Sunday, May 27, 2007

[WiMAX] Fundamentals of WiMAX - (3)

Today we are talking about one of the two core technologies of WiMAX: Multiple Antenna Techniques (let's abbreviate in MAT). Another important one is OFDM, which will be deferred to next topic. Like all 3G systems picked CDMA, we can anticipate that perhaps almost all upcoming 4G systems will employ OFDM and MIMO (Multiple Input Multiple Output) techniques. (Recent an interesting news is that Qualcomm just gave a tough talk on the patents of 4G. )

What can MAT provide? Generally speaking, MAT supports
1) spatial diversity,
2) beamforming,
3) and spatial multiplexing.

In old times (1970s), people only use multiple antenna in receiver owing to the cost and size consideration. In this case, with selection combining or maximal ratio combining (MRC), the performance under fading environment can be improved significantly due to receive diversity.

Later, people noticed that using MAT with beamforming can concentrate the power of the desired signal in a specific direction and lower down the interference to other links away from this direction. This is especially useful for multi user network. They call it smart antenna technique. One example is the TD-SCDMA system in 3G area.

In the late of 1990s, a key breakthrough was made for MAT. Transmit diversity schemes were invented, such as the space-time block code (STBC). Now the promising area of MAT is the spatial multiplexing.

For the receive diversity, you can find it from most of the wireless communication books. So we will focus our concentration on the transmit diversity and beamforming.

1) Transmit diversity
There are two types of transmit diversity, one is open-loop, another one is closed-loop. Open-loop means at the transmitter you don't need a feedback for the channel state from the receiver. On the contrary, closed-loop requires it. Thus you should guarantee the channel is with reciprocity. In some cases, you can assume the channel to be reciprocal, although it isn't.

Open-loop transmit diversity
The most popular open-loop transmit diversity scheme is STBC, where a special coding scheme (called Alamouti code, name of the inventor, who is currently the CTO of Intel's Mobility Wireless Group) is used in the transmitter. In short, a Nt*Nr STBC achieves the same diversity order and data rate as a Nr*Nt receive diversity system with MRC, but with a 10*log_10(Nt) transmit power penalty because of Nt transmit antennas.

For WiMAX or other OFDM-based system, the space/time coding can be coded over adjacent subcarriers rather than time slots.

Closed-loop transmit diversity
In this case, channel state information(CSI) is required at the transmitter. Because channel changes quickly in a highly mobile scenario, it is difficult to track the channel state in time and feedback it to the transmitter. Therefore, closed-loop transmit diversity tends to be suitable for fixed or low-mobility scenarios. Two important closed-loop schemes are: transmit selection diversity (TSD) and linear diversity precoding (LDP).

In TSD, only a subset of Nt transmit antennas are selected with good CSI, thereby comparing with STBC, TSD has less transmit power penalty because fewer transmit signals are sent.

In LDP, a linear precoder is used at the transmitter and a linear postcoder is applied at the receiver. They are used to improve the link reliability and can achieve higher SNR than the open-loop schemes.

Friday, May 25, 2007

Free talk - Analogy between wireless network and society

Today when I was on the metro bus, an idea jumped into my mind: there is some similarity between the wireless network and the human society.

For example, a sensor network with data aggregation is as a community, a cellular network is like a country ruled by a dictatorial government, a pure ad hoc network can be considered with Anarchism (means abolition of governments), and a network with multiple access points is like a society with democracy (some nodes are 'elected' as access points).

The interesting thing is, what is good for the administration of the society, republic, federal , democracy or autocracy? Similarly, which network topology is effective for a wireless network? Why the existing mobile networks use the cellular architecture? Why most 802.11 networks are configured in IBSS (Infrastructure Basic Service Set)? Think about it...

Saturday, May 12, 2007

[Wireless] Why OFDM for WiMAX and B3G?

In the beginning of 3G (Third Generation) era, CDMA technique was widely employed for most systems. For instance, 3GPP selected WCDMA (Wideband CDMA) for FDD mode, TD-SCDMA for TDD mode; 3GPP2 extended cdmaone to CDMA2000. The main reason is to support higher data rate wireless service in 3G systems than that in 2G systems.

Wideband CDMA system has a large bandwidth, which can be used to achieve frequency diversity. However, there exists a contradiction between the large bandwidth and high data rate which can be shown in the following simple diagram:

Wide bandwidth -> frequency selective channel -> causes large delay spread in time domain
High data rate -> small symbol interval -> experiences serious Inter Symbol Interference

For CDMA system, in order to combat this multipath delay spread, usually it has to resort to the sophisticated RAKE receivers, which resolves multipath using muliple fingers.

However, when wireless system goes to the broadband era, wideband CDMA may not be adequate. Even 3GPP proposed HSPDA/HSUPA to support data rate up to 14.4M/5.8Mbps in 5MHz channel , 3GPP2 proposed 1xEV-DO for data rate 3.1M/1.8Mbps (revision A), 4.9M/1.8Mbps (revision B) in 1.25MHz channel, their capability of supporting much higher data rate is limited. One reason is that they have to use multiple codes simultaneously, which may result in self-interference.

That's why in B3G (Beyond 3G), LTE (Long-term evolution) of 3GPP and AIE (Air Interface Evolution) of 3GPP2 both pick OFDM combining MIMO instead of CDMA. Similarly, in IEEE802.11 series, the low date rate is working on CDMA (802.11b) but high date rate uses OFDM (802.11g and 802.11a). WiMAX is also using OFDM.

Tuesday, May 8, 2007

[WiMAX] Fundamentals of WiMAX - (2)

In this part, we are going to give an overview of the wireless channel that may be experienced by WiMAX.

Generally speaking, the basic and important issues of wireless channel contain three aspects: pathloss, large-scale Shadowing and small-scale fading. You can find detailed descriptions of them in most of the books regarding wireless communications (chapter 4 and 5 of [1]). In summary, pathloss is inversely propotional to the distance between the transmitter and the receiver, Shadowing is mainly caused by obstructions within the propagation environment and is often log-normal distributed, small-scale fading is caused by multipath reflections (modeled by delay spread or coherence bandwidth) or motion between the transmitter and the receiver (modeled by Doppler frequency shift or coherence time).

So we will skip those fundamentals issues and assume that you have already known them very well. However, channel also varies over space if multiple antennae are used. Multiple antenna techniques emerged as a breakthrough in the last several years and some traditional books may lack of description on this part, thereby next we will give an outline of the spatial aspect of wireless channels.



Angular Spread

Like delay spread or Doppler spread, a parameter used to model the channel in space is the angular spread. It refers to the statistical distribution of the arriving signal's angle. A small angular spread means the received signal's energy is more focused, which implies less statistical diversity; contrarily, large angular spread means more statistical diversity available.

Coherence Distance

The dual of angular spread is coherence distance. A coherence distance Dc means that any physical positions separated by Dc have an uncorrelated received signal amplitude and phase. Coherence distance is often related to the wave length. Thus, higher frequency system has shorter coherence distance.

Coherence distance is often used to specify how far apart antennae should be separated to make received signals statistically independent. Small coherence distance means antenna array can be easily applied to provide rich diversity. This is particularly useful for mobile terminals which has the size constraint. Large coherence distance implies that large separation between two antennas is needed. In this case, it would be preferable to use beamforming with antenna array.

Channel model for OFDM

Since WiMAX is using OFDM, the channel model with multidimension correlation should be considered. When channel is highly frequency selective, it requires a large number of closely spaced subcarriers to combat the ISI (Inter Symbol Interference). On the other hand, when the channel is with large Doppler frequency shift (high mobility), it will degrade the subcarrier orthogonality. In the frequency domain, it will cause significant ICI (Inter Carrier Interference) as subcarriers become more closely spaced.

Emperical model

An emperical channel model for MIMO was proposed and developed by 3GPP (Third Generation Partnership Project). It is described in the technical report TR 25.996 [2]. This model can be used for systems such as WiMAX, 802.11n and 802.20 which use MIMO techniques.

Reference:
[1] Theodore S. Rappaport, "Wireless Communications: Principles and Practice (2nd Edition)," Prentice Hall PTR, December, 2001
[2] Spatial channel model for Multiple Input Multiple Output (MIMO) simulations http://www.3gpp.org/ftp/Specs/archive/25_series/25.996/

Sunday, May 6, 2007

[WiMAX] Fundamentals of WiMAX - (1)

WiMAX (Worldwide Interoperability for Microwave Access) is one kind of wirless broadband access technology. In my country, China, WiMAX is going to be deployed for the 2008 Olypmic games. Actually, in China there is another technology called McWill (Multicarrier Wireless internet Local Loop), which is a synchronous CDMA system using Smart Antenna , SWAP signaling , synchronous CDMA and Software Radio. This McWill is developed by a Chinese company. I remembered in the end of 2006, the news said McWill would be used for 2008 Olympic games. However, just after several months, the latest news says that WiMAX is authorized by the committe of the Olympic games and China Mobile is goint to deploy it in 2008. It looks like McWill lost the game. It is really a pity.

Ok, let us go back to our topic and concentrate on techniques. From today, I'm going to give a series of introduction of WiMAX, mainly focuses on its fundamental issues, such as overview, history, technicall challeges, physical layer, MAC layer etc. I think it also helps me consolidate my knowledge on WiMAX.

History of WiMAX

Before WiMAX, there already exist many wireline broadband access techniques, such as DSL(Digital Subscriber Line), cable modem, FTTP(Fiber to the premises ) and VDSL. This market grows quickly. When I came to the US in 2001, the rate of Verizon's is about 768Kbps. Now I'm using Comcast's cable modem with rate 5Mbps. Recently, Verizon is promoting its FIOS system which is said to support rate at 15-40Mbps.

Later, people found it is necessary to provide broadband access service in wireless, because it offers additional benifits and convenience. For example, it may be easier to install than wire-line access, the cost may be lower, it can be portable and mobile.

Generally speaking, wireless broadband access can be categorized into two types:
1) Fixed wireless broadband,
2) Mobile broadband.

As for WiMAX, it has evolved through four stages. [1]
1) Narrowband WLL (Wireless Local Loop) system
2) First generation LOS (Line of Sight) broadband system
3) Second generation NLOS (Non Line of Sight) broadband system
4) Standard-based broadbank system.

Next, let us take a quick look of those four stages.

1) Narrowband WLL

It is mainly based on DECT or CDMA. As I know, WLL on CDMA at 450MHz has been using in China for a long while.

2) LOS

In 1990s, LMDS(Local Multipoint Distribution System), works on 2.5 or 3.5GHz. Operator: Nextlink. Now it is almost dead. [2]

In late 1990s, MMDS(Multichannel Multipoint Distribution System), works on 2.5GHz. Operators: MCI, Sprint.

Starting in 2006, MMDS frequencies in the 2110-2155 MHz range are being re-allocated for the Advanced Wireless Services (AWS-1). [3]

3) NLOS

It is proposed to overcome the LOS issue, and improve link performance under multipath conditions. It may use OFDM, CDMA or multiantenna techniques.

4) Standard-based

It is the well-known IEEE 802.16x [4], a seriers of standards for Wireless Metropolitan Area Network (WMAN).

  • December 2001, IEEE 802.16 standards completed for > 11GHz.
    Wireless WAN-SC, uses single carrier modulation techniques. MAC layer is with TDM.
  • January 2003, IEEE 802.16a standard completed.
    Works in the 2GHz to 11GHz range, enables NLOS deployments by using OFDM. MAC is using OFDMA.
  • June 2004, IEEE 802.16-2004 standard completed and approved.
    It replaces 802.16, 802.16a, 802.16c with a single standard. It is also called Fixed WiMAX.
  • December 2005, IEEE802.16e-2005 approved.
    Supports vehicular mobility applications. It is also referred to as Mobile WiMAX.

Similar to 802.11x, IEEE established the specifications for WiMAX but left to the industry the task of converting them into an interoperable standard and the task of certification. WiMAX Forum (analogous as Wi-Fi Alliance) was formed to do those tasks.



References:
[1] J.G.Andrews, Arunabha Ghosh and Rias Muhamed, "Fundamentals of WiMAX-understanding Broadband Wireless Networking", Prentice Hall, Feb, 2007

[2] http://en.wikipedia.org/wiki/MMDS

[3] http://en.wikipedia.org/wiki/LMDS

[4] http://en.wikipedia.org/wiki/Wimax