Trust me, even with complete understanding of these principles solving the problems in network is difficult!! Lets say for safer side that with understanding of these principles, life will be easier!!
Wireless propagation can be split into three parts

1. Mechanisms of Propagation
2. Large scale Path Loss
3. Small scale Fading

There are numerous great books describing these phenomena. I am just going to present a brief idea so that one is not overwhelmed with the amount of information presented by enormous resources.

Reflection : When the electromagnetic wave impinges upon an object with very large dimensions as compared to its wavelength.(like buildings, walls)

Diffraction : When  the path between Transmitter and Receiver is obstructed by a surface with sharp irregularities.

Scattering : When the medium has objects smaller compared to the wavelength.


Now lets get to Propagation.If you go the wiki page of Radio Propagation, it says, " Radio propagation is the behavior of radio waves when they are transmitted, or propagated from one point on the Earth to another, or into various parts of the atmosphere."
Large scale propagation models deal with predicting mean signal strength for an arbitrary Transmitter-Receiver separation distance, they are called large scale as they characterize signal strength over a large distances. One most important formula one needs to know is the Friis equation. This describes the signal strength over the distance. 

 Small scale propagation models characterize the rapid fluctuations of received signal strength over very short distances or short duration of time.
Small scale fading can be further classified into Flat and frequency selective fading. Another way pf classification is fast and slow fading.

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Follow Up : Spectrum

Now that we have reviewed the first generation systems and before we move to the concepts and principles that enable these systems, Lets learn more about spectrum.

According to Wikipedia, Spectrum is is a condition that is not limited to a specific set of values but can vary infinitely within a continuum. Complex isn't it? Its a Latin word that means image.. We are purely interested in Radio spectrum and its properties.

Radio Spectrum : Consists radio frequencies , i.e. frequencies less than 300GHz. It is important to note that the generation of radio waves is strictly regulated by the governments / regulatory bodies all over the world. We will discuss the need of this mandate when we talk about Interference.

To begin with ITU has divided the entire radio spectrum (Ranging from 3kHz to 300GHz) into 9 bands. The band numbers start from 4. The signals with frequencies less than 3kHz are used for submarine communication due to the obvious reasons.

Every band has a band plan.It is rather interesting to study the band plans. It provides huge amount of information about the band usage. It actually has some scenarios which I hadn't even thought of!


Spectrum Management is going to be a very important word for us due to the limited resources and ever increasing demands..Spectrum Planning, Digital dividends will have more and more importance in the coming years.

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The First Generation

The first generation mobile systems were developed in the United states, European nations and Japan around the same time. This is the only "Analog Generation" in the mobile telephony.
As we are talking about 1980s, this was the time when telephone systems were deployed only for voice. There are two most important characteristics of the first generation or 1G systems.

1. They were analog systems
2. There was no standard accepted across the globe.

There were several different standards and systems in use for the 1G systems. For example, NTT, AMPS, ETACS, NMT-450, NMT- 900 etc. NTT(Nippon Telephone and Telegraph was the first cellular system and was deployed in 1979.
Lets have a closer look at these different systems.

1. AMPS : The Advanced Mobile Phone System (AMPS) was commercially deployed by AT&T Bell labs in the year 1983 in and around Chicago.These systems used Frequency division Multiple Access(FDMA). The bandwidth was 30kHz. The Downlink frequency used 869-894 MHz where as the Uplink frequency was 824-829 MHz. AMPS implemented Frequency Division Duplexing(FDD). There were total of 832 channels (416 to each operator as only two operators were permitted in every market).These 416 channels were divided as 21 control channels and 395 channels for voice traffic.
Later systems with AMPS standards were deployed in South America and Asia as well.

2. ETACS : The Total Access Communication System was launched in UK and had following specifications. This system was deployed with a channel bandwidth of 25kHz,  D/L frequency of 916-948 MHz and U/L of 871-904 MHz. Frequency modulation was used for this FDMA , FDD system. The number of channels here were 1240.

3. NTACS : This is a second European standard with D/L frequency of 860-870 MHz and U/L of 915-925 MHz and a channel bandwidth of 12.5 kHz. Similar to other first generation systems this was a FDMA, FDD system using frequency modulation with 400 channels.

4. NMT 450 : This Nordic Mobile Telephone system was launched in Norway, Sweden and some other countries. The most important feature of this system was handover support. The automatic handover could support international roaming. The general specification include frequency band from 450-470MHz, channel bandwidth of 25kHz. This system could support 200 channels. Other specifications are similar to any other 1G system.

5. NMT 900 : This system is similar to the NMT 450 discussed above with following modifications channel bandwidth was decreased to 12.5kHz and frequency bands used were 890-960 MHz. 1999 channel were supported.

So these are the first generation systems. I feel it is important to know about all the previous generations if we want to innovate in the current standards.

In a nutshell the first generation systems were FDMA , FDD and frequency modulated systems. These analog systems were designed for voice only.

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Follow Up : The History of Wireless Systems

There are two fantastic books about the history of wireless. As I earlier said, if one wants to innovate or revolutionize anything, it is necessary to understand the origin. For innovation in wireless technology and standards, it is important to know the origin of the wireless systems. In the the post for this month, I have already discussed the history. As this is the follow up post, I will be writing about the two books I found particularly interesting. There are number of books available on this subject but I loved these two.

1. History of Wireless By : T. K. Sarkar, Robert Mailloux , Arthur A. Oliner, M.Salazar-Palma , Dipak L. 
                                          Sengupta

2.The History of Wireless: How Creative Minds Produced Technology for the Masses By Ira Brodsky

I am just trying to give a summary of these.

The first book, History of wireless. This book contains everything!! The chronology of events is all inclusive and it starts from 2637BC, since the development of magnetism. I am still reading the Maxwell's equations part of it. The authors have covered the theoretical and historical details starting from the development of magnetism covering telegraphy, radio upto modern wireless systems.

The second book is an interesting read. It is almost like a novel. Different chapters for different developments. The author starts with the magic/science debate behind the wireless and then after describing all these technical developments he talks about the future of communication. This book is a must read for any tech-enthusiast.

Another great video from Ericsson covering major milestones

http://www.youtube.com/watch?v=X5jPoQzEh-M

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And it all began..

Lets go back and trace the history of the world of wireless. Did it all begin with with Guglielmo Marconi showing us how the radio works? No it goes a generation back..

It all started with Maxwell's equations!! I am sure everyone remembers Maxwell's equations..I always struggled to understand then and most importantly solve problems based on those in my exams..:):)

So in 18th centure James Clerk Maxwell laid the foundation of modern communication systems. He proved the existence of the existence of Electromagnetic waves.  

This set of equations is the foundation for development and evolution of wireless communication systems.

Then later in the same century, Prof Hertz validated the Maxwell's equations by certain experiments. The eighteenth century ended on a very high note as Guglielmo Marconi sent morese radio signals over a distance more than a mile. Who would have thought that this distance of a mile is going to change to hundreds of miles in the coming century. Marconi demonstrated the wireless telegraph to British post office and hence became the father of long distance transmission of radio transmission.

The nineteenth century came with the betterment of radio communications.

In the first year of the century Marconi achieved another "impossible tasks" when he showed the transmission of radio signal across the Atlantic ocean. Now there was a new possibility on the horizon "Wireless communication"..

Later first voice over radio was shown in the United states. Around 1920s to 1940s this voice transmission over radio waves was mostly used in the police cars. The world wars were another strong driving force in the development and evolution of wireless communication.

From late 1940s a completely new system of mobile telephony was introduced. There onwards the the mobile phones have seen PSTN, AMPS(first generation) , GSM, IS-95 etc..Then in 1990s Qualcomm came up with the CDMA technology and the third generation came into picture. As we are already witnessing the fourth generation and moving towards the advanced long term evolution the communication standards have traveled from voice to high speed data traffic.

Also the number of connected devices is said to reach 15 billion by 2015!!

Thats more than twice the current world population!!

Now as we are getting higher data rates and increasing voice and data traffics we have a brand new set of challenges and many problems to solve.

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