Saturday, December 1, 2012

Hack Apple TV for Watching Chinese TVs and Videos

[Hack Patriot Box Office for Watching Chinese Videos and TVs]
Figure 1.  Apple TV
Apple TV (ATV) is a multi-function set-top box designed by Apple Inc. originally as a networked media player for streaming multimedia contents to a television or an external monitor.  Now, it seemingly evolves into a iPod Touch without a touch screen but with a relatively inexpensive MSRP price tag of $99.  (Indeed, if you have an old generation iPod Touch, you may think about mirroring or airplay it onto a television instead.)  On March 7, 2012, Apple Inc. released a 3rd generation ATV (ATV3)  that includes a 32 nm ARM Cortex-A9 Apple A5 SoC (the same process as used in a 5th generation iPod Touch, a new iPad 2 and an iPad Mini) and 512MB mobile DDR2 memory (the same amount of memory as used in an iPhone 4).  An ATV3 has a buit-in 6 W power supply with no cooling fan necessary.  Unfortunately, until now, there is no public released jailbreak available for an ATV3, so modified software or apps such as XBMC cannot run on it.

Because Apple TV (ATV) is such an elegant device with a small footprint and intuitive to use functionalities, I know many people want to use it for watching all kinds of internet videos, e.g., those Chinese TVs and videos widely available over the internet.  Hereafter, a small hack or trick will be explained for this purpose.  A nice feature of this hack is that no modification on either the hardware or the software is necessary on your Apple TV.  No jailbreaking is required and therefore, the manufacture warranty is fully preserved.  

In this blog, I will explain a set of configurations applicable to an ATV3 with Apple TV iOS 4.4+ updates, which include the app, iTunes Movie Trailers.  The set of configurations include several ATV client side configurations and two optional web server side configurations.  If you can fluently read Chinese, here are references to this hack:
Before starting the configurations, it may be necessary for you to: 
  1. verify that your ATV is powered and properly connected to your television;
  2. verify that you can navigate the Apple TV interface;
  3. verify that your ATV is properly connected to your network;
  4. verify that the network is properly configured.  
For "How to start your ATV?," you may check online references such as  or

In fact, the original Trailer app is a link pointing to a XML file hosted on a apple server:  Apparently, a Chinese ATV fan figured a way to set up his own DNS server@ and to configure an ATV to direct the original Trailer link to his own XML file.  In the following, the set of ATV client side configurations teach you how to set up your Apple TV and redirect he original Trailer link to a new XML file hosted on a Chinese server.  After this, another set of ATV server side configurations will teach you how to further personalize the new XML file.

ATV Client Side Configurations

  1. ATV Network Configurations.
    1. From the main Apple TV menu,  choose Setting -> General ->  Network -> Configure TCP/IP -> Manual
    2. Don't change and keep your workable "IP Address"
    3. Don't change and keep your workable "Subnet Mask"
    4. Don't change and keep your workable "Router"
    5. When prompted, change "DNS Address" to ""
    6. Press the Menu button on the remote once to return to the previous screen.
  2. ATV iTune Configuration
    1. From the main Apple TV menu, choose Settings -> iTunes Store. 
    2. Confirm "Location" is "United States".  Otherwise, scroll to highlight "United States" and select it.
    3. Press the Menu button on the remote once to return to the previous screen.
  3. Find an UID of your ATV and/or register for an account for later server side configurations

    1. From the main Apple TV menu, choose Trailers and you may find that a homepage of the Apple app, Trailers, becomes a new homepage, on which there is a "Apple TV" link listed in the middle next to three popular Chinese video sites such as iQiyi, Sohu video and PPTV listed on the top.
    2. From the main Apple TV menu, choose Trailers -> Apple TV -> Personal -> Personal Links
    3. Find a sentence "个人 xxxxxxxxx " and this xxxxxxxx is the UID for your ATV.
    4. Alternatively from the main Apple TV menu, choose Trailers -> Apple TV -> Personal, you can register an account

Web Server Side Configurations (Optional)

After you finish the above client side configuration on your Apple TV,  you should be able to find several pre-loaded apps or links displayed after you click the Trailer app. Among the pre-loaded apps, there is a "Personal app" or "个人 app" in the middle of the screen.  This "Personal app" or "个人 app" provides a directory storing links to your favorite programs or TV sources.  Here are two approaches for configuring your "Personal app" or "个人 app:"  
  1. Approach 1:  If you have found your UID from your ATV and want to use it for the server side configurations, go to 
  2. Approach 2: If you have registered an account on your ATV, you can go to

Friday, April 27, 2012

How to Improve Forward Link Positioning for Cellular Networks? III. Hearability and Accuracy

How to Improve Forward Link Positioning ... ? I. Introduction
1x HDP Enhancements
Enhanced Location Based Services Support in cdma2000
Enhance Downlink Positioning in WiMAX/16m
How Wide A Widband Channel Should Be?
IEEE ICC 2008 Tutorial, Location Based Services for Mobiles
Location Based Services for Mobiles: I. Introduction

Hearability Issue

Hearability of a forward-link positioning system usually is quantified by how many reference signals a terminal may utilize to make a positioning fix in a pre-defined positioning duration.  In theory, a terminal need measure parameters of only 4 different reference signals for a precise three-dimension fix.  However, more reference signals a terminal can use, more diversity benefits a terminal may use for a more accurate positioning fix.

A hearability issue of a cellular positioning network generally is a dimension limitation issue.  It mostly is due to limitations of network geometry and network deployment.  In other words, it is a network issue.  For example, for a given cellular network, say a CDMA2000 1x RTT network or a WCDMA network, its hearability mainly depends on a network topology of the cellular network and a frequency reuse factor of the cellular network.  The network topology including network sectorization may affect achievable DoP values for positioning.  The frequency reuse factor may have a significant impact on co-channel interference to a terminal, which in turn relates to the positioning accuracy achievable by the terminal.  Hearability of an exemplary CDMA2000 1x RTT network is shown in Figure 1.

Figure 1. The hearability of CDMA2000 1x Pilots for AFLT, IEEE ICC 2008 "cdma2000 Highly Detectable Pilot" 
On the other hand, since major considerations for an actual deployment of cellular network base stations are voice and data service capacity, environmental impact and financial limitations, etc.,  a mobile phone network usually is not optimized for mobile positioning in nature.

Accuracy Issue

Positioning accuracy of cellular network forward link positioning is a dimension limitation issue.  Mainly the accuracy is limited by a frequency reuse factor and available bandwidth.  In general, given a certain positioning duration, wider bandwidth received reference signals have, more uncorrelated signal samples a terminal can obtain.  On the other hand, it is known that an achievable SNR highly depends on the frequency reuse factor of a cellular network.  More particularly, a CRLB of the achievable positioning accuracy is asymptotically linear to the number of uncorrelated signal samples and SNR value in dB.

In addition, from a signal processing or a receiver design perspective, a correlation between received signal samples largely depends on a sampling frequency on the received signals and achievable multipath resolution.  Multipath resolution is a function of both a channel delay profile and a bandwidth of received signals.  For example, a statistic delay profile of an exemplary cellular network is shown in Figure 2.  Additional discussions on the statistic delay profile can be found in another blog, "How Wider A Wideband Channel Should be?".   In general, wider the bandwidth of a transmit signal is and  higher the multipath resolution of a channel is achievable.

Figure 2. A statistic model of delay spread. 

Additional Reference

[1] E. Sousa, V. Jovanovic, C. Daigneault, “Delay spread measurements for the digital cellular channel in Toronto”, IEEE Trans. on Vehicular Technology, Nov 1994
[2] J. Ling, D. Chizhik, D. Samardzija, R. Valenzuela, “Wideband and MIMO measurements in wooded and open areas”, Lucent Bell Laboratories,
[3] K. Baum, “Frequency-Domain-Oriented Approaches for MBWA: Overview and Field Experiments”, Motorola Labs, IEEE C802.20-03/19, March 2003
[4] L. Greenstein, V. Erceg, Y. S. Yeh, M. V. Clark, “A New Path-Gain/Delay-Spread Propagation Model for Digital Cellular Channels,” IEEE Transactions on Vehicular Technology, VOL. 46, NO.2, May 1997, pp.477-485.
[5] A. Algans, K. I. Pedersen, P. Mogensen, “Experimental Analysis of the Joint Statistical Properties of Azimuth Spread, Delay Spread, and Shadow Fading,” IEEE Journal on Selected Areas in Communications, Vol. 20, No. 3, April 2002, pp. 523-531.
[6] Spatial Channel Model AHG (Combined ad-hoc from 3GPP & 3GPP2), “Spatial Channel Model Text Description ”, 3GPP, 2003

Friday, April 13, 2012

How to Improve Forward Link Positioning for Cellular Networks? II. Fundamentals

How to Improve Forward Link Positioning ... ? I. Introduction
1x HDP Enhancements
Enhanced Location Based Services Support in cdma2000
Enhance Downlink Positioning in WiMAX/16m
How Wide A Widband Channel Should Be?
IEEE ICC 2008 Tutorial, Location Based Services for Mobiles
Location Based Services for Mobiles: I. Introduction

In general,  how to improve forward link positioning in a cellular network is a mix of dimension limitation issues and interference issues such as co-channel interferences.  It also relates to the well-known near-far issue.  In an actual development and deployment of a cellular network, both link budget and design target of the cellular network should be considered for improving forward link positioning.  Due to the complexity nature of this engineering issue, it is necessary to study it from various aspects and understand scalability of each involved engineering factor.  In the following, this topic is studied respectively from information theory, receiver design and engineering deployment perspectives.

A Information Theory Perspective

Information theory usually tells us how far we may go under a certain set of limitations.  More particularly, it may also help us understand how an optimal performance may scale with various combination of different inputs.  From an information theory perspective, a lower bound on the second order statistic of a signal parameter estimation is the well-known Cramer-Rao Lower Bound (CRLB), which in turned is decided by an inverse of Fisher information for unbiased cases.  Basically, this means that the achievable accuracy is dominated by the number of available uncorrelated samples and an achievable SNR value during a given signal estimation duration.  Appearantly when more uncorrelated signal samples and higher SNR are available, a more accurate positioning fix is possible. However, a CRLB may move up or down in different directions when the number of signal samples and/or a received SNR value change.  As shown in Figure 1, a root mean squared error (RMSE) in dB of positioning accuracy may have different linear relationships with SNR in dB and the number of samples. 

Figure 1. The scalability of CRLB

Asymptotically, this relationship may also be expressed as 

lg (  ΔRMSE  ) ~ O( SNRdB )  or  O( Nsample )

A Signal Processing Perspective

From a multiuser signal processing perspective, a near-far problem means a simple increase of forward link signal power may not help improve the SNR for forward link positioning.

A Link Budget Perspective

A standard representation of cellular networks can be sown in Figure 2, where the frequency reuse factor is K = 3. From a link budget perspective, the SINR and coverage of received reference signals are determined by both cochannel interference (CCI) and path loss. CCI is a function of both the distance to neighbor cells and frequency or time reuse factor K.  In general, the strength PCCI of CCI linearly decreases with the increasing of the reuse factor K. Its strength in dB is linear to the log of the average cell size of a cellular network. This can be shown as

PCCI  ~ O( K )  and  lg( PCCI ) ~ O( cell size in dB )

Figure 2. An illustration of cellular networks with a frequency reuse factor of 3.

Monday, April 2, 2012

How to Improve Forward Link Positioning for Cellular Networks? I. Introduction

How to Improve Forward Link Positioning ... ? III. Hearability and Accuracy
1x HDP Enhancements
Enhanced Location Based Services Support in cdma2000
Enhance Downlink Positioning in WiMAX/16m
How Wide A Widband Channel Should Be?
IEEE ICC 2008 Tutorial, Location Based Services for Mobiles
Location Based Services for Mobiles: I. Introduction

There are many ways locating a mobile in a cellular network. The basic methods include dead reckon, proximity sensing, signal signature tracking, trilateration, multilateration, triangulation, etc. Some major approaches for cellular networks can be illustrated in Figure 1.

Figure 1. Mobile Positioning Techniques, IEEE ICC 2008 "Location Based Services for Mobiles"

Positioning performance is understood through quantifying the two parameters, hearability and accuracy. Hearability indicates how many hearable good references a terminal can track in a single snapshot. A typical number should be not less than 4. Accuracy is a well-known parameter for understanding the stability of each fix.  Usually it is quantified through the 2nd order statistics of fixes. A comparison of several mobile positioning schemes is shown in Figure 2.

Figure 2. A comparison of mobile positioning schemes
For cellular network forward link positioning, its performance is determined by 1) reference signal bandwidth, 2) channel delay spread profile, 3) co-channel interference  and 4) dilution of precision ( DoP ) or network topology.

Saturday, March 24, 2012

Evolving Random Access Channel: III Power Control p.k. Access Timing

Access Channel Enhancements for 1x Rel. F
One Eighth Rate Access Probes for Smart Terminals
Evolving Random Access Channel: I Introduction
Evolving Random Access Channel: II Slotted ALOHA Models

During the discussion of evolving access channels for machine-to-machine (M2M) communication liked services, two major requirements are defined. As stated by 3GPP2 system requirement document, one is "[a]n M2M service shall not impact any ... existing applications and services that are currently being supported". The other one is "[t]he scalability of M2M Device deployment shall be supported allow flexible M2M Device and service deployment scenarios for large numbers of M2M Devices."  More details of these two requirements can be found in 3GPP2 Machine-to-Machine Communication System Requirements, S.R0146-0 and 3GPP M2M Service Requirements, ETSI TS 102-689. Due to the nature of M2M traffic pattern and QoS requirement, recent studies find out that uplinks, especially existing access channels, will become the bottleneck when M2M services are widely deployed in the future. Evolving access channel once again becomes a hot topic since it was optimized for short message service (SMS) in 1990s.

Minimize Collision and Interference: Power Control and Access Timing

Of the two requirements, let's start from how to minimize the impact of new on legacy services at first. Doing so is not only required by service providers but also because these two requirements don't always conflict with each other. From random access channel fundamentals, it is not hard to understand two key factors determining the collision and interference between probes, access timing and power control. Access timing information is about when access probes will start using access channel. Access timing is not only the key assumption for many ALOHACSMA MAC protocols, but also important for new access probes to decide the best time using the shared access channel with the minimum overlap to legacy probes. The second factor is perfect power control. This becomes critical is because when access channel is loaded in the future,  it is impossible to completely avoid the overlapping between access probes even though the perfect timing information of the channel might be available. At this time, perfect power control on new access probes becomes necessary in reducing interference to legacy services. Based on these two factors, two optimal access channel designs will be presented and discussed in the following.

Optimal Access Probes: Dumb Probes p.k. Smart Probes.

If the perfect access timing information of legacy access probes is available, one optimal access channel design is to use very short and smart access probes, which know the activities of the other probes and are able to try its best to arrive inside some access gap. In order to have little overlap with other access probes, it is nature for a smart access probe to be short. While it is necessary to maintain a minimum payload during a short period, a smart and short access probe usually demands high transmit power and data rate. How smart probes work with legacy probes can be illustrated in Figure 1.

Figure 1. A traditional Thinking of Improving Access Probe Design
Though smart probes can have minimum impact on legacy probes, it is very hard to be implemented. In reality, it is extremely challenging for an access probes to know the instant access timing information of other probes, even though they may be co-located next to each. In a typical scenario for mobile communication, each mobile has no timing information of any other mobile without the help of base stations.  When the perfect access timing information is unavailable, one optimal access channel scheme is to send very long and dumb access probes instead. Here “dumb” means each of these access probes has no any access timing information of other probes. However, in order to minimize any impact to other probes, including current and future probes, each dumb probe still wants to take advantages of access gaps of access channel. To achieve this, each probe has to be transmitted at a very low data rate and span a very long duration with perfect power control.  A low transmit data rate spanning a long duration usually means a low transmit power. Though a dumb probe does look "slow" and cautious, it also means potentially more diversity opportunities are achievable.
Figure 2. A New Thinking of Improving Access Probe Design
Now as you can see, there are two different starting points for designing optimal access probes for minimizing the impact to legacy services. For smart probes, timing information is the challenge and key.For dumb probes, perfect power control is the key. A comparison of these two optimal designs can be found in the following table.

Table 1. A Comparison of Dumb Probes and Smart Probes

Monday, March 5, 2012

RTP Packetization for H.264 NAL Units

H.264 Network Abstract Layer Header
How to Broadcast Multimedia Contents?

RFC3984 (RTP Payload Format for H.264 Video) defines 3 modes: 0 (single NAL unit mode), 1 (non-interleaved mode), and 2 (interleaved mode).  Mode 0 requires you either use UDP fragmentation or tell the encoder not to generate NALs larger than MTU-X. Mode 1 lets you do fragmentation. Details of how to set up a fragmentation unit packet can be found in the RFC. Basically the fragmentation information is on the front. Small NAL units, e.g. SPS and PPS packets, can be aggregated together using single-time aggregation packets (STAPs). Each packet requires normal RTP headers with incremented sequence numbers but the same timestamp. Though a mark on the last RTP packet of a frame is expected,  it is not guaranteed. Mode 2 lets you fragment, combine, and interleave the transmission order to change how a burst loss will affect a stream, among other things.

Friday, March 2, 2012

Evolving Random Access Channel: II Slotted ALOHA Models

Access Channel Enhancements for 1x Rel. F
One Eighth Rate Access Probes for Smart Terminals
Evolving Random Access Channel: I Introduction
Evolving Random Access Channel: III Dumb Access Probes and Smart Access Probes

It is well known that the access channel design of 3G cellular mobile network is based on ALOHA models. More specifically it is based on a single-channel single-hub slotted ALOHA plus open-loop power control.  Indeed, there are many possible variations of slotted ALOHA.  Considering access channel configuration, there are the cases with a single shared access channel and the cases with multiple access channels.  Considering network configuration, there are single hub models and multi-hub models. Though multi-channel slotted ALOHA models have been intensively studied in computer engineering domain, a ALOHA model with more than one participating hub and more than one access channel hasn't received enough attention so far. I guess one reason is the original ALOHA models were proposed for satellite communications, where typically it is a single-cell or single-hub service scenario. Another reason I guess is the current computer engineering focuses more on network topology and MAC layer above issues instead of related channel modeling and PHY layer challenges. My feeling is the study of a multi-hub model with PHY-layer macro-diversity usually belongs to the stronghold of communication engineering. Conceptually the access channel of a cellular network is also more closely related to a multi-hub slotted ALOHA model, which has either single acess channel or multiple access channels.

Access Channel Design Challenges

It is well-known that there is a tradeoff between access capacity and throughput in ALOHA models, including both slotted and pure ALOHA.  Basically say, after a certain point, more access load will result in lower access success rate, which usually in turn results in more access delay due to retransmission. This tradeoff can be illustrated in Figure 1.
Figure 1. The Access Channel Access Capacity and Throughput Dilemma
In addition, a traditional CDMA2000 access channel can be modeled as a classic slotted ALOHA, i.e., single-channel single-hub slotted ALOHA. In this design, each base station only detects the access probes with its own access channel mask. Though it is well-known that additional macro-diversity gain is achievable with multiple base stations detect one access probe and the standards also doesn't prevent a base station chipset manufacture from implementing a base station which can detects the access probes to both itself and neighbor base stations, the resulted implementation complexity increase, in both PHY layer, MAC and upper layers, can be very high. One simple illustration of this tradeoff of diversity gain and complexity increase can be shown in Figure 2.
Figure 2. The Access Channel Search Complexity and Achievable Macro-Diversity Dilemma

Then one question is, what is the big deal of macro-diversity for access channel? It helps reduce transmit power, therefore, reduce interference and improve battery life. It helps mitigate network imbalance. It can also help network positioning.

Macro-diversity can help reduce network imbalance. There are two kinds of related network imbalance issues. One is forward link and reverse link imbalance. The other one is the reverse link load imbalance among base stations. Network imbalance is not only because of the non-uniform distribution of terminals but also because it is hard for a terminal to make a good decision on which base station it should point its access probes to without knowing base station side rise over thermal (RoT) condition.  This is important especially when a network user capacity is high, the cell coverage is large and the network is heavily and non-uniformly loaded.  Further more, since the existing access procedure simply asks a mobile to point its access probes to a sector typically with the strongest forward link pilot channel (F-PICH), this approach apparently is suboptimal. Therefore a new network load aware access probing mechanism is necessary in this case.

Access Channel Design Models

Table 1. A Comparison of Different Slotted ALOHA Models for  Mobile Network Access Channel Design

New Access Channel Proposal

Table 2. A Comparison of Different Access Channel Designs for Mobile Network

Wednesday, February 29, 2012

Evolving Random Access Channel: I Introduction

Access Channel Enhancements for 1x Rel. F
One Eighth Rate Access Probes for Smart Terminals
Evolving Random Access Channel: II Slotted ALOHA Models
Evolving Random Access Channel: III Dumb Access Probes and Smart Access Probes

Random access channel (RACH) is an contention-based uplink common channel resource, which is shared by multiple access terminals. The typical application scenarios, which trigger random access procedure, include
  • initialize access from power up or radio link failure,
  • initialize access from idle state,
  • initialize access to the handover target,
  • initialize access when uplink synchronization is unavailable.
In addition, it can also be used by a terminal for registration update, location update and to send a small amount of data on the uplink. A key feature of random access channel is that messages are not scheduled and there is no certainty of collisions. This is different to a dedicated channel, which is exclusively assigned to one user at each time.

In 3GPP, it is defined as a common transport channel in the uplink, which is one-to-one mapped onto physical random access channels (PRACHs).  In current UMTS standard, RACH can also be used to send small amount of data on the uplink, which is quite different from the approach taken by 3GPP. In 3GPP2, there are reverse link access channel (R-ACH) defined in IS-95 and reverse link enhanced access channel (R-EACH) in IS-2000. 

Thursday, February 2, 2012

Evolved Macro-Diversity: CDMA2000 and UMTS

Macro-diversity typically means a special communication mode between a single mobile station and multiple base stations in a cellular network. It has been in CDMA standards as soft handoff since the beginning. The people in the industry usually think "soft handoff" and "macro-diversity" are interchangeable in most scenarios. Recently it has been employed for CDMA2000 BCMCS and UMTS MBS too. The basic idea is to coordinate multiple base stations to deliver the same data stream to a mobile receiver in the down links and receive the signals from a mobile station from multiple base stations. Macro-diversity is possible for CDMA soft handoff because there is no hybrid automatic repeat request (HARQ) for voice data and no fast retransmission is necessary due to the strict delay requirement of voice service. The benefits of doing soft handoff on voice service include reduced transmission power and seamless mobility. On the other hand, it also challenges the mobile station's capability to handle additional multipaths.

In the scenario of high-rate data delivery, HARQ is necessary for taking advantage of inaccurate channel estimation as well as channel fluctuation. Since the channels between the mobile and each base station in its active list are generally different and there is no fast link among involved base stations, therefore it is inherently difficult to do soft handoff for high-rate data delivery service.  In addition, the amount of fading resulted from soft-combining multiple channel may diminish and this may result less achievable time diversity gain. Therefore, macro-diversity is only applicable for the data delivery services, where there is no HARQ, for example, BCMCS and UMTS MBS. For the case of the macro-diversity of soft over the air combining, the achievable ergodic capacity is

C1 = B * E{ log2[ 1 + ( S1 + S2) / N ] }

However, things changed a little bit more recently. the simultaneous communication between multiple base stations and a single mobile is proposed for EV-DO Rev. C in a new term, single-carrier multi-link. For LTE-Advanced (LTE Release 10/11) , it is called Coordinated Multi-Point transmission and reception (CoMP).  When two data streams from two base stations are independent from each, it essentially is a way of spatial multiplexing, in which interference cancellation is one of the key receiver element for the mobile station to achievable maximum throughput. However, considering the independent fast multiuser scheduling and HARQ are used by each access network or eNodeB, it is very challenging for mobiles to do successive interference cancellation. For the case of the spatial multiplexing without interference cancellation, the achievable ergodic capacity is

C2 = B * E{ log2[ 1 + S1 / ( N + S2 ) ] } + B * E{ log2[ 1 + S2 / ( N + S1 ) ] } ≤ C1

Tuesday, January 31, 2012

Hack Patriot Box Office for Watching Chinese Videos and TVs

As requested by friends, this blog is dedicated to explain a simple way for watching Chinese TV programs and movies on a television or monitor for free using a special media player instead of a computer.  Compared with many well-known approaches watching Chinese TVs on a computer, this one seems much more operation-friendly as well as eco-friendly.  It is attractive for many Chinese families who may not always be technical savvy.  In addition, as I know, the operation cost or power consumption of this kind of MIPS-based media players is relatively low.  Its total power consumption usually is no more than 15 watts.  Meanwhile, a typical power consumption of a PC CPU, even some mobile or laptop CPUs,  itself is known to be between 50 watts and 100 watts.  Even a power efficient Intel Atom CPU usually is inside a range between 5 and 20 watts, as I remember.  Nowadays a typical home desktop demands a power supply of no less than 250 watts, not to mention that a powerful game desktop or work station easily require a power supply of at least 400 watts.

The media player which I am introducing here is a Patriot Box Office High-Definition Media Player PCMPBO25 which a Realtek RTD1073DD SoC based networked player and made by Patriot Memory.  It has about 128MB SPI flash, 128MB DDR2 SDRAM and a 400MHz MIPS core.  One nice thing about this player is Patriot Memory hosts a very OPEN and friendly support forum for it and shares a lot of details of its firmware.  This not only makes this player hard to be bricked but also enables many mods and hacks.  One simple mod I am going to introduce here is to update its firmware for watching Chinese movies and TV programs.

The procedure for updating its firmware has been posted on Patriot Memory support forum.  Now, it is slightly modified and reposted here for your convenience.
  1. Watch over composite hookup if possible ( Comment: Though I have hacked many PBOs with watching over HDMI without any issues so far, YMMV);
  2. Download firmware;
  3. Unzip/unrar;
  4. Copy "install.img" file to the ROOT directory of an FAT32 FORMATTED USB drive ONLY;
  5. Put the USB drive into the front usb port of Patriot Box Office;
  6. Power up both PBO and TV and choose the PBO input on TV menu;
  8. The screen will black out for about a few seconds until the update process initiates. The whole update process may reboot & resume, please DO NOT remove the usb drive UNTIL the screen goes back to the setup page; and
  9. (Optional) Additional update might be necessary if the remote doesn't work after the above update.
As far as I know, the newest unofficial firmware supporting China videos, movies and TVs is the one posted on HDP Fans Forum [11/2011].

Friday, January 20, 2012

What Is The Next for Mobile System Design? I: A Single-Cell Model Perspective on Downlinks

Interference Cancellation: A Short Overview
How to Broadcast Multimedia Contents?

[Note] Due to the asymmetry between the uplinks and downlinks of a mobile network, there are different considerations, tradeoffs and techniques for designing each directions. In general, with the recent advance on uplink interference cancellation and management techniques, mobile network is usually limited by downlinks due inter-cell interference, especially when delay is a key part of the equation. On this blog, my focus will be on downlinks. How to evolve mobile system uplinks will be discussed in separated blogs.

Mobile system design usually starts from our understanding of wireless channels and the services customers are demanding. The properties of various wireless channels can help us understand the system design limitation we are facing and the potentials we may achieve.  For example, COST 231 model, which was developed by European COST Action 231. Its variations are the most popular radio propagation models used in almost every wireless standardization body, including 3GPP, 3GPP2 and IEEE. Its modifications include COST 231-Hata Model and COST 231-Walfisch-Ikegami Model. One nice thing of COST 231 channel model is it helps us understand the tradeoff between reception and coverage we are facing in a typical single-cell environment.
Figure 1. Spectral Efficiency and Coverage Tradeoff

As shown in Figure 1, with a 300-meter-tall transmitter antenna, we can see that the path-loss changes 0.66 dB at every 90% coverage change, 1.39 dB at every 80% coverage change, 2.22 dB at every 70% coverage change and 3.18 dB at every 60% coverage change. In general, if you want more coverage, then you may lose some capacity especially on the cell-edge.  Otherwise, you have to shrink your coverage.

Figure 2.  What we want to achieve.
As shown in Figure 2, though there is a fundamental tradeoff between coverage and performance we are constantly facing in our system design, customers always desire their mobile network having both better coverage and higher performance for less. Now the challenge to us is how to push up the system design boundary. There are at least three major approaches available to push the envelope. They are 1) interference cancellation and management, 2) multi-antenna technology and 3) cells cooperation and relay.

Figure 3.  Mobile System Design Options
Interference cancellation (IC) and management are the key ingredients for mobile network to achieve optimal performance. There are many ways to do interference cancellation, linear ICs (decorrelating detector, MMSE IC) and Nolinear ICs (joint detection, decision feedback IC ). Interference management can be done in time, frequency and space domain. OFDMA-liked multiplexing scheme is friendly to interference management.  MIMO can help meet the demand of high data rate and high link quality. It can not only help improve link quality through spatial diversity and beamforming but also help achieve higher data  throughput using spatial multiplexing and multiuser MIMO. The third weapon is heterogeneous transmission and deployment, which can help improve the network throughput as well as cell-edge user experience. Cooperation between cells is not something very new. Starting from 2G/IS-95, there has been soft handoff for macro-diversity. Additionally in 3G, we did it for broadcast multicase service over mobile networks, e.g., CDMA2000 BCMCS or UMTS MBS. However, all these cell cooperations are coordinated by MSC.  More recently, LTE-Advanced standardized X2 interface between eNodeBs belonging to the same MME.  This makes neighboring cells cooperation, such as corrdinated multi-point transmission and reception (CoMP), inter-cell interference coordination (ICIC) and relay, a reality. Similarly in CDMA2000 EV-DO Rev. C, there is a feature called single-carrier multi-link (SCML), which essentially extends the capability of multi-carrier devices in a single-carrier environment.