Wideband CDMA Air Interface
Orthogonal Variable Spreading Factors (OVSF)
Channel-Structure and Synchronization with UMTS
Synchronization
Advantages of W-CDMA over GPRS/EDGE
Roll Out of UMTS
UMTS Licensing
Development of first UMTS Devices

The 3G successor to GSM was called UMTS. This standard is described in more detail below.

UMTS Terrestrial Radio Network (UTRAN)

With 3G comes a new access network. The previous GSM access via base station with the base transceiver station and the base station controller can no longer be used. A parallel new access network was required. This is called UMTS Terrestrial Radio Network (UTRAN).

In order to distinguish from GSM components, the key elements are also renamed. The base transceiver station is now called Node-B (network node B). The end device is no longer called Mobile Station (MS) but User Equipment (UE).

Wideband-CDMA Air Interface

W-CDMA can transmit both packet data and voice, as we will see below. Therefore, there is no need for a special packet control unit like GPRS. The Node-B is controlled using a new Radio Network Controller (RNC).

The core element of UMTS is W-CDMA. CDMA as a technology has already been discussed as IS-95. However, the decision was made to have a significantly higher bandwidth for UMTS; instead of 1.25 MHz, W-CDMA has a bandwidth of 5 MHz (hence wideband). Since bandwidth is directly related to bit rate or chip rate, W-CDMA has a fixed chip rate of 3.84 Mchip/s.

The so-called spreading factor (SF), i.e. the length of a sequence with which the data is spread, is essential for the number of channels as well as the actual transmission bit rate. The higher the SF, the better the protection of the bits and more channels can be transmitted at the same time. However, the bit rate is low in this case. In return, higher bit rates come at the price of lower protection of the bits and a smaller number of possible channels.

In IS-95, Walsh functions were used for spreading with a fixed length of 64. This results in a fixed data rate. As described in the IMT chapter, this was later extended to 128.

ORTHOGONAL VARIABLE SPREADING FACTORS (OVSF)

A flexible approach is chosen for W-CDMA, i.e. the spreading factor is variable. For this purpose, a whole series of codes are used which are called Orthogonal Variable Spreading Factors (OVSF). For each SF there is a set of spreaders which have the same orthogonal properties as the Walsh codes and are therefore suitable for CDMA. The OVSF are shown in the following figure up to an SF of 8. For W-CDMA the SF goes up to 256 as the maximum spread length.

The OVSF have a tree structure. I.e. C(1,1) develops into C(2,1) and C(2,2). C(4,3) develops into C(8,5) and C(8,6).

Individual users can now be assigned different SF levels depending on their transfer rate requirements. The lowest degree is SF = 8. This can be used 32 times until it reaches the length of 256. However, larger SF degrees created behind the code used can no longer be used for other users. This makes it possible to transmit different bit rates, as the following table shows.

Spreading Factor	Data rate (kbit/s)	Data rate after Channel Coding (kbit/s)	Application
8	960	384	Packet Data
16	480	128	Packet Data
32	240	64	Packet Data/Video
64	120	32	Packet Data
128	60	12, 28	Voice, Packet Data, Location Update, SMS
256	30	5, 15	Voice

This results in a maximum data rate of 384 kbit/s with a spreading factor of 8. This is the highest rate that UMTS R99 can deliver.

As with IS-95, spreading with OFSF codes is done to reach the final chip rate of 3.84 Chip/s. A scrambling with a pseudo noise sequence follows. Above all, this ensures that there is an optimal spectral distribution of the signal. In addition, the corresponding PN sequence can be used for identification. Spreading and scrambling therefore have different functions in the downlink and uplink.

In the downlink from the base station to the end device, the OVSR code is used to set the data rate and also to identify the user. Scrambling is used to identify the base station.

In the uplink from the end device to the base station, spreading with the OVSR signal only serves to identify the data rate. The participants are separated using different spreading signals.

Channel Structure and Synchronization

In GSM, the first of the eight time slots were reserved for control channels. The assignment was easy. In IS-95, the control channels were determined by fixed Walsh codes. With UMTS, the organization of the channels is a little more complicated. First of all, there are three levels:

• Logical level:
  describes the information that is to be transferred.
• Transport level:
  Here the control data is prepared and channel coded according to the following level
• Physical level:
  These are the actual channels used, which differ by different codes

There are two channels in which information is distributed:

Primary Common Control Physical Channel P-CCPCH: This channel is used to communicate information about the cell and its environment. The P-CCPCH sends with the simple code 256.1, i.e. only 1 before scrambling.

Secondary Common Control Physical Channel P-CCPCH: Paging is done via this channel (Logical Paging Channel PCH). Furthermore, this physical channel also contains fast control information (Fast Associated Control Channel), for example for establishing a connection.

The following channels are used for data transmission:

Dedicated Physical Data Channel (DPDCH): This is the traffic channel for voice and packed data and contains parallel control information (Dedicated Control Channel) in order to prepare information about reception and upcoming cell changes. As described, the channels differ in their corresponding spreading codes.

Dedicated Physical Control Channel (DPCCH): This is an extra control channel to control the transmission power. As with IS-95, UMTS also has to regulate the transmission power quickly (1500 times per second). See the near-far problem in IS-95.

In addition to the DPDCH and the DPCCH, the following channel is defined in the uplink:

Physical Random Access Channel (PRACH): The mobile device initiates a connection to the network via this channel.

Synchronization

Before a connection between UE and Nobe-B can be established, synchronization must be established. UMTS is embedded in a frame structure which consists of 15 slots (time slots). The Nobe-B sends 2560 chips in each slot. 15 slots therefore take 10 ms. The first 256 chips of the slots form two special synchronization channels, the Primary Synchronization Channel P-SCH and the Secondary Synchronization Channel S-SCH. On the P-SCH, all Node-Bs send the same synchronization sequence. This makes it possible for the mobile device to synchronize with the slots by autocorrelating (comparing) the input signal of the strongest Node-B with the known synchronization sequence.

Once the start of the slot is detected, the S-SCH can be decoded in which 15 different, known sequences are sent. This allows the start of a frame to be detected. If this is known, the PN sequence of Node B can be identified using another auxiliary channel, the C-PICH (Common Pilot Channel). There are 64 different PN sequences with which a Node-B can send. If the PN sequence is known, the P-CCPCH can finally be decoded and all the information necessary for logging in can be read and saved.

Advantages of W-CDMA over GPRS/Edge

A significant (and of course expected) advantage of UMTS over GPRS/EDGE is of course the data rate. UMTS allows data rates of up to 384 kbit/s. However, under favorable circumstances, EDGE can also achieve data rates of up to 236.8 kbit/s. Therefore, UMTS does not really represent a quantum leap in transmission speed.

However, a key difference is the response time when interacting in the network. With GPRS, establishing packet switching is complex and time-consuming. It takes on average 500 – 1000 ms (i.e. up to 1 second) for the network to respond to a request. This is seen as very disturbing or disruptive when working on the Internet. This gets worse when a cell turnover occurs. There may be an interruption of several seconds here. The reason for these response times is that the channels for transmission have to be released again and again in order to save capacity. Once the channels have been assigned, they then have to be set up again.

The biggest advantage of UMTS over GPRS/EDGE was the response time, which was significantly reduced.

With W-CDMA it was possible to keep the channels for much longer without immediately affecting the capacity. There are enough codes available. If necessary, it is possible to fall back on codes with a large SF. This means the response time was significantly shorter without around 200 ms.

Roll Out of UMTS

As described, the first UMTS standard was adopted as R99 at the end of 1999. At that time there was still an euphoria about mobile communications. Every year new innovative devices came onto the market with new functions and possibilities (see Feature Phone). Mobile communications seemed to be developing into a money printing machine.

So UMTS initially promised a golden future. The following functions were promised on the mobile phone in the short term.

• “Real Internet Browsing” (beyond WAP)
• Video calls
• Location-based services (such as navigation etc.)
• TV

UMTS Licensing

There was also euphoria among the national organizations that issued the licenses for the new UMTS bands. For example, in Germany (as well as in other European countries) the frequency ranges were auctioned off. Six parties bought spectrum in Germany and paid almost 50 billion euros for it. Each mobile operator not only had to pay around 8 billion for the license, but also had to commit to installing a network within 2 years.

50 billion euros. If you assume (very optimistically) that you can win 50 million users for UMTS services, you have to generate 1000 euros per user in order to even recoup the costs of the licenses. This did not take into account the fact that a completely new mobile network had to be installed, another billion-dollar investment. In retrospect, it is strange that a business model could be build on this bad starting conditions, especially since it could not be expected that customers would pay significantly more for UMTS services. In fact, two participants in Germany gave up and gave back their licenses. (Which were then reassigned later).

In March 2000, the so-called dotcom bubble burst and with it the belief in the unlimited growth of digital Internet services.

One reason for the optimism was probably the absolute belief in the success of the Internet and the companies associated with it. The share prices of the companies involved in this soared to unprecedented heights, reaching their peak in March 2000. Then the share prices suddenly began to fall and finally crashed. There was talk of the “bursting of the DotCom bubble”. Many companies had to file for bankruptcy. Large companies had to lay off employees in order to survive. It was not a good time to invest. The decline continued until 2003 and it would take another 15 years for technology prices to return to 2000 levels.

Development of first UMTS devices

UMTS was a major challenge for developers of chips and devices. New baseband circuits had to be developed which, in addition to the GSM functionality, also had to provide UMTS functionality. For some manufacturers, the development of CDMA-based receivers and transmitters was new territory and required a lot of effort. Qualcomm had better conditions here. They were probably the best at mastering CDMA technology through IS-95 and were able to quickly expand it to W-CDMA. In addition, it was easier for Qualcomm to integrate GSM functionality into their chipsets than it was for the Europeans to integrate W-CDMA into GSM chipsets. The MSM6200 was released in 2002 and became a dominant system solution for UMTS. Thus, at least for the chipsets, Qualcomm managed to enter the European systems and Qualcomm developed into the leading manufacturer of baseband processors.

For all (other) manufacturers, system development was a major challenge. Not only did the hardware have to be built and mastered, the SW or the so-called firmware, that ran on the DSP, became a big problem.

There was also the general question whether a large investment in UMTS was even worth it. Would there even be a market for devices? Why should a user, who has previously used a GSM phone buy a UMTS device? A UMTS device would even consume more power, when it was used for voice calls. What made matters even worse was that UMTS was not a global market. UMTS devices could not be used outside of Europe because different frequencies were used. The question „Why UMTS“ was legitimate and open. What was the “killer application” that was supposed to convince users to use the new technology?And, above all, who would pay for it.

It took until 2004 for the UMTS network to go into operation in Germany and for the first mobile devices to come onto the market. Initially there were only a few telephones that did not differ significantly from normal GSM devices. They were rather heavier and, above all, more expensive. One of their special features was video telephony. However, this only worked with another UMTS device under favorable circumstances and at a high minute price. In 2005, only 2.5% of mobile subscribers used UMTS. Even in 2007 the proportion had only grown to 10%. There was also competition from the emerging WLAN, which became more and more popular and was also publicly available in so-called hotspots. WLAN had significantly higher bit rates than UMTS and was practically free. UMTS also had to improve in terms of performance (speed).