3G in the Eye of RF Engineer

Gujarat Telecom Circle

3 Steps to 3G: GSM Transition to W-CDMA.

KALPESH B JESALPURA JTO CMTS


UMTS Network Architecture :The Radio Network Controller (RNC) controls the radio resources of the Node Bs that are connected to it. The RNC is analogous to a BSC in GSM. Combined, an RNC and the Node Bs that are connected to it are known as a Radio Network Subsystem (RNS). The interface between a Node B and an RNC is the Iub interface. Unlike in GSM, where BSCs are not connected to each other, in the UMTS RAN (officially, the UMTS Terrestrial Radio Access Network, or UTRAN), an interface exists between the RNCs. This interface is termed Iur. The primary purpose of this interface is to support inter-RNC mobility and soft handover between Node Bs connected to different RNCs.

MSC is divided into an MSC server (Call Server) and a media gateway (MGW). The Call server contains all of the mobility management and call control logic that would be contained in a standard MSC. It does not, however, contain a switching matrix. The switching matrix is contained within the MGW, which is controlled by the MSC server and can be placed remotely from the MSC. Control signaling for circuit-switched calls is between the RNC and the MSC server. The media path for circuit-switched calls is between the RNC and the MGW. Typically, an MGW will take calls from the RNC and KALPESH B JESALPURA JTO CMTS

Gujarat Telecom Circle routes those calls towards their destinations over a packet backbone. packet data traffic from the RNC is passed to the SGSN and from the SGSN to the GGSN over an IP backbone. Given that data and voice can both use IP transport within the core network, a single backbone can be constructed to support both types of service. This can mean significant capital and operating expenses compared to the construction and operation of separate packet and circuit switched backbone networks.

W-CDMA Parameters



Comparison of IS-2000 and W-CDMA

NOTE :- Third Generation Partnership (3GPP) works on UMTS, which is based on WCDMA, and 3GPP2 works on CDMA2000.

Comparison of GSM and W-CDMA



UMTS Speech Service UMTS uses the Adaptive Multirate (AMR) speech coder. This is actually several coders in one and provides coding rates of 12.2 Kbps, 10.2 Kbps, 7.95 Kbps, 7.40 Kbps, 6.70 Kbps, 5.90 Kbps, 5.15 Kbps, and 4.75 Kbps. The AMR coder allows for the speech bit rate to change dynamically during a call. As we shall describe later, the higher the bit rate of any service, the smaller the effective footprint of a cell. Thus, a user at the edge of a cell could change from a high speech-coding rate to a lower speech-coding rate to effectively extend the coverage for speech service. Each AMR speech frame is 20 ms in duration and it is possible to change the speech-coding rate from one speech frame to the next. Thus, the coding rate could change as often as every 20 ms, although that is unlikely to ever happen in reality. The AMR coder also supports voice activity detection (VAD) and discontinuous transmission (DTX), with comfort noise generation.

WCDMA Basics



If multiple users transmit simultaneously on the same frequency, then the stream of data from each user needs to be spread according to a different pseudo-random sequence. In other words, each user data stream needs to be spread according to a different spreading code. At the receiving end, the stream of data from a given user is recovered by despreading the set of received signals with the appropriate spreading code. Of course, what is being despread is the complete set of signals received from all users that are transmitting. Imagine, for example, two users (A and B) that are transmitting on the same frequency, but with two different spreading codes. If, at the receiving end, the received signal is despread with the spreading code applicable to user A, then the original data stream from user A is recovered. The data stream that is recovered does have some noise created by the fact that the received signal also contains user data from user B. The noise, however, is small. Similarly, if the received signal is dispread according the spreading code used by user B, then the original data stream from user B is recovered, with a little noise generated by the presence of user A’s data within the spread signal. Provided that the rate of the spreading signal (the chip rate) is far larger than the user data rate, then the noise (that is, the interference) generated by the presence of other users will be sufficiently small to not inhibit the recovery of the data steam from a given user. Of course, as the number of simultaneous users increases, so does the interference and it eventually becomes impossible to recover a specific user’s data with any confidence. The ratio of the chip rate to the user data symbol rate is known as the spreading factor. The capability to recover a given user’s signal is directly influenced by the spreading factor. The higher the spreading factor, the greater the capability to recover given user’s signal. In terms of transmission and reception, a higher spreading factor has an equivalent effect as transmitting at a higher power. Thus, the magnitude of the spreading factor can be considered a type of gain and is known as the processing gain. In dB, Processing gain = 10 X 10Log10 (spreading rate/user rate). The gain due to Despreading of the signal over wideband noise is the Processing Gain KALPESH B JESALPURA JTO CMTS


The WCDMA air interface of UMTS (hereafter simply WCDMA) has a nominal bandwidth of 5 MHz. The chip rate in WCDMA is 3.84 X 106 chips/second (3.84 Mcps). In theory, for a speech service at 12.2 Kbps (and, for now, assuming no extra bandwidth for error correction), the spreading factor would be 3.84 X 106/12.2 X 103 = 314.75. This would equate to a processing gain of 25 dB. In reality, however, WCDMA does include extra coding for error correction. Consequently, a spreading factor as high as 314.75 is not supported, at least not in the uplink. The supported uplink spreading factors are 4, 8, 16, 32, 64, 128, and 256. The highest spreading factor (256) is used mostly by the various control channels.

Uplink Spreading Factors and Data Rates At first glance, it appears that the lowest spreading factor (4) provides a gross rate of only 960 Kbps and a usable rate of only 480 Kbps. This does not meet the requirements of IMT-2000, which states that a user should be able to achieve speeds of 2 Mbps. In order to meet that requirement, UMTS supports the capability for a given user to transmit up to six simultaneous data channels. Thus, if a user wants to transmit user data at a user rate greater than 480 Kbps, then multiple channels are used, each with a spreading factor of four. With six parallel channels, each at a spreading factor of four, a single user can obtain speeds of over 2 Mbps. KALPESH B JESALPURA JTO CMTS


Downlink Spreading Factors and Data Rates In the downlink, the same spreading factors are available, with a spreading factor of 512 also possible. One difference between the uplink and downlink, however, is the number of bits per symbol. In “Universal Mobile Telecommunications Service (UMTS),” the uplink effectively uses one bit per user symbol, while the downlink effectively uses two bits per user symbol. Consequently, for a given spreading factor, the user bit rate in the downlink is greater than the corresponding bit rate in the uplink. The user rate in the downlink is not quite twice that in the uplink, however, due to differences in the way that control channels and traffic channels are multiplexed on the air interface. An important capability of WCDMA is that user data rates do not need to be fixed. In WCDMA, channels are transmitted with a 10-ms frame structure. It is possible to change the spreading factor on a frame-by-frame basis. Thus, within one frame, the user data rate is fixed, but the user data rate can change from frame to frame. This capability means that WCDMA can offer bandwidth on demand. Note : - Rate changes every 10 ms do not apply to AMR speech as each speech packet is 20 ms in duration, so that the speech rate can change every 20 ms if needed, but not every 10 ms.



The WCDMA Air Interface In other words, if a single user (user A) is transmitting both user data and control information, the base station must first separate the set of transmissions from user A from the transmissions of all other users. It must then separate the control information from the user data. First, each individual data stream is spread to the chip rate by the application of a spreading code, also known as a channelization code, and which operates at the chip rate of 3.84 Mcps. Then the combined set of spread signals is scrambled by the application of a scrambling code, which also operates at the chip rate. The channelization spreads the individual data streams and hence increases the required bandwidth. Since the scrambling code also operates at the chip rate, however, it does not further increase the required bandwidth. At the receiving end, the combined signal is first descrambled by application of the appropriate scrambling code. The individual user data streams are then recovered through the application of the appropriate channelization codes.

Uplink Spreading, Scrambling, and Modulation A physical channel is what carries the actual user data or control information over the air interface. A physical channel can be considered a combination of frequency, scrambling code, and


Gujarat Telecom Circle channelization code, and in the uplink, as we shall describe later, the relative phase is also significant. Channelization Codes :As mentioned, the channelization codes are used to separate multiple streams of data from a given user, whereas the scrambling codes are used to separate transmissions from different users. The channelization codes are known as Orthogonal Variable Spreading Factor (OVSF) codes.

Orthogonal Variable Spreading Factor (OVSF)

NOTE : - Correlation of any 2 OVSF code will always result in equal no. of 1’s and 0’s

General Development of the OVSF 0 1 0 0

0 0 0 0

2X2

0 1 0 1

0 0 1 1

4X4

0

Repeat Horizontally

0

0 1 1 0

0 0 0 0

0 1 0 1

0 0 1 1

0 1 1 0

0 0 0 0

0 1 0 1

0 0 1 1

0 1 1 0

0 0 0 0

0 1 0 1

0 0 1 1

0 1 1 0

1 1 1 1

1 0 1 0

1 1 0 0

1 0 0 1

8X8 Repeat

Reverse Diagonally

Vertically

0 KALPESH B JESALPURA JTO CMTS

1

Gujarat Telecom Circle Scrambling Codes :Once the different channels have been spread with appropriate channelization codes, they are combined, as shown in Figure below, and then scrambled by a particular scrambling code. Two types of scrambling codes exist—Long and Short scrambling codes.

The long scrambling codes used in WCDMA are known as Gold codes and are constructed from the modulo 2 addition of portions of two binary m-sequences. The portions used are segments of length 38,400. Because the long scrambling codes are generated from msequences, they have pseudorandom characteristics. The short scrambling codes also have pseudorandom characteristics. These, however, are much shorter, at a length of length 256 chips. Long scrambling codes are used in the case where the base station uses a rake receiver. Short scrambling codes can be used when the base station uses advanced multi-user detection techniques such as a Parallel Interference Cancellation (PIC) receiver. KALPESH B JESALPURA JTO CMTS

Gujarat Telecom Circle PN Sequences: Gold Codes

Uplink Scrambling codes can be long or short codes • Long codes are complex valued Gold codes and are a 38400 chip segment from a 225 chip code, duration 10ms (1 frame) – There are 16,777,216 codes… • Short codes are complex valued S(2) codes and 256 chips long, duration 66.67ms – There are also 16,777,216 codes

Some important restrictions apply to the use of channelization codes. That is because, in the case where more than one channel is being transmitted, the chosen channelization codes must be orthogonal. KALPESH B JESALPURA JTO CMTS


For example, consider the channelization code Cch,4,0. This code is simply the sequence 1,1,1,1 repeated over and over, with each sequence of four bits repeated 960,000 times per second. Consider the channelization code Cch,8,0. This is simply the sequence 1,1,1,1,1,1,1,1 repeated over and over, with each sequence of eight bits repeated 480,000 times per second. Clearly, if one data stream from a given user is spread with the code Cch,4,0, and a second data stream from the same user is spread with the code Cch,8,0, the net effect is that they are spread in the same way and cannot be distinguished at the receiver. Consequently, channelization codes must be selected in a manner that ensures that each channel is spread differently. Uplink Modulation Uplink Modulation WCDMA uses Quadrature Phase Shift Keying (QPSK) modulation in the uplink.

Uplink Spreading and Modulation


Gujarat Telecom Circle Downlink Spreading, Scrambling, and Modulation Downlink includes pilot channels, synchronization channels, channels used for the broadcast of system information, channels used for the paging of subscribers, and so on. Downlink Spreading With the exception of the synchronization channels (SCHs), the downlink channels are spread to the chip rate and scrambled, each channel to be spread is split into two streams—the I branch and the Q branch. The even symbols are mapped to the I branch and the odd symbols are mapped to the Q branch. The I branch is treated as a stream of real-valued bits, whereas the Q branch is treated as a stream of imaginary bits. Each of the two streams is spread by the same channelization code. The spreading code/channelization code to be used is taken from the same code tree as used in the uplink—that is, OVSF codes that are chosen to maintain the orthogonality between different channels transmitted from the same base station. On the downlink, however, each channel (with the exception of the synchronization channel) is subjected to a serial-to-parallel conversion, as shown in below. For a given spreading factor, the serial-to-parallel conversion effectively doubles the data rate of the physical channel. In other words, half of the channel’s data is carried on the I branch with half on the Q branch, and both of these are spread with the same spreading factor.


Gujarat Telecom Circle If, for example, we have a spreading factor of 8, then the data rate on the I channel is 480 Kbps and the data rate on the Q channel is also 480 Kbps. The net data rate is 960 Kbps—twice that achieved on the uplink for the same value of spreading factor. In reality, however, the data rate on the downlink is not quite twice that on the uplink. This is due to the fact, as is explained later, that control information is timemultiplexed with a user data on the downlink. This reduces the net throughput for a given downlink data channel. Nonetheless, for a given spreading factor on the downlink, the effective throughput is significantly greater than the corresponding throughput on the uplink for the same spreading factor. Downlink Scrambling The downlink scrambling codes are used to separate the transmissions of one cell from those of another. The downlink scrambling codes are Gold codes similar to the long scrambling codes used in the uplink. As is the case for the long codes used on the uplink, the codes used on the downlink are limited to a 10-ms duration. Downlink Scrambling codes are complex valued Gold codes 18 • They have a 38400 chip segment from a 2 chip code, duration 10ms (1 frame)

Therefore, the available downlink scrambling codes are separated into 512 groups. Each group contains one primary scrambling code and 15 secondary scrambling codes. Thus, 512 primary scrambling codes exist and 7,680 secondary scrambling codes exist, for a total of 8,192 downlink scrambling codes. A cell is allocated one, and only one, primary scrambling code, which, of course, has 15 secondary scrambling codes associated with it. A given base station will use its primary scrambling code for the transmission of channels that need to be heard by all terminals in the cell. Thus, paging messages need to be scrambled by the cell’s primary scrambling code. For that matter, all transmissions from the base station can simply use the cell’s primary scrambling code. After all, it is the scrambling code that identifies the cell, while the various channelization codes are used to separate the various transmissions (physical channels) within the cell. KALPESH B JESALPURA JTO CMTS

Gujarat Telecom Circle A cell can, however, choose to use a secondary scrambling code for channels that are directed to a specific user and do not need to be decoded by other users. In general, it is a good idea for all transmissions from a cell to use the cell’s primary scrambling code, as this helps to minimize interference.

512 primary scrambling codes are available. These are divided into 64 groups, each consisting of 8 scrambling codes, as shown in table below: Downlink spreading and scrambling are applied to all downlink physical channels transmitted on a cell, with the exception of the synchronization channel (SCH). This channel is added to the downlink stream, as shown in Figure below. In fact, as explained later in this chapter, the SCH contains two subchannels—the primary SCH and secondary SCH. The reason why these are transmitted without scrambling is the fact that they are the first channels decoded by a terminal. If they were scrambled, then the terminal would first have to know the scrambling code of the base station just to synchronize.



Downlink Modulation The downlink uses QPSK modulation. The process in the downlink is the same as that shown in Figure above for the uplink. Downlink Spreading and Modulation



Spread Spectrum Basics Spreading and Scrambling

At the transmitter, the information source provides symbols • The symbols are applied to a spreading code • The resulting chip-rate spread signal is applied to a Scrambling Code • The resulting chip-rate spread/scrambled signal modulates the transmitter The Receiver recovers the signal and the same scrambling code descrambles it • Next the spreading code de-spreads the signal, yielding the original symbol-rate data



Glossary GSM : - Global System for Mobile communications WCDMA :- Wideband Code Division Multiple Access DS-CDMA :- Direct-Sequence CDMA UMTS : - Universal Mobile Telecommunications System UTRA : - Universal Terrestrial Radio Access GPRS :- General Packet Radio Service EDGE : - Enhanced Data Rates for Global Evolution ETSI :- European Telecommunications Standards Institute FDD :- Frequency Division Duplex TDD :- Time Division Duplex TDMA :- Time Division Multiple Access CDMA : - Code Division Multiple Access FDMA :- Frequency Division Multiple Access 3GPP : - Third Generation Partnership (3GPP works on UMTS, which is based on WCDMA, and 3GPP2 works on CDMA2000.) RAN :- Radio access network UTRAN :- UMTS Terrestrial Radio Access Network UE : - User Equipment ME : - Mobile Equipment USIM :- UMTS Subscriber Identity Module HSDPA : - High Speed Downlink Packet Access


3G in the Eye of RF Engineer

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